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ASME Conference Presenter Attendance Policy and Archival Proceedings

2018;():V001T00A001. doi:10.1115/ICEF2018-NS1.
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This online compilation of papers from the ASME 2018 Internal Combustion Engine Division Fall Technical Conference (ICEF2018) represents the archival version of the Conference Proceedings. According to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference by an author of the paper, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Large Bore Engines

2018;():V001T01A001. doi:10.1115/ICEF2018-9505.

In recent years, to satisfy the more and more stringent energy efficiency and pollutants emission regulations of ship, which had been issued by the International Marine Organization (IMO), the combustion improvement of the two-stroke low-speed diesel engines has been paid much attention. The phenomenological combustion model, as an effective and economic approach, is widely used for parametric study on diesel engine combustion process. However, the fuel of two-stroke low-speed diesel engine is heavy oil, and there are few researches focused on the modeling of heavy oil spray. Therefore, a spray model that can describe the heavy oil spray evolution is needed. In this study, a one-dimensional discrete diesel spray model based on the conservation of the momentum flux and mass flow rate along the spray axis is modified for heavy oil. By in-depth analysis of physical properties of diesel and heavy oil, viscosity is found to be the main factor that results in the difference of the fuel concentration and velocity distribution over the spray cross-sectional area. According to the turbulent jet theory, the Schmidt number, which represents the capability of mass and momentum diffusion, proves to be inversely related to fuel viscosity. In order to involve the viscosity effects into the one-dimensional diesel spray model, the relation between viscosity and Schmidt number is derived as a simple formulation to account for the fuel concentration and velocity distribution. The calculation of heavy oil spray penetration is validated by the experiment data, and the results shows that the improved spray model has the capability to predict the propagation of heavy oil spray.

Commentary by Dr. Valentin Fuster
2018;():V001T01A002. doi:10.1115/ICEF2018-9523.

The present publication describes investigations on a lean burn gas engine equipped with a variable intake valve train and demonstrates how steady state engine performance can be improved in comparison to a conventional state-of-the-art application with constant Miller timing.

As the knock border represents a significant limitation of the operating range of gas engines, the engine specific knock limit was derived from measurements on a single cylinder research engine and transferred to a 1D simulation model of the corresponding multicylinder engine; a large bore, two stage turbocharged gas engine in the 5 MW power range with a variable intake valve train. Special attention was given to the setup of the simulation model to improve prediction quality and reduce simulation effort.

An optimal strategy using the flexibility of a variable intake valve train for engine operation is presented that is capable of accommodating fluctuating gas qualities, which are described by the methane number. The operating strategy was derived from the 1D simulation model. The better performance than with a state-of-the-art strategy will be quantified in terms of engine efficiency while knocking combustion caused by low methane numbers is prevented.

Since ambient temperatures in certain regions where the engine is operated do not remain stable throughout the year and ambient pressure varies depending on sea level, these issues must also be addressed. The temperature and density of the intake air have a large influence on the performance of the turbocharging unit and thus overall engine efficiency. The simulation results show the engine’s behavior under varying ambient conditions and outline potential strategies for improvement made possible by using variable valve timing on the intake side.

Topics: Valves , Trains , Gas engines
Commentary by Dr. Valentin Fuster
2018;():V001T01A003. doi:10.1115/ICEF2018-9569.

Marine diesel engines usually operate on a highly boosted intake pressure. The reciprocating feature of diesel engines and the continuous flow operation characteristics of the turbocharger (TC) make the matching between the turbocharger and diesel engine very challenging. Sequential turbocharging (STC) technology is recognized as an effective approach in improving the fuel economy and exhaust emissions especially at low speed and high torque when a single stage turbocharger is not able to boost the intake air to the pressure needed. The application of STC technology also extends engine operation toward a wider range than that using a single-stage turbocharger.

This research experimentally investigated the potential of a STC system in improving the performance of a TBD234V12 model marine diesel engine originally designed to operate on a single-stage turbocharger. The STC system examined consisted of a small (S) turbocharger and a large (L) turbocharger which were installed in parallel. Such a system can operate on three boosting modes noted as 1TC-S, 1TC-L and 2TC. A rule-based control algorithm was developed to smoothly switch the STC operation mode using engine speed and load as references. The potential of the STC system in improving the performance of this engine was experimentally examined over a wide range of engine speed and load. When operated at the standard propeller propulsion cycle, the application of the STC system reduced the brake specific fuel consumption (BSFC) by 3.12% averagely. The average of the exhaust temperature before turbine was decreased by 50°C. The soot and oxides of nitrogen (NOx) emissions were reduced respectively. The examination of the engine performance over an entire engine speed and torque range demonstrated the super performance of the STC system in extending the engine operation toward the high torque at low speed (900 to 1200 RPM) while further improving the fuel economy as expected. The engine maximum torque at 900 rpm was increased from 1680Nm to 2361 Nm (40.5%). The average BSFC over entire working area was improved by 7.4%. The BSFC at low load and high torque was significantly decreased. The application of the STC system also decreased the average NOx emissions by 31.5% when examined on the propeller propulsion cycle.

Commentary by Dr. Valentin Fuster
2018;():V001T01A004. doi:10.1115/ICEF2018-9582.

Pistons for heavy duty diesel applications endure high thermal loads and therefore result in reduced durability. Pistons for such heavy duty applications are generally designed with an internal oil gallery — called the piston cooling gallery (PCG) — where the intent is to reduce the piston crown temperatures through forced convection cooling and thereby ensure the durability of the piston. One of the key factors influencing the efficiency of such a heat-transfer process is the volume fraction of oil inside the piston cooling gallery — defined as the filling ratio (FR) — during engine operation.

As a part of this study, a motoring engine measurement system was developed to measure the piston filling ratio of an inline-6 production heavy duty engine. In this system, multiple high precision pressure sensors were applied to the piston cooling gallery and a linkage was designed and fabricated to transfer the piston cooling gallery oil pressure signal out of the motoring engine. This pressure information was then correlated with the oil filling ratio through a series of calibration runs with known oil quantity in the piston cooling gallery. This proposed method can be used to measure the piston cooling gallery oil filling ratio for heavy duty engine pistons. A preliminary transient Computational Fluid Dynamics (CFD) analysis was performed to identify the filling ratio and transient pressures at the corresponding transducer locations in the piston cooling gallery for one of the motoring test operating speeds (1200 RPM). A mesh dependency study was performed for the CFD analysis and the results were compared against those from the motoring test.

Commentary by Dr. Valentin Fuster
2018;():V001T01A005. doi:10.1115/ICEF2018-9602.

The following article details a method for the optimization and improved use of the internal combustion engine as main propulsion. The focus here is not on new propulsion systems or combustion processes, but on the characterization of the typical usage of existing systems in order to enable better utilization.

As one major potential for improvement, the transient machinery operation is examined and discussed in this article. Higher fuel consumption and higher emissions occur compared with stationary engine operation in that operation mode. Experimental data from test bed (“Caterpillar MaK 6M20”) measurements are presented which explain the consequences of transient operation. Furthermore, appropriate analyzing methods to evaluate this operation mode are shown. Finally, a modelling approach is presented using the data for calibration and validation of an engine simulation model.

The most significant part to predict real transient efficiency and emissions is the in-cylinder process and especially its combustion process. Therefore, the simulation model does not use engine maps but a mostly physically based engine model by using thermodynamic approaches and chemical reaction kinetics. The specific application of that simulation model for four-stroke medium-speed engines covers the behavior of transient operation during ship maneuverings since it is developed for integration into a ship engine simulator.

Commentary by Dr. Valentin Fuster
2018;():V001T01A006. doi:10.1115/ICEF2018-9670.

Carbon capture has been deemed crucial by the Intergovernmental Panel on Climate Change if the world is to achieve the ambitious goals stated in the Paris agreement. A deeper integration of renewable energy sources is also needed if we are to mitigate the large amount of greenhouse gas emitted as a result of increasing world fossil fuel energy consumption. These new power technologies bring an increased need for distributed fast dispatch power and energy storage that counteract their intermittent nature. A novel technological approach to provide fast dispatch emission free power is the use of the Argon Power Cycle, a technology that makes carbon capture an integral part of its functioning principle. The core concept behind this technology is a closed loop internal combustion engine cycle working with a monoatomic gas in concert with a membrane gas separation unit. By replacing the working fluid of internal combustion engines with a synthetic mixture of monoatomic gases and oxygen, the theoretical thermal efficiency can be increased up to 80%, more than 20% over conventional air cycles. Furthermore, the absence of nitrogen in the system prevents formation of nitrogen oxides, eliminating the need for expensive exhaust gas after-treatment and allowing for efficient use of renewable generated hydrogen fuel. In the case of hydrocarbon fuels, the closed loop nature of the cycle affords to boost the pressure and concentration of gases in the exhaust stream at no penalty to the cycle, providing the driving force to cost effective gas membrane separation of carbon dioxide. In this work we investigated the potential benefits of the Argon Power Cycle to improve upon current stationary power generation systems regarding efficiency, air pollutants and greenhouse gas emissions. A cooperative fuel research engine was used to carry out experiments and evaluate engine performance in relation to its air breathing counterpart. A 30% efficiency improvement was achieved and results showed a reduction on engine heat losses and an overall increase on the indicated mean effective pressure, despite the lesser oxygen content present in the working fluid. Greenhouse gas emissions were reduced as expected due to a substantial increase in efficiency and nitric oxides were eliminated as it was expected. Numerical simulation were carried out to predict the performance and energy penalty of a membrane separation unit. Energy penalties as low as 2% were obtained capturing 100% of the carbon dioxide generated.

Commentary by Dr. Valentin Fuster
2018;():V001T01A007. doi:10.1115/ICEF2018-9671.

Dual-Fuel (DF) engines offer great fuel flexibility combined with low emissions in gas mode. The main source of energy in this mode is provided by gaseous fuel, while the Diesel fuel acts only as an ignition source. For this reason, the reliable autoignition of the pilot fuel is of utmost importance for combustion in DF-engines. However, the autoignition of the pilot fuel suffers from low compression temperatures caused by Miller valve timings. These valve timings are applied to increase efficiency and reduce nitrogen oxide emissions. Previous studies have investigated the influence of injection parameters and operating conditions on ignition and combustion in DF-engines using a unique periodically chargeable combustion cell. Direct light high-speed images and pressure traces clearly revealed the effects of injection parameters and operating conditions on ignition and combustion. However, these measurement techniques are only capable of observing processes after ignition. In order to overcome this drawback, a high-speed shadowgraph technique was applied in this study to examine the processes prior to ignition. Measurements were conducted to investigate the influence of compression temperature and injection pressure on spray formation and ignition. Results showed that the autoignition of Diesel pilot fuel strongly depends on the fuel concentration within the spray. The high-speed shadowgraph images revealed that in the case of very low fuel concentration within the pilot spray only the first-stage of the two-stage ignition occurs. This leads to large cycle-to-cycle variations and misfiring. However, it was found that a reduced number of injection holes counteracts these effects. The comparison of a Diesel injector with 10-holes and a modified injector with 5-holes showed shorter ignition delays, more stable ignition and a higher number of ignited sprays on a percentage basis for the 5-hole nozzle.

Topics: Fuels , Engines , Diesel , Ignition
Commentary by Dr. Valentin Fuster
2018;():V001T01A008. doi:10.1115/ICEF2018-9682.

Large natural gas engines that introduce premixed fuel and air into the engine cylinders allow a small fraction of fuel to evade combustion, which is undesirable. The premixed fuel and air combust via flame propagation. Ahead of the flame front, the unburned fuel and air are driven into crevices, where conditions are not favorable for oxidation. The unburned fuel is a form of waste and a source of potent greenhouse gas emissions. A concept to vent unburned fuel into the crankcase through built-in slots in the liner during the expansion stroke has been tested. This venting process occurs before the exhaust valve opens and the unburned fuel sent into the crankcase can be recycled to the intake side through a closed crankcase ventilation system. The increased communication between the cylinder and the crankcase changes the ring pack dynamics, which results in higher oil consumption. Oil consumption was measured using a sulfur tracer technique. Careful design is required to achieve the best tradeoff between reductions in unburned hydrocarbon emissions and oil control.

Commentary by Dr. Valentin Fuster
2018;():V001T01A009. doi:10.1115/ICEF2018-9704.

Lean-burn combustion dominates the current reciprocating engine R&D efforts due to its inherent benefits of high BTE and low emissions. The ever-increasing push for high power densities necessitates high boost pressures. Therefore, the reliability and durability of ignition systems face greater challenges. In this study, four ignition systems, namely, stock Capacitive discharge ignition (CDI), Laser ignition, Flame jet ignition (FJI), and Nano-pulse delivery (NPD) ignition were tested using a single cylinder natural gas engine. Engine performance and emissions characteristics are presented highlighting the benefits and limitations of respective ignition systems. Optical tools enabled delving into the ignition delay period and assisted with some characterization of the spark and its impact on subsequent processes. It is evident that advanced ignition systems such as Lasers, Flame-jets and Nano-pulse delivery enable extension of the lean ignition limits of fuel/air mixtures compared to base CDI system.

Commentary by Dr. Valentin Fuster
2018;():V001T01A010. doi:10.1115/ICEF2018-9710.

The balancing of the electric grid has become more challenging due to the expansion of fluctuating renewable energy sources for electric power generation. The importance of power plants driven by internal combustion engines will increase since they can react flexibly and quickly to changes in the energy demand. With regard to the emission of pollutants and CO2, gas fueled engines are favored for gensets. However, it is more challenging to meet the dynamic load requirements with a gas engine than with a conventional diesel engine because the load acceptance of the gas engine is limited by the occurrence of knocking combustion.

Dual fuel engines are a good compromise between these two engine concepts; they can use gaseous fuel during steady state engine operation and increase the diesel share during transient modes to improve the dynamic behavior. The high number of degrees of freedom of dual fuel combustion concepts requires advanced operating strategies.

The aim of this paper is to investigate and evaluate strategies to improve the transient behavior of a 20-cylinder large bore diesel-gas engine (displacement 6.24 dm3 per cylinder) for a genset application. In the investigations, the latest turbocharging technology is applied in combination with a turbine waste gate. A wide range diesel injector that covers the whole diesel injection range of approximately 1 % to 100 % diesel fraction1 of the rated power fuel mass provides the basis for the most flexible diesel injection. A 1D simulation tool was used to model and optimize the genset in transient operation. The combustion process was simulated with Vibe heat release rate models. The optimized transient engine operating strategies were validated on a highly dynamic single cylinder research engine test bed.

The paper provides a comparison of different strategies that use these technologies to improve the dynamic behavior of the genset in island mode operation during a 50 % load step. Key to meeting the challenging requirements is an optimized diesel injection strategy or even a switch from gas operation mode to diesel operation mode during the load step. Based on the results of simulation and engine testing, potential ways to minimize engine speed drop and recovery time after the load demand increase are evaluated.

Commentary by Dr. Valentin Fuster
2018;():V001T01A011. doi:10.1115/ICEF2018-9765.

Experiments were conducted on a large bore, medium speed, single cylinder, diesel engine to investigate operation with substitution ratio of natural gas varying from 0 to 93% by energy. As reported in a previous publication [1], these data were used to validate an analytical methodology for predicting performance and emissions under a broad spectrum of energy substitution ratios. For this paper, these experimental data are further analyzed to better understand the performance and combustion behavior under natural gas substitution ratios of 0%, 60% and 93%. These results show that by transitioning from diesel to 60% dual-fuel (60% NG substitution ratio), an improvement in the NOx-efficiency trade-off was observed that represented a ∼3% improvement in efficiency at constant NOx. Further, the transition from 60% dual-fuel to 93% dual-fuel (93% NG substitution ratio) resulted in additional efficiency improvement with a simultaneous reduction in NOx emissions. The data suggest that this improvement can be attributed to the premixed nature of the high substitution ratio. Furthermore, the results show that high cycle-to-cycle variation was observed for the 93% dual-fuel combustion tests. Further analysis, along with diesel injection rate measurements, show that the observed extreme sensitivity of the combustion event can be attributed to critical parameters such as diesel fuel quantity and injection timing. Results suggest a better understanding of the relative importance of combustion system components and operating conditions in controlling cycle-to-cycle variation of combustion process.

Topics: Combustion , Fuels , Engines , Diesel
Commentary by Dr. Valentin Fuster

Fuels

2018;():V001T02A001. doi:10.1115/ICEF2018-9566.

Microemulsions are sustainable alternatives to fos sil fuels, which could possibly be used without any modifications in current engines and storage-transportation-supply infrastructure. Our current work attempts to examine the usability of butanol-diesel-water microemulsion fuels in a diesel engine. A small percentage of water is desirable, as it reduces the NOx and smoke emissions. The microemulsion regions were mapped out in ternary phase diagrams, and the fuel was characterized as per ASTM D975, and further examined for its performance in a diesel engine. The formulated microemulsions satisfied the ASTM standards, and had properties (density, viscosity, flash points, cloud points, copper strip corrosion rating, sulfur content, and ash percent) close to those of neat diesel. The percentage change in property ε was calculated as [|(εdiesel − εmicroemulsion)|/εdiesel] × 100. The calorific values for the microemulsion fuels showed a maximum reduction of 8.31% as compared to that of neat diesel. The brake thermal efficiency, however, increased by 15.38% for the same, with respect to the value for neat diesel (2% higher overall efficiency of the engine). The brake specific fuel consumption was also lowered by 5.04%, and the maximum reduction in emissions of CO, unburnt HC, CO2, and NOx were observed to be 53.48%, 67.40%, 30.82%, and 41.72%, respectively, relative to those of neat diesel. The present experimental investigations thus suggest that the microemulsions could be used as a sustainable cleaner alternative to diesel.

Commentary by Dr. Valentin Fuster
2018;():V001T02A002. doi:10.1115/ICEF2018-9585.

High reactivity gas-to-liquid kerosene (GTL) was investigated with port fuel injection (PFI) of low reactivity n-butanol to conduct reactivity controlled compression ignition (RCCI). In the preliminary stage, the GTL was investigated in a constant volume combustion chamber, and the results indicated a narrower negative temperature coefficient (NTC) region than ultra-low sulfur diesel (ULSD#2). The engine research was conducted at 1500 RPM and various loads with early n-butanol PFI and dual DI pulses of GTL at 60 crank angle degrees (CAD) before top dead center (TDC) and at a timing close to TDC. Boost and PFI fractions (60% by mass n-butanol) were kept constant in order to analyze the fuel reactivity effect on combustion. Conventional diesel combustion (CDC) mode with a single injection and the same combustion phasing (CA50) was used as an emissions baseline for RCCI. RCCI increased ignition delay and combustion duration decreased compared to CDC. Results showed that in order to maintain CA50 for RCCI within 1 CAD, GTL mass required for the first DI pulse to be 15% lower than that of ULSD#2 at higher loads. Peak heat release rate decreased for GTL by 25% given the high volatility and low viscosity of GTL. In general, using GTL, NOx and soot levels were reduced across load points by up to 15% to 30%, respectively, compared to ULSD RCCI, while maintaining RCCI combustion efficiency at 93–97%. Meanwhile, reductions of 85% in soot and 90% in NOx were determined when using RCCI compared to CDC. The more favorable heat release placement of GTL led to increased thermal efficiency by 3% at higher load compared to ULSD#2. The higher volatility and increased reactivity for GTL achieved lower UHC and CO than ULSD#2 at lower load. The study concluded that GTL offered advantages when used with n-butanol for this RCCI fueling configuration.

Topics: Combustion , Emissions
Commentary by Dr. Valentin Fuster
2018;():V001T02A003. doi:10.1115/ICEF2018-9595.

Because of the potential to reduce NOx and PM emissions simultaneously and the utilization of biofuel, diesel/compressed natural gas (CNG) dual-fuel combustion mode with port injection of CNG and direct injection of diesel has been widely studied. While in comparison with conventional diesel combustion mode, the dual-fuel combustion mode generally leads lower thermal efficiency, especially at low and medium load, and higher carbon monoxide (CO) and total hydrocarbons (THC) emissions. In this work, n-butanol was blended with diesel as the pilot fuel to explore the possibility to improve the performance and emissions of dual-fuel combustion mode with CNG. Various pilot fuels of B0 (pure diesel), B10 (90% diesel/10% n-butanol by volume basis), B20 (80% diesel/20% n-butanol) and B30 (70% diesel/30% n-butanol) were compared at the CNG substitution rate of 70% by energy basis under the engine speeds of 1400 and 1800 rpm. The experiments were carried out by sweeping a wide range of pilot fuel start of injection timings based on the same total input energy including pilot fuel and CNG. As n-butanol was added into the pilot fuel, the pilot fuel/CNG/air mixture tends to be more homogeneous. The results showed that for the engine speed of 1400 rpm, pilot fuel with n-butanol addition leads to a slightly lower indicated thermal efficiency (ITE). B30 reveals much lower NOx emission and slightly higher THC emissions. For the engine speed of 1800 rpm, B20 can improve ITE and decrease the NOx and THC emissions simultaneously relative to B0.

Topics: Combustion , Fuels , Engines
Commentary by Dr. Valentin Fuster
2018;():V001T02A004. doi:10.1115/ICEF2018-9615.

A novel ship propulsion concept employs natural gas to reduce ship emissions and improve overall ship propulsion efficiency. This concept proposes a serial integration of Solid Oxide Fuel Cell (SOFC) and a natural gas engine, while anode-off gas (gas at the fuel cell exhaust) is used in the natural gas engine. This study focusses on SOFC-gas engine integration by experimentally analyzing the effects of adding hydrogen, which is the main combustible component of the fuel cell anode-off gas, in marine natural gas engines. The overall challenge is to employ the anode-off gas to improve the performance of marine natural gas engines. To study the effects of anode-off gas combustion in natural gas engines, experiments with hydrogen addition in a marine natural gas engine of 500 kW rated power were performed. Natural gas was replaced with 10 % and 20 % of hydrogen, by volume, without any penalties in terms of output power.

We found that the high combustion rate of hydrogen improved combustion stability, which allowed for better air-excess ratio control. Thus allowing leaning to higher air-excess ratios and extending the, otherwise, limited operating window. Hydrogen addition also improved brake thermal efficiency by 1.2 %, while keeping NOx emissions below the maritime emission regulations. The improvement in engine efficiency with a larger operating window may help improve the load-taking capabilities of marine natural gas engines.

Topics: Hydrogen , Gas engines
Commentary by Dr. Valentin Fuster
2018;():V001T02A005. doi:10.1115/ICEF2018-9640.

In order to establish a pathway to evaluate chemical kinetic mechanisms (detailed or reduced) in a real engine environment, a GT Power model of the well-studied Cooperative Fuels Research (CFR) engine was developed and validated against experimental data for primary reference fuel blends between 60 and 100 under RON conditions. The CFR engine model utilizes a predictive turbulent flame propagation sub-model, and implements a chemical kinetic solver to solve the end-gas chemistry. The validation processes were performed simultaneously for thermodynamic and chemical kinetic parameters to match IVC conditions, burn rate, and knock prediction. A recently published kinetic mechanism was implemented in GT-Power, and was found to over-predict the low temperature heat release for iso-octane and PRF blends, leading to advanced knock onset phasing compared to experiments. Three reaction rates in the iso-octane and n-heptane pathways were tuned in the kinetic mechanism in order to match experimental knock-point values, yielding excellent agreement in terms of the knock onset phasing, burn rate, and the thermodynamic conditions compared to experiments. This developed model provides the initial/boundary conditions of the CFR engine under RON conditions, including IVC temperature and pressure, MFB profile, residual fraction and composition. The conditions were then correlated as a function of CFR engine compression ratio, and implemented in a 0-D SI engine model in Chemkin Pro in order to demonstrate an application of the current work. The Chemkin Pro and GT-Power simulations provided nearly identical results despite significant differences in heat transfer models and chemical kinetic solvers. This work provides the necessary framework by which robust chemical kinetic mechanisms can be developed, evaluated, and tuned, based on the knocking tendencies in a real engine environment for PRF blends.

Commentary by Dr. Valentin Fuster
2018;():V001T02A006. doi:10.1115/ICEF2018-9647.

In this experimental examination, an attempt was made to improve the performance and diminish the exhaust emissions by adding titanium oxide (TiO2) nanoparticles into J30D5H blend (5% by volume n-hexane, 30% by volume jojoba methyl ester, and 65% by volume diesel fuel) under various engine loads and a constant speed of 2000 rpm. The titanium oxide nanoparticles were added to J30D5H blend at two proportions, including 25 mg/l and 50 mg/l by using an ultrasonic technique. The addition of TiO2 into J30D5H led to a significant improvement in the engine performance, where the brake specific fuel consumption was reduced by 12%, while the brake thermal efficiency was increased by 15% compared to J30D5H blend. The combustion consequences for the J30D5H blend with nanoparticles addition exhibited that the peak pressure and maximum heat release rate were increased by approximately 4.5% and 2%, respectively. Moreover, the CO and UHC emissions were reduced by 20% and 50%, respectively. Nevertheless, the NOx emission was increased by about 15% with adding TiO2 into J30D5H blend.

Commentary by Dr. Valentin Fuster
2018;():V001T02A007. doi:10.1115/ICEF2018-9657.

The charge cooling effect of methanol was studied and compared to that of iso-octane. The reduction in compression work due to fuel evaporation and the gain in expansion work were evaluated by the means of in-cylinder pressure measurements in a HD CI engine. A single injection strategy was utilized to obtain a longer premixing period to adequately capture the cooling effect. The effect was clear for both tested fuels, however, methanol generally caused the pressure to reduce more than iso-octane near TDC. It was found that the contribution of reduced compression work to the increased net indicated efficiency is negligible. Regarding the expansion work, a slower combustion with higher pressure was obtained for methanol in comparison to that of iso-octane due to the cooling effect of fuel evaporation. As a result from this, a lower heat transfer loss was obtained for methanol, in addition to the significantly lower NOx emissions.

Commentary by Dr. Valentin Fuster
2018;():V001T02A008. doi:10.1115/ICEF2018-9673.

Major interests in the automotive industry include the use of alternative fuels and reduced fuel usage to address fuel supply security concerns and regulatory requirements. The majority of previous internal combustion engine (ICE) control strategies consider only the First Law of Thermodynamics (FLT). However, FLT is not able to distinguish losses in work potential due to irreversibilities, e.g., up to 25% of fuel exergy may be lost to irreversibilities. To account for these losses, the Second Law of Thermodynamics (SLT) is applicable. The SLT is used to identify the quality of an energy source via availability since not all the energy in a particular energy source is available to produce work; therefore optimal control that includes availability may be another path toward reduced fuel use. Herein, Model Predictive Control (MPC) is developed for both FLT and SLT approaches where fuel consumption is minimized in the former and availability destruction in the latter. Additionally, both include minimization of load tracking error. The controls are evaluated in the simulation of a single cylinder naturally aspirated compression ignition engine that is fueled with either 20% biodiesel and 80% diesel blend or diesel only. Control simulations at a constant engine speed and changing load profile show that the SLT approach results in higher SLT efficiency, reduced specific fuel consumption, and decreased NOx emissions. Further, compared to use of diesel only, use of the biodiesel blend resulted in less SLT efficiency, higher specific fuel consumption, and lower NOx emissions.

Commentary by Dr. Valentin Fuster
2018;():V001T02A009. doi:10.1115/ICEF2018-9729.

Supporting chemical kinetics model development with robust experimental results is the job of shock-tube, rapid compression machine, and other apparatus operators. A key limitation of many of these systems is difficulty with preparation of a fuel vapor-air mixture for heavy liquid fuels. Previous work has suggested that the Cetane Ignition Delay (CID) 510 system is capable of providing data useful for kinetics validation. Specifically, this constant-volume combustion chamber (1) can be characterized by a single bulk temperature, and (2) uses a high-pressure diesel injector to generate rapid fuel-air mixing and thus create a homogeneous mixture well before ignition.

In this study, initial experiments found relatively good agreement between experiments and kinetic models for n-heptane and poor agreement for iso-octane under nominally the same ignition delay ranges for ambient conditions under which the mixture is determined to be effectively homogeneous. After excluding potential non-kinetic fuel properties as causes, further experiments highlight the high pressure sensitivity of the negative temperature coefficient (NTC) behavior. While this challenge is well known to kinetic mechanism developers, the data set included in this work (n-heptane at 5 bar and iso-octane at 5–20 bar, each for various equivalence ratios) can be added to those used for validation.

The results and system characterization presented demonstrate that this combustion system is capable of capturing kinetic effects decoupled from the spray process for these primary reference fuels. Future work can leverage this capability to provide kinetics validation data for most heavy, exotic, or otherwise difficult to test liquid fuels.

Commentary by Dr. Valentin Fuster
2018;():V001T02A010. doi:10.1115/ICEF2018-9733.

With advanced engines pushing the limits of fuel efficiency, rapid development and improvement of engines increasingly rely on insights from simulations. Reliable simulations require fuel models that consist of a fuel surrogate and its kinetic mechanism. As complexity and sources of fuels vary, a good surrogate needs to be tailored for the specific test fuel. A simple surrogate, typically consisting of 1 to 3 components, can match a single property of the real fuel, such as ignition quality or average molecular weight. More complex surrogates with 4 to 7 components can capture many properties simultaneously. While simple surrogates are good for estimating ignition in engines they require some compensation for the mismatch of the fuels’s physical properties. Complex surrogates can be used to directly represent real fuels in both laboratory experiments and simulations.

We have developed a surrogate blending methodology to identify surrogates with a desired degree of complexity. This involves methods that estimate properties for fuel blends, including ignition quality, sooting propensity, distillation curve, as well as other physical and chemical properties that are important to combustion behavior in simulations. We have assembled and developed a rich library of over 60 fuel components from which we can formulate surrogates to represent most gasoline, diesel, gaseous fuels, renewable fuels, and several additives. The components cover a carbon number range from 1 to 20, and chemical classes including linear and branched alkanes, olefins, aromatics with one and two rings, alcohols, esters, and ethers. As part of the library, we have assembled self-consistent and detailed reaction mechanisms for all the components. The mechanisms also include comprehensive NOx creation and destruction pathways, molecular weight growth kinetics leading to the formation of polycyclic aromatic hydrocarbons (PAH), and a detailed soot-surface mechanism. The mechanisms have been validated extensively using over 500 published sets of experimental kinetics data from a wide range of facilities and diagnostic methods. Over the past decade, the validation suite has been used to improve the kinetics database such that good predictions and agreement to data are achieved for the fuel components and fuel-component blends, within experimental uncertainties. This effectively eliminates the need to tune specific rate parameters when employing the kinetics mechanisms in combustion simulations.

For engine simulations, the master mechanisms have been reduced using a combination of available reduction methods while strictly controlling the error tolerances for targeted predictions. These include several directed relation graph (DRG) based methods and sensitivity analysis. Iteratively using these reduction methods has resulted in small mechanisms for efficiently incorporating the validated kinetics into computational fluid dynamics (CFD) applications. The surrogate formulation methodology, the comprehensive fuel library, and mechanism reduction strategies suggested in this work allow the use of CFD to explore design concepts and fuel effects in engines with reliable predictions.

Topics: Combustion , Fuels
Commentary by Dr. Valentin Fuster
2018;():V001T02A011. doi:10.1115/ICEF2018-9747.

A continued challenge to engine combustion simulation is predicting the impact of fuel-composition variability on performance and emissions. Diesel fuel properties, such as cetane number, aromatic content and volatility, significantly impact combustion phasing and emissions. Capturing such fuel property effects is critical to predictive engine combustion modeling. In this work, we focus on accurately modeling diesel fuel effects on combustion and emissions. Engine modeling is performed with 3D CFD using multi-component fuel models, and detailed chemical kinetics. Diesel FACE fuels (Fuels for Advanced Combustion Engines) have been considered in this study as representative of street fuel variability. The CFD modeling simulates experiments performed at Oak Ridge National Laboratory (ORNL) [1] using the diesel FACE fuels in a light-duty single-cylinder direct-injection engine. These ORNL experiments evaluated fuel effects on combustion phasing and emissions. The actual FACE fuels are used directly in engine experiments while surrogate-fuel blends that are tailored to represent the FACE fuels are used in the modeling. The 3D CFD simulations include spray dynamics and turbulent mixing.

We first establish a methodology to define a model fuel that captures diesel fuel property effects. Such a model should be practically useful in terms of acceptable computational turnaround time in engine CFD simulations, even as we use sophisticated fuel surrogates and detailed chemistry. Towards these goals, multi-component fuel surrogates have been developed for several FACE fuels, where the associated kinetics mechanisms are available in a model-fuels database. A surrogate blending technique has been employed to generate the multi-component surrogates, so that they match selected FACE fuel properties such as cetane number, chemical classes such as aromatics content, T50 and T90 distillation points, lower heating value and H/C molar ratio. Starting from a well validated comprehensive gas-phase chemistry, an automated method has been used for extracting a reduced chemistry that satisfies desired accuracy and is reasonable for use in CFD. Results show the level of modeling necessary to capture fuel-property trends under these widely varying engine conditions.

Commentary by Dr. Valentin Fuster

Advanced Combustion

2018;():V001T03A001. doi:10.1115/ICEF2018-9524.

Experimental study on knocking characteristics in a direct injection turbo-charged gasoline engine was carried out. The thermodynamic analysis was conducted to investigate effects of the combustion phasing and the burning rate on the knocking behavior. The localization of knock events and the characterization of the early flame kernel propagation were conducted with the fiber optic sensor.

The advanced combustion phasing and the slower combustion speed generally increased the knocking probability. However, not only quasi-dimensional thermodynamic combustion characteristics but also the spatial parameter such as the flame propagation direction significantly affected the knocking occurrence. From the fiber optic sensor test results, knocking onset location was found to be closely correlated with the flame propagation direction and mainly observed in the opposite side to the main flame propagation direction. The flame propagation direction leaning to the exhaust side was identified to be favorable for the knocking mitigation because the end gas location on hotter exhaust side could be avoided.

Engine tests for various squish designs and tumble port designs were implemented to study the effect of the in-cylinder flow, which significantly affects previously discussed knocking-related parameters. As a result, tumble and squish flow significantly increased combustion speed and advanced combustion phasing. Fuel consumption could be also reduced due to suppressed knocking combustion. In addition, new tumble port design enabled the flame propagation to have favorable leaning direction.

Commentary by Dr. Valentin Fuster
2018;():V001T03A002. doi:10.1115/ICEF2018-9559.

Ozone assisted combustion has shown promise in stabilizing combustion and extending operating range of internal combustion engines. However, it has been reported that sensitivity of ozone quantity on combustion varies significantly dependent on combustion modes. For example, auto-ignition driv3en combustion in homogeneous charge compression ignition (HCCI) engine was found to be highly sensitive to the ozone concentration, and up to 100 PPM was found to be sufficient to promote combustion. On the other hand, flame propagation in spark-ignited (SI) engine has been reported to be much less sensitive to the ozone amount, requiring ozone concentration about 3000∼6000 PPM to realize any benefit in the flame speed. A better understanding on the ozone sensitivity is required for combustion device design with ozone addition. In this study, a Damköhler number analysis was performed to analyze the vast difference in the ozone sensitivity between auto-ignition and flame propagation. The analysis showed that, for ozone to be effective in flame propagation, the contribution of ozone on chemistry should be large enough to overcome the diffused radical from the oxidation layer. It is expected that similar analysis will be applicable to any additives to provide an understanding of their effect.

Commentary by Dr. Valentin Fuster
2018;():V001T03A003. doi:10.1115/ICEF2018-9560.

An optimal combustion phasing leads to a high combustion efficiency and low carbon emissions in diesel engines. With the increasing complexity of diesel engines, model-based control of combustion phasing is becoming indispensable, but precise prediction of combustion phasing is required for such strategies. Since cylinder-to-cylinder variations in combustion can be more significant with advanced combustion techniques, this work focuses on developing a control-oriented combustion phasing model that can be leveraged to provide cylinder-specific estimates. The pressure and temperature of the intake gas reaching each cylinder are predicted by a semi-empirical model and the coefficients of this intake pressure and temperature model are varied from cylinder-to-cylinder. A knock integral model is leveraged to estimate the SOC (start of combustion) and the burn duration is predicted as a function of EGR fraction, equivalence ratio of fuel and residual gas fraction in a burn duration model. After that, a Wiebe function is utilized to estimate CA50 (crank angle at 50% mass of fuel has burned). This cylinder-specific combustion phasing prediction model is calibrated and validated across a variety of operating conditions. A large range of EGR fraction and fuel equivalence ratio were tested in these simulations including EGR levels from 0 to 50%, and equivalence ratios from 0.5 to 0.9. The results show that the combustion phasing prediction model can estimate CA50 with an uncertainty of ±0.5 crank angle degree in all six cylinders. The impact of measurement errors on the accuracy of the prediction model is also discussed in this paper.

Commentary by Dr. Valentin Fuster
2018;():V001T03A004. doi:10.1115/ICEF2018-9561.

Proper Orthogonal Decomposition (POD) offers an approach to quantify cycle-to-cycle variation (CCV) of the flow field inside the internal combustion engine cylinder. POD decomposes instantaneous flow fields (also called snapshots) into a series of orthonormal flow patterns (called POD modes) and the corresponding mode coefficients. The POD modes are rank-ordered by decreasing kinetic energy content, and the low-order, high-energy modes are interpreted as constituting the large-scale coherent flow structure that varies from engine cycle to engine cycle. Various POD-based analysis techniques have thus been proposed to characterize engine flow field CCV using these low-order modes. The validity of such POD-based analyses rests, as a matter of course, on the reliability of the underlying POD results (modes and coefficients). Yet a POD mode can be disproportionately skewed by a single outlier snapshot within a large data set, and an algorithm exists to define and identify such outliers. In this paper, the effects of a candidate outlier snapshot on the results of POD-based conditional averaging and quadruple POD analyses are examined for two sets of crank angle-resolved flow fields on the mid-tumble plane of an optical engine cylinder recorded by high-speed particle image velocimetry. The results with and without the candidate outlier are compared and contrasted. In the case of POD-based conditional averaging, the presence of the outlier scrambles the composition of snapshot subsets that define large-scale flow pattern variations, and thus substantially alters the coherent flow structures that are identified; for quadruple POD, the shape of coherent structures as well as the number of modes to define them are not significantly affected by the outlier.

Commentary by Dr. Valentin Fuster
2018;():V001T03A005. doi:10.1115/ICEF2018-9568.

European and US emission legislation on diesel compression ignition engines has pushed for the development of new types of combustion concepts to reduce hazardous pollutants and increase fuel efficiency. Partially premixed combustion (PPC) has been proposed as one solution to future restrictions on emissions while providing high gross indicated efficiency. The conceptual idea is that the time for the mixing between fuel and air will be longer when ignition delay is increased by addition of high amounts of exhaust gas recirculation (EGR). Increased air-fuel mixing time will lead to lower soot emissions and the high EGR rates will reduce both NOx emissions and combustion flame temperature, which decreases the overall heat transfer.

Previous research in heavy-duty gasoline PPC has mostly focused on emissions and efficiency at low and medium load in single-cylinder engines. In this paper a Volvo D13 heavy-duty single-stage VGT engine with a newly developed Wave piston was run at medium and high engine load with a variation in fuel injection pressure. The Wave piston was specifically designed to enhance air-fuel mixing and increase combustion velocity. Two fuels were used in the experiments, PRF70 and Swedish MK1 diesel. Soot-NOx trade-off, combustion characteristics and efficiency were compared for both fuels at 1000 and 2000 Nm engine torque. The results show that at high load the combustion behavior with respect to rate of heat release and heat transfer is very similar between the fuels and no major difference in indicated efficiency could be observed. Peak gross indicated efficiencies were reported to be around 49 % for both fuels at 1000 Nm and slightly above 50 % at 2000 Nm. The new Wave piston made it possible to obtain 1 g/kWh engine-out NOx emissions while still complying with Euro VI legislation for particulate emissions. Soot emissions were generally lower for PRF70 compared to MK1 diesel. We could also conclude that gas exchange performance is a major issue when running high load PPC where high Λ and EGR is required. The single-stage VGT turbocharger could not provide sufficient boost to keep Λ above 1.3 at high EGR rates. This penalized combustion efficiency and soot emissions when reaching Euro VI NOx emission levels (0.3–0.5 g/kWh).

Commentary by Dr. Valentin Fuster
2018;():V001T03A006. doi:10.1115/ICEF2018-9580.

Natural gas as an alternative fuel in engine applications substantially reduces both pollutant and greenhouse gas emissions. High pressure dual fuel direct injection of natural gas and Diesel pilot has the potential to minimize methane slip from gas engines and increase the fuel flexibility, while retaining the high efficiency of a Diesel engine. Speed and load variations as well as various strategies for emission reduction entail a wide range of different operating conditions. The influence of these operating conditions on the ignition and combustion process is investigated on a rapid compression expansion machine. By combining simultaneous Shadowgraphy and OH* imaging with heat release rate analysis, an improved understanding of the ignition and combustion process is established. At high temperatures and pressures the reduced pilot ignition delay and lift-off length minimize the effect of natural gas jet entrainment on pilot mixture formation. A simple geometrical constraint was found to reflect the susceptibility for misfiring. At the same time natural gas ignition is delayed by the early pilot ignition close to the injector tip. The shape of heat release is only marginally affected by the operating conditions and mainly determined by the degree of premixing at the time of gas jet ignition. Luminescence from the sooting natural gas flame is generally only detected after the flame extends across the whole gas jet at peak heat release rate. Termination of gas injection at this time was confirmed to effectively suppress soot formation, while a strongly sooting pilot seems to intensify soot formation within the natural gas jet.

Topics: Natural gas , Diesel
Commentary by Dr. Valentin Fuster
2018;():V001T03A007. doi:10.1115/ICEF2018-9589.

Detection of combustion related phenomena such as misfire, knock and sporadic preignition is very important for the development of electronic controls needed for the gasoline direct injection engines to meet the production goals in power, fuel economy, and low emissions.

This paper applies several types of combustion ionization sensors, and a pressure transducer that directly sense the in-cylinder combustion, and the knock sensor which is an accelerometer that detects the impact of combustion on engine structure vibration. Experimental investigations were conducted on a turbocharged four cylinders gasoline direct injection engine under operating conditions that produce the above phenomena. One of the cylinders is instrumented with a Piezo quartz pressure transducer, MSFI (Multi sensing fuel injector), a standalone ion current probe, and a spark plug applied to act as an ion current sensor. A comparison is made between the capabilities of the pressure transducer, ion current sensors, and the knock sensor in detecting the above phenomena. The signals from in-cylinder combustion sensors give more accurate information about combustion than the knock sensor. As far as the feasibility and cost of their application in production vehicles the spark plug sensor and MSFI appear to be the most favorable, followed by the Standalone mounted sensor which is an addition to the engine.

Commentary by Dr. Valentin Fuster
2018;():V001T03A008. doi:10.1115/ICEF2018-9590.

Engine performance and emissions of a six-stroke Gasoline Compression Ignition (GCI) engine with wide range of Continuously Variable Valve Duration (CVVD) control were numerically investigated at low engine load conditions. For the simulations, an in-house 3-D CFD code with high fidelity physical sub-models was used and the combustion and emissions kinetics were computed using a reduced kinetics mechanism for a 14-component gasoline surrogate fuel. Double injections were employed to effectively form the local fuel/air mixtures with optimal reactivity. Several valve timing and duration variations through the CVVD control were considered under both positive valve overlap (PVO) and negative valve overlap (NVO) conditions. Effects of intake-valve re-breathing between the first expansion and the second compression strokes were also investigated.

Close attention was paid to understand the effects of two additional strokes of the engine cycle on the thermal and chemical conditions of charge mixtures that alter ignition, combustion and energy recovery processes. Double injections were found to be necessary to effectively utilize the additional two strokes for the combustion of overly mixed lean charge mixtures during the second power stroke (PS2). It was found that combustion phasing in both power strokes is effectively controlled by the intake valve closure (IVC) timing since it affects the effective compression ratio. Engine operation under NVO condition with fixed exhaust valve opening (EVO) and IVC timings tends to advance the ignition timing of the first power stroke (PS1) but has minimal effect on the ignition timing of PS2. Re-breathing was found to be an effective way to control the ignition timing in PS2 at a slight expense of the combustion efficiency.

The operation of a six-stroke GCI engine could be successfully simulated and the operability range of the engine could be substantially extended by employing the CVVD technique. In addition, the control of valve timings could successfully control the thermodynamic and compositional conditions of in-cylinder mixtures that enable to control the combustion phasing.

Commentary by Dr. Valentin Fuster
2018;():V001T03A009. doi:10.1115/ICEF2018-9609.

Heavy-duty compression-ignition (CI) engines converted to natural gas (NG) spark ignition (SI) operation have the potential to increase the use of NG in the transportation sector. A 3D numerical simulation was used to predict how the conventional CI combustion chamber geometry (i.e., re-entrant bowl and flat head) affects the combustion stability, performance and emissions of a single-cylinder CI engine that was converted to SI operation by adding a low-pressure gas injector in the intake manifold and a spark plug in place of the diesel injector. The G-equation based 3D CFD simulation investigated three different combustion chamber configurations that changes the size of the squish region at constant compression ratio and clearance height. The results show that the different flame propagation speeds inside and outside the re-entrant bowl can create a two-zone combustion phenomenon. More, a larger squish region increased flame burning speed, which decreased late-combustion duration. All these findings support the need for further investigations of combustion chamber shape design for optimum engine performance and emissions in CI engines converted to NG SI operation.

Commentary by Dr. Valentin Fuster
2018;():V001T03A010. doi:10.1115/ICEF2018-9610.

Gasoline compression ignition (GCI) is a cost-effective approach to achieving diesel-like efficiencies with low emissions. Traditional challenges with GCI arise at low-load conditions due to low charge temperatures causing combustion instability and at high-load conditions due to peak cylinder pressure and noise limitations. The fundamental architecture of the two-stroke Achates Power Opposed-Piston Engine (OP Engine) enables GCI by decoupling piston motion from cylinder scavenging, allowing for flexible and independent control of cylinder residual fraction and temperature leading to improved low load combustion. In addition, the high peak cylinder pressure and noise challenges at high-load operation are mitigated by the lower BMEP operation and faster heat release for the same pressure rise rate of the OP Engine. These advantages further solidify the performance benefits of the OP Engine and demonstrate the near-term feasibility of advanced combustion technologies, enabled by the opposed-piston architecture.

This paper presents initial results from a steady state testing on a brand new 2.7L OP GCI multi-cylinder engine. A part of the recipe for successful GCI operation calls for high compression ratio, leading to higher combustion stability at low-loads, higher efficiencies, and lower cycle HC+NOx emissions. In addition, initial results on catalyst light-off mode with GCI are also presented. The OP Engine’s architectural advantages enable faster and earlier catalyst light-off while producing low emissions, which further improves cycle emissions and fuel consumption over conventional engines.

Commentary by Dr. Valentin Fuster
2018;():V001T03A011. doi:10.1115/ICEF2018-9611.

Heavy-duty compression-ignition (CI) engines converted to natural gas (NG) operation can reduce the dependence on petroleum-based fuels and curtail greenhouse gas emissions. Such an engine was converted to premixed NG spark-ignition (SI) operation through the addition of a gas injector in the intake manifold and of a spark plug in place of the diesel injector. Engine performance and combustion characteristics were investigated at several lean-burn operating conditions that changed fuel composition, spark timing, equivalence ratio, and engine speed. While the engine operation was stable, the reentrant bowl-in-piston (a characteristic of a CI engine) influenced the combustion event such as producing a significant late-combustion, particularly for advanced spark timing. This was due to an important fraction of the fuel burning late in the squish region, which affected the end of combustion, the combustion duration, and the cycle-to-cycle variation. However, the lower cycle-to-cycle variation, stable combustion event, and the lack of knocking suggest a successful conversion of conventional diesel engines to NG SI operation using the approach described here.

Commentary by Dr. Valentin Fuster
2018;():V001T03A012. doi:10.1115/ICEF2018-9613.

Natural gas is widely used in sequentially port fuel injection engine to meet stringent emission regulation. Lean burn operation is one of the ways to improve spark-ignition engine fuel economy. The instability in the combustion process of the lean burn engine is one of the major challenges for engine research. In this study, the performance and combustion characteristics of a lean burn sequential injection compressed natural gas (CNG) engine were investigated numerically using computational fluid dynamics (CFD) modeling over a wide range of air/fuel equivalence ratio. A detailed chemical kinetic mechanism was used for natural gas combustion along with laminar flame speed model to capture lean burn operating condition within the combustion chamber. Combustion pressure, indicated mean effective pressure (IMEP), and heat release were analyzed for performance analysis, whereas flame development angle (CA 10), combustion duration, thermal efficiency were taken for combustion analysis. The results show that on increasing air/fuel equivalence ratio at a given spark timing, IMEP decreases as the lean burn mixture produces less amount of gross power output due to insufficient available energy. Moreover, lower burning velocity characteristic of natural gas extends the combustion duration, where a substantial amount of total energy released after top dead center. It is also seen that optimum spark timing (MBT) for maximum IMEP advances with an increase in air/fuel equivalence ratio due to late ignition timing under lean burn condition. CFD model successfully captures the effect of dilution to illustrate the considerations to design future combustion engine for spark ignited natural gas engine.

Commentary by Dr. Valentin Fuster
2018;():V001T03A013. doi:10.1115/ICEF2018-9618.

Natural gas is traditionally considered as a promising fuel in comparison to gasoline due to the potential of lower emissions and significant domestic reserves. These emissions can be further diminished by using noble gases, such as argon, instead of nitrogen as the working fluid in internal combustion engines. Furthermore, the use of argon as the working fluid can increase the thermodynamic efficiency due to its higher specific heat ratio. In comparison to pre-mixed operation, the direct injection of natural gas enables the engine to reach higher compression ratios while avoiding knock. Using argon as the working fluid increases the in-cylinder temperature at top dead center and enables the compression ignition of natural gas. In this numerical study, the combustion quality and ignition behavior of methane injected into a mixture of oxygen and argon has been investigated using a three-dimensional transient model of a constant volume combustion chamber. A dynamic structure large eddy simulation model has been utilized to capture the behavior of the non-premixed turbulent gaseous jet. A reduced mechanism consists of 22-species and 104-reactions were coupled with the CFD solver. The simulation results show that the methane jet ignites at engine-relevant conditions when nitrogen is replaced by argon as the working fluid. Ignition delay times are compared across a variety of operating conditions to show how mixing affects jet development and flame characteristics.

Commentary by Dr. Valentin Fuster
2018;():V001T03A014. doi:10.1115/ICEF2018-9625.

One of the pending issues regarding Homogeneous Charge Compression Ignition (HCCI) engines is high load operation limit constrained by excessive pressure rise rates (PRRs). The present study investigates various measures to reduce combustion harness in a residual-affected HCCI engine. At the same time, the impact of those measures on efficiency and emissions is assessed. Experimental research was performed on a single cylinder engine equipped with a fully-flexible valvetrain mechanism and direct gasoline injection. The HCCI combustion mode with exhaust gas trapping was realized using negative valve overlap and fuel reforming, achieved via the injection of a portion of fuel during exhaust re-compression.

Three measures are investigated for the PRR control under the same reference operating conditions, namely: (i) variable intake and exhaust valve timing, (ii) boost pressure adjustment and (iii) split fuel injection to control the amount of fuel injected for reforming. Variable exhaust valve timing enabled control of the amount of trapped residuals, and thus of the compression temperature. The reduction in the amount of trapped residuals, at elevated engine load, delays auto-ignition, which results in a simultaneous reduction of pressure rise rates and nitrogen oxides emissions. The effects of intake valve timing are much more complex, because they include the variability in the amount of intake air, the thermodynamic compression ratio as well as the in-cylinder fluid flow. It was found, however, that both early and late intake valve openings delay auto-ignition and prolong combustion. Additionally, the reduction of the amount of fuel injected during exhaust re-compression further delays combustion and reduces combustion rates. Intake pressure reduction has by far the largest effect on peak pressure reduction yet is connected with excessive NOx emissions. The research successfully identifies air-path and injection techniques, which allow for the control of combustion rates and emissions under elevated load regime, thus shorting the gap towards the real-world application of HCCI concepts.

Commentary by Dr. Valentin Fuster
2018;():V001T03A015. doi:10.1115/ICEF2018-9628.

Turbocharged gas engines for combined heat and power units are optimized to increase efficiency while observing and maintaining legitimate exhaust gas emissions. In order to do so, the charge motion is raised. This study investigates the influence of passive prechamber spark plugs in high turbulent combustion chambers. The subjects of investigation are two different gas engine types, one of them running on sewage gas the other one on biogas. The occurring charge motions initiated by the cylinder heads are measured by integrative determination of swirl motion on a flow bench. In addition, three different passive prechamber spark plugs are characterized by a combustion analysis. Each of the three spark plugs comes with a different electrode or prechamber geometry. The resulting combustion and operating conditions are compared while the equal brake mean effective pressure and constant NOx-emissions are sustained. The results of the combustion analysis show a rising influence of the spark plug with increasing air-to-fuel-ratio induced by charge motion. Furthermore, clear differences between the spark plugs are determined: electrode arrangement and prechamber geometry help to influence lean misfire limits, engine smoothness, start behavior and ignition delay. The results indicate the capability of spark plugs to increase lifetime and engine efficiency.

Commentary by Dr. Valentin Fuster
2018;():V001T03A016. doi:10.1115/ICEF2018-9634.

It is well known that ammonia (NH3) combustion does not produce carbon dioxide (CO2) causing global warming. Therefore, NH3 has received much attention as an alternative diesel fuel for internal combustion engines. On the other hand, it has been reported that the exhaust gas of diesel engine fumigated with NH3 contains unburned NH3 with toxicity for humans and nitrous oxide (N2O) with strong global warming effect. Hence the NH3 and N2O emissions should be reduced to prevent the human health damage and global warming.

The aim of this study was to develop the combustion strategies for reducing the unburned NH3 and N2O emissions on diesel engine fumigated with NH3. The experimental results indicated that the higher temperature combustion of NH3 prevents the N2O production and allows itself to react well. From the numerical simulation results, hydrocarbon combustion decomposes NH3 and N2O in ignition processes.

Commentary by Dr. Valentin Fuster
2018;():V001T03A017. doi:10.1115/ICEF2018-9635.

This study aims to develop an exhaust gas temperature increase technique of a lean burn gas engine, to improve the performance of the waste heat recovery devices that potentially can be installed in the future. This paper shows the exhaust gas temperature increase technique using an EGR device.

In our experiments, the lean burn gas engine has the rated power output of 400 kW with spark-ignition and pre-chamber systems. The EGR device was developed and installed to the gas engine. The experimental results showed that the exhaust gas temperature was increased to +30 °C at the EGR rate of 15 % with maintained NOx emission and CA MFB 50% by decreasing the relative air/fuel ratio (Λ) and advancing the ignition timing (θig). In addition, the gross generation efficiency was slightly increased with increasing the EGR rate. This result was explained using three factors; the internal engine efficiency, the combustion efficiency, and the recirculated energy rate.

Commentary by Dr. Valentin Fuster
2018;():V001T03A018. doi:10.1115/ICEF2018-9648.

Ultra-lean burn with high turbulence has high potential for improving thermal efficiency and reducing NOx emissions in spark-ignition engines. Formation of initial flame kernel in high-turbulence flow by advanced ignition technologies is crucial for successful implementation of the ultra-lean burn concept.

In this study, a four-coil ignition system is designed to enable temporally flexible discharge, including the single strike, multi-strike and continuous discharge with the discharge energy range from 100 to 300 mJ. The performance of the different discharge strategies on igniting the lean methane-air mixture is evaluated in an optically accessible constant volume vessel. The initial mixture pressure of 3.0 MPa and temperature of 388 K are set to simulate typical conditions near TDC (top dead center) of turbocharged large-bore natural gas engines. Both the flow and quiescent conditions around the spark plug are taken into account with and without gas flows in the vessel. The flame kernel formation and developing processes are captured by using the Schlieren imaging technique with a high-speed CMOS video camera, while evolution of both the voltage and current in the circuit are well monitored by the high-voltage probe and current clamp.

With the continuous discharge ignition, the lean limit is remarkably extended in the case of the flow condition, while it is changed only slightly under the quiescent condition, compared with the other strategies. Analysis of the current and voltage waveforms shows that the continuous discharge strategy can enable a steadier and longer discharging period than the other strategies, regardless of conditions with and without gas flow. Besides, the continuous discharge strategy can accelerate the initial flame propagation compared with the other strategies. Once the flame kernel is successfully established, an increase in the discharge energy of single strike has no obvious effects on the flame development, but it is necessary for maintaining the lean limit. Although, in principle, the multi-strike discharge strategy can increase the ignition energy released to the mixture, the current waveform is prone to be interrupted with the discharge channel strongly distorted by the gas flow under the high-pressure condition. The flame propagation speed of the ultra-lean mixture is rather slow under the high ambient pressure quiescent condition compared with the high ambient pressure flow condition. Enhancement of turbulent flow in the mixture is very crucial for realizing the highly efficient and stable combustion of the lean mixture.

Topics: Ignition , Methane
Commentary by Dr. Valentin Fuster
2018;():V001T03A019. doi:10.1115/ICEF2018-9649.

Over the last few decades, emissions regulations for internal combustion engines have become increasingly restrictive, pushing researchers around the world to exploit innovative propulsion solutions. Among them, the dual fuel low temperature combustion (LTC) strategy has proven capable of reducing fuel consumption and while meeting emissions regulations for oxides of nitrogen (NOx) and particulate matter (PM) without problematic aftertreatment systems. However, further investigations are still needed to reduce engine-out hydrocarbon (HC) and carbon monoxide (CO) emissions as well as to extend the operational range and to further improve the performance and efficiency of dual-fuel engines.

In this scenario, the present study focuses on numerical simulation of fumigated methane-diesel dual fuel LTC in a single-cylinder research engine (SCRE) operating at low load and high methane percent energy substitution (PES). Results are validated against experimental cylinder pressure and apparent heat release rate (AHRR) data. A 3D full-cylinder RANS simulation is used to thoroughly understand the influence of the start of injection (SOI) of diesel fuel on the overall combustion behavior, clarifying the causes of AHRR transition from two-stage AHRR at late SOIs to single-stage AHRR at early SOIs, low temperature heat release (LTHR) behavior, as well as high HC production.

The numerical campaign shows that it is crucial to reliably represent the interaction between the diesel spray and the in-cylinder charge to match both local and overall methane energy fraction, which in turn, ensures a proper representation of the whole combustion. To that aim, even a slight deviation (∼3%) of the trapped mass or of the thermodynamic conditions would compromise the numerical accuracy, highlighting the importance of properly capturing all the phenomena occurring during the engine cycle.

The comparison between numerical and experimental AHRR curves shows the capability of the numerical framework proposed to correctly represent the dual-fuel combustion process, including low temperature heat release (LTHR) and the transition from two-stage to single stage AHRR with advancing SOI. The numerical simulations allow for quantitative evaluation of the residence time of vapor-phase diesel fuel inside the combustion chamber and at the same time tracking the evolution of local diesel mass fraction during ignition delay — showing their influence on the LTHR phenomena. Oxidation regions of diesel and ignition points of methane are also displayed for each case, clarifying the reasons for the observed differences in combustion evolution at different SOIs.

Commentary by Dr. Valentin Fuster
2018;():V001T03A020. doi:10.1115/ICEF2018-9653.

Gasoline Direct Injection Homogeneous Charge Compression (GDI-HCCI) combustion is achieved by closing early the exhaust valves for trapping hot residual gases combined with direct fuel injection. The combustion is chemically controlled by multi-point auto-ignition which its main combustion phase can be controlled by direct injection timing of fuel. This work investigates the effect of single pulse injection timing on a supercharged GDI-HCCI combustion engine by using a four-stroke single cylinder engine with a side-mounted direct fuel injector.

Injection of primary reference fuel PRF90 under the near-stoichiometric-boosted condition is studied. The fuel is injected during negative valve overlap (NVO) or recompression period for fuel reformation under low oxygen concentration and the injection is retarded to intake stroke for the homogeneous mixture. It is found that the early fuel injection in NVO period advances the combustion phasing compared with the retarded injection in the intake stroke. Noticeable slower combustion rate from intake stroke fuel injection is obtained compared with the NVO injection due to charge cooling effect. Zero-dimensional combustion simulations with multiple chemical reaction mechanisms are simulated to provide chemical understanding from the effect of fuel injection timing on intermediate species generations. The species such as C2H4, C3H6, CH4, and H2 are found to be formed during the NVO injection period from the calculations. The effects of single pulse injection timings on combustion characteristics such pressure rise rate, combustion stability, and emissions are also discussed in this study.

Commentary by Dr. Valentin Fuster
2018;():V001T03A021. doi:10.1115/ICEF2018-9664.

Diesel engines have been widely used due to the higher reliability and superior fuel conversion efficiency. However, they still generate significant amount of carbon dioxide (CO2) and particulate matter (PM) emissions. Natural gas is a low carbon and clean fuel that generates less CO2 and PM emissions than diesel during combustion. Replacing diesel by natural gas in internal combustion engines help reduce both CO2 and PM emissions. Natural gas – diesel dual fuel combustion is a practical and efficient way to replace diesel by natural gas in internal combustion engines. One concern for dual fuel combustion engines is the diesel injector tip temperature increase with increasing natural gas fraction.

This paper reports an experimental investigation on the diesel injector tip temperature variation and combustion performance of a natural gas – diesel dual fuel engine at medium and high load conditions. The natural gas fraction was changed from zero to 90% in the experiment. The results suggest that the injector tip temperature increased with increasing natural gas fraction at a given diesel injection timing or with advancing the diesel injection timing at a given natural gas fraction. However, the injector tip temperature never exceeded 250 °C in the whole experimental range. The effect of natural gas fraction on combustion performance depended on engine load and diesel injection timing.

Commentary by Dr. Valentin Fuster
2018;():V001T03A022. doi:10.1115/ICEF2018-9691.

Lean-burn engines are important due to their ability to reduce emissions, increase fuel efficiency, and mitigate engine knock. In this study, the surface roughness of spark plug electrodes is investigated as a potential avenue to extend the lean flammability limit of natural gas. A nano-/micro-morphology modification is applied on surface of the spark plug electrode to increase its surface roughness. High-speed Z-type Schlieren visualization is used to investigate the effect of the electrode surface roughness on the spark ignition process in a premixed methane-air charge at different lean equivalence ratios. In order to observe the onset of ignition and flame kernel behavior, experiments were conducted in an optically accessible constant volume combustion chamber at ambient pressures and temperatures. The results indicate that the lean flammability limit of spark-ignited methane can be lowered by modulating the surface roughness of the spark plug electrode.

Commentary by Dr. Valentin Fuster
2018;():V001T03A023. doi:10.1115/ICEF2018-9701.

The Achates Power Inc. (API) Opposed Piston (OP) Engine architecture provides fundamental advantages that increase thermal efficiency over current poppet valve 4 stroke engines. In this paper, combustion performance of diesel and gasoline compression ignition (GCI) combustion in a medium duty, OP engine are shown.

By using GCI, NOx and/or soot reductions can be seen compared to diesel combustion at similar or increased thermal efficiencies. The results also show that high combustion efficiency can be achieved with GCI combustion with acceptable noise and stability over the same load range as diesel combustion in an OP engine.

Commentary by Dr. Valentin Fuster
2018;():V001T03A024. doi:10.1115/ICEF2018-9706.

To enable efficient exhaust waste energy recovery (WER), it is important to characterize the exergy available in engine exhaust flows. In a recent article (Mahabadipour et al. (2018), Applied Energy, Vol. 216, pp. 31–44), the authors introduced a new methodology for quantifying crank angle-resolved exhaust exergy (including its thermal and mechanical components) for the two exhaust phases, viz., the “blowdown” phase and the “displacement” phase. The present work combines experimental measurements with GT-SUITE simulations to investigate the effect of exhaust back-pressure (Pb) on crank angle-resolved exhaust exergy in a single-cylinder research engine (SCRE). To this end, Pb values of 1, 1.4, and 1.8 bar are considered for conventional diesel combustion on the SCRE. Furthermore, the effect of boost pressure (Pin) between 1.2 to 2.4 bar on the thermal and mechanical components of exhaust exergy are reported at different Pb. The exergy available in the blowdown and the displacement phases of the exhaust process are also quantified. Regardless of Pin, with increasing Pb, the cumulative exergy percentage in the blowdown phase reduced uniformly. For example, at Pin = 1.5 bar and 1500 rpm engine speed, the cumulative exergy percentage in the blowdown phase decreased from 34% to 17% when Pb increased from 1 bar to 1.8 bar. The percentage of fuel exergy available as exhaust exergy was quantified. For instance, this normalized cumulative exergy in the exhaust increased from 10% to 21% when Pb increased from 1 bar to 1.8 bar at 1200 rpm. Finally, although the present work focused on exhaust exergy results for diesel combustion in the SCRE, the overall methodology can be easily adopted to study exhaust exergy flows in different engines and different combustion modes to enable efficient exhaust WER.

Commentary by Dr. Valentin Fuster
2018;():V001T03A025. doi:10.1115/ICEF2018-9707.

Computational simulations of engine combustion processes are increasingly relied upon to lead the design of advanced IC engines. Both computational fluid dynamics (CFD) simulations as well as thermodynamics-based phenomenological 0D or 1D gas dynamics simulations are examples of current simulation strategies. Before simulations can be utilized to guide the design process, they must be validated with experimental results. Typically, the experimental data used for validation of computational simulations include in-cylinder pressure and apparent heat release rate (AHRR) histories. However, the process of comparison of experimental and simulated pressure and AHRR curves is largely qualitative; therefore, the validation process is mostly visual. In the present work, the authors introduce a framework for quantifying uncertainties in experimental pressure data, as well as uncertainties in the “average” AHRR curve that is derived from ensemble-averaged cylinder pressure histories. Predicted AHRR curves from CFD simulations are also quantitatively compared with the experimental AHRR bounded by “uncertainty bands” in the present work.

Commentary by Dr. Valentin Fuster
2018;():V001T03A026. doi:10.1115/ICEF2018-9723.

Gasoline compression ignition (GCI) using a single gasoline-type fuel for port fuel and direct injection has been shown as a method to achieve low-temperature combustion with low engine-out NOx and soot emissions and high indicated thermal efficiency. However, key technical barriers to achieving low temperature combustion on multi-cylinder engines include the air handling system (limited amount of exhaust gas recirculation (EGR)) as well as mechanical engine limitations (e.g. peak pressure rise rate). In light of these limitations, high temperature combustion with reduced amounts of EGR appears more practical. Furthermore, for high temperature GCI, an effective aftertreatment system allows high thermal efficiency with low tailpipe-out emissions. In this work, experimental testing was conducted on a 12.4 L multi-cylinder heavy-duty diesel engine operating with high temperature GCI combustion using EEE gasoline. Engine testing was conducted at an engine speed of 1038 rpm and brake mean effective pressure (BMEP) of 14 bar. Port fuel and direct injection strategies were utilized to increase the premixed combustion fraction. The impact on engine performance and emissions with respect to varying the injection and intake operating parameters was quantified within this study. A combined effect of reducing heat transfer and increasing exhaust loss resulted in a flat trend of brake thermal efficiency (BTE) when retarding direct injection timing, while increased port fuel mass improved BTE due to advanced combustion phasing and reduced heat transfer loss. Overall, varying intake valve close timing, EGR, intake pressure and temperature with the current pressure rise rate and soot emissions constraint was not effective in improving BTE, as the benefit of low heat transfer loss was always offset by increased exhaust loss.

Commentary by Dr. Valentin Fuster
2018;():V001T03A027. doi:10.1115/ICEF2018-9734.

Non-premixed combustion of directly-injected natural gas offers diesel-like performance and efficiency with lower fuel costs and reduced greenhouse gas emissions. To ignite the fuel, a separate ignition source is needed. This work reports on the initial development of a new hot-surface based ignitor, where a small quantity of natural gas is injected and ignited by a hot element. This generates a robust pilot flame to ignite the main gas injection. A series of experimental tests were conducted to evaluate the sensitivity of the pilot flame formation process to hot surface temperature and geometry and to gas pilot injection geometry. Tests were conducted in a constant-volume combustion chamber at up to 6 bar with hot surface temperatures up to 1750 K. Reacting-flow computational fluid dynamics (CFD) evaluation is used to help interpret the results and to extrapolate to engine-relevant pressures. The results show that hot surface temperatures around 1500 K can minimize the pilot ignition time. An injector geometry where the pilot gas jets are angled such that they impinge on the hot surface but retain sufficient momentum to convect mass into the main chamber helps to ensure rapid and stable ignition. The CFD results indicate that, at engine pressures, a stable gas pilot flame could be established within 1–2 ms using the proposed injector geometry. These results will be used to underpin further development activities on this concept.

Commentary by Dr. Valentin Fuster
2018;():V001T03A028. doi:10.1115/ICEF2018-9746.

Non-intrusive measurements are always desirable in flame research, particularly in the study of internal combustion engines where intrusive measurements are usually not applicable. With the use of digital image processing and color analysis, the imaging system can be turned into an abstract multi-spectral system to determine the characteristics of flame emission. First this study conducts a precise calibration to make up a spectral correlation between the camera spectrum responses and the radical emissions of an ethanol diffusion flame. The color model of HSV is used to represent the camera spectrum responses. The actual wavelength of each radical of the diffusion flame has also been examined using a spectrograph. Subsequent experiment is the application of the spectral correlation into a direct injection spark ignition optical engine to research the combustion behavior. Two fuel injectors, different in nozzle configuration, were utilized and tested individually. The high-speed imaging system films hundreds of engine combustion cycles, and each cycle covers the propagation from the flame ignition stage towards the end of combustion. In those cycles, the presence of radicals of interest was captured and represented by Hue degree.

Commentary by Dr. Valentin Fuster
2018;():V001T03A029. doi:10.1115/ICEF2018-9753.

Dual-fuel strategies can enable replacement of diesel fuel with low reactivity biofuels like hydrous ethanol. Our previous work has shown that dual-fuel strategies using port injection of hydrous ethanol can replace up to 60% of diesel fuel on an energy basis. However, they yield negligible benefits in NOx emissions, soot emissions, and brake thermal efficiency (BTE) over conventional single fuel diesel operation. Pretreatment of hydrous ethanol through steam reforming before mixing with intake air offers the potential to both increase BTE and decrease soot and NOx emissions. Steam reforming can upgrade the heating value of the secondary fuel through thermochemical recuperation (TCR) and produces inert gases to act as a diluent similar to exhaust gas recirculation. This study experimentally investigated a novel thermally integrated steam reforming reactor that uses sensible and chemical energy in the exhaust to provide the necessary heat for hydrous ethanol steam reforming. An off-highway diesel engine was operated at three speed and load settings with varying hydrous ethanol flow rates reaching fumigant energy fractions of up to 70%. The engine achieved soot reductions of close to 90% and minor NOx reductions; however, carbon monoxide and unburned hydrocarbon emissions increased. A first law energy balance using the experimental data shows that efficient TCR effectively upgraded the heating value of the secondary fuel. Overall, hydrous ethanol steam reforming using TCR can lead to 23% increase in fuel heating value at 100% conversion, a limit approached in the conducted experiments.

Commentary by Dr. Valentin Fuster
2018;():V001T03A030. doi:10.1115/ICEF2018-9760.

There are many articles and papers published about the developments in engine downsizing as an effective means in reducing vehicle fuel consumption while improving engine performance. The increase in performance of gasoline turbo charged direct injected (GTDI) engines, in conjunction with diverse vehicle platform performance targets (i.e. towing capability) and higher gear transmissions pushes the engine to operate with higher torques at lower engine speeds. This operating condition has increased the propensity of an abnormal combustion event, known as Low Speed Pre-Ignition (LSPI) or Stochastic Pre-Ignition (SPI). The power cylinder unit (PCU) components exposed to this pre-ignition event can experience failure. The engine manufacturers, as well as MAHLE, continue to ensure engine and PCU component survivability against LSPI by performing life cycle robustness testing. MAHLE’s research of LSPI continues to focus on the robustness of PCU components in the presence of LSPI events, as well as investigating design developments that have the potential to minimize the propensity of LSPI to occur. The test procedure development for evaluating natural LSPI events will be presented. Various test results and parameter sensitivities that were documented during this procedure development, along with the many challenges associated with engine performance repeatability will be discussed. Parameters that were found to influence LSPI propensity, as well as parameters that were found not to influence LSPI propensity will be discussed.

Topics: Engines
Commentary by Dr. Valentin Fuster
2018;():V001T03A031. doi:10.1115/ICEF2018-9762.

The application of a Thermal Barrier Coating (TBC) to combustion chamber surfaces within a Low Temperature Combustion (LTC) engine alters conditions at the gas-wall boundary and affects the temperature field of the interior charge. Thin, low-conductivity, TBCs (∼150μm) exhibit elevated surface temperatures during late compression and expansion processes. This temperature ‘swing’ reduces gas-to-wall heat transfer during combustion and expansion, alters reaction rates in the wall affected zones, and improves thermal efficiency. In this paper, Thermal Stratification Analysis (TSA) is employed to quantify the impact of Thermal Barrier Coatings on the charge temperature distribution within a gasoline-fueled Homogeneous Charge Compression Ignition (HCCI) engine. Using an empirically derived ignition delay correlation for HCCI-relevant air-to-fuel ratios, an autoignition integral is tracked across multiple temperature ‘zones’. Charge mass is assigned to each zone by referencing the Mass Fraction Burn (MFB) profile from the corresponding heat release analysis. Closed-cycle temperature distributions are generated for baseline (i.e., ‘metal’) and TBC-treated engine configurations. In general, the TBC-treated engine configurations are shown to maintain a higher percentage of charge mass at temperatures approximating the isentropic limit.

Commentary by Dr. Valentin Fuster
2018;():V001T03A032. doi:10.1115/ICEF2018-9771.

Lean or diluted combustion has been considered as an effective strategy to improve the thermal efficiency of spark ignition engines. Under lean or diluted conditions, the combustion speed is reduced by the diluting gas. In order to speed up the combustion, in-cylinder flow is intentionally enhanced to promote the flame propagation. However, it is observed that the flow may make the spark ignition process more challenging due to the shortened discharge duration, the frequent re-strikes of spark plasma and the more complicated interactions between the flow and the flame.

In this research, the effects of spark discharge current level and discharge duration on flame kernel development and flame propagation of lean methane air mixture are investigated under flow velocity of about 25 m/s and background pressure of 4 bar abs in an optical combustion chamber. A dual coil ignition system and an in-house developed current management module are used to create different discharge current levels. The average discharge current levels range from 55 mA, 190 mA, up to 250 mA. Detached flame kernel is observed under some test conditions. The flame propagation speed with the detached flame is generally slower than the flame developed from a flame kernel attached to the spark plug. The flame detachment is related to both the discharge current level and the discharge duration. When the discharge current level is high at 250 mA, the detached flame is observed at shorter discharge duration of 0.8 ms, while when the discharge current is low at 190 mA, detached flame can happen at longer discharge duration of 1.3 ms.

Various discharge current and discharge durations are adopted to initiate the combustion in a single-cylinder engine operating with lean gasoline air mixture. It is shown from the results that a higher discharge current level and longer discharge duration are beneficial for controlling the combustion phasing and improving the operation stability of the engine.

Commentary by Dr. Valentin Fuster
2018;():V001T03A033. doi:10.1115/ICEF2018-9772.

Engine-out NOx emissions from diesel engines continue to be a major topic of research interest. While substantial understanding has been obtained of engine-out (i.e. before any aftertreatment) NOx formation and reduction techniques, not least EGR which is now well established and fitted to production vehicles, much less data are available on cycle resolved NOx emissions. In this work, crank-angle resolved NO and NOx measurements have been taken from a high-speed light duty diesel engine at test conditions both with and without EGR. These have been combined with 1D data of exhaust flow and this used to form a mass average of NO and NOx emissions per cycle. These results have been compared with combustion data and other emissions. The results show that there is a very strong correlation (R2 > 0.95) between the NOx emitted per cycle and the peak cylinder pressure of that cycle. In addition, the crank-angle resolved NO and NOx measurements also reveal that there is a difference in NO : NO2 ratio (where NO2 is assumed to be the difference between NO and NOx) during the exhaust period, with proportionally more NO2 being emitted during the blowdown period compared to the rest of the exhaust stroke.

Commentary by Dr. Valentin Fuster

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