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The benefits of pre-chamber combustion (PCC), such as improved engine efficiency and reduced NOx emissions, are primarily observed when operating at lean conditions with an active pre-chamber, where auxiliary fuel is supplied directly to the pre-chamber. Estimating the pre-chamber excess air ratio (λ) is important in the active pre-chamber concept to gain insights into the pre-chamber combustion phenomenon. Experimental investigations were performed using a narrow-throat pre-chamber at global-λ 1.6, 1.8, and 2.0. The fraction of fuel energy injected in the pre-chamber over the total fuel energy was fixed at 3%, 7%, and 13% for each global-λ. The mixture formation process inside the pre-chamber is first simulated using the 1-D simulation software GT-Power to analyze the pre-chamber λ at the ignition timing. However, the 1-D results were unable to reproduce the experimental observations on the pre-chamber pressure buildup accurately. Upon simulating the same conditions using the 3-D CFD software CONVERGE, the pre-chamber λ estimated from the CFD model is well-correlated to the experimental data. The CFD results indicate that the amount of fuel trapped in the pre-chamber at the inlet valve closing timing is over-predicted by the 1-D simulations. A correlation between the injected and the trapped fuel in the pre-chamber is proposed by theoretical scavenging models and applied to the 1-D simulation results to improve pre-chamber λ prediction accuracy.

Lean operation increases the efficiency of the Otto-cycle internal combustion engine and decreases its emissions. However, increasing the air-fuel ratio beyond stoichiometry requires higher ignition energy to maintain the stable operation of the engine. The pre-chamber emerges as one of the promising enablers of lean operation, providing much larger energy into the main combustion chamber than simple a spark plug at multiple sites to increase combustion stability. Pre-chambers are classified into two categories based on their fuel input; active pre-chambers, with a dedicated fuel injection system, and passive pre-chambers, which are solely charged with the main chamber air-fuel mixture through nozzle holes. Therefore, the passive pre-chamber type is favorable for existing engines because of its compact design and limited modification requirements. Nevertheless, passive pre-chambers have issues with igniting very lean mixtures. In this study, a single-cylinder light-duty engine is used to study the possibility of extending the lean limit of the passive pre-chamber using a split direct injection (DI) strategy and indirect enrichment of the pre-chamber mixture. The results of the split injection method were then compared to port fuel injection (PFI) measurements. Also, another set of experiments was performed with a standard spark plug using PFI and split DI for comparison. The results showed an increase in the lean limit of passive pre-chamber operation when using the split DI strategy compared to PFI, from λ = 1.5 to 1.7. However, increased soot production was observed when using the split injection strategy.

We studied the relationship between pre-chamber jet and main chamber ignition in the pre-chamber combustion (PCC) of an optical engine, fueled with methane and equipped with an active pre-chamber with two rows of orifices. Acetone planar laser-induced fluorescence (PLIF) and OH* chemiluminescence imaging techniques were simultaneously applied to visualize the pre-chamber jet and the reaction zone in the main chamber, respectively. The pre-chamber fueling was constant and the main chamber fueling was increased to form an ultra-lean case and a lean case with global excess air ratios (λ) of 2.3 and 1.8, respectively. Results indicate that a higher pressure difference between pre-chamber and main chamber ( ▵ P) produces larger pre-chamber jet penetration speed; the maximum pre-chamber jet penetration speed appears at timing around the peak ▵ P. Over enrichment of the pre-chamber charge reduces the peak ▵ P and thus does not favor a faster pre-chamber jet discharge. In addition to the main pre-chamber jet, a weaker post jet discharge process is visualized; the former is due to the pre-chamber combustion while the latter due to the ▵ P fluctuation and the cylinder volume expansion. The post pre-chamber jet is accompanied by a post reaction zone in the ultra-lean case (λ=2.3) and there are two unburned regions in the main chamber: one is around the pre-chamber nozzle and the other between the adjacent reaction zones. These two unburned regions are consumed by flame propagation in the lean case (λ=1.8). The weak pre-chamber jet from the upper-row orifice does not produce any distinct reaction zone, indicating that the pre-chamber orifice location and arrangement on the nozzle also matters in the pre-chamber design. The pre-chamber jet penetration length is longer than that of the reaction zone during pre-chamber discharge; the penetration length difference between the pre-chamber jet and reaction zone decreases with increasing main chamber fueling.

Multiple injection strategies can be used for controlling the heat release rate in an engine, particularly in compression ignition engines. This can mitigate the heat transfer losses and overcome the limitation related to the maximum pressure allowed for a particular engine. Controlling heat release with repetitive injections requires precise characterization of the fuel injection rates. In such a configuration, the injector used should be characterized for its hydraulic delay, rate of injection, and the effect of dwell timing with multiple injections. This study investigates the fuel injection behavior of a high-flow-rate solenoid injector operated with single and double injections. Two characterization methods, the momentum flux, and the Bosch tube are used and compared to investigate their suitability with the multiple injection strategies. Experiments with single injection are conducted by varying the Energizing Timing (ET) from 0.5 up to 2 ms. The tests with multiple injections (i.e., double injections) are conducted with a fixed ET of 0.5 ms, while the dwell times (δt) are varied from 0.1 up to 1 ms. All tests are performed at 500, 1000, 1500, and 2000 bar rail pressures. Depending on the injection pressure, the injector’s needle could not fully close with short dwell times and the injections are merged. The momentum flux method has faster ramp-up and decaying and more oscillations in the quasi-steady-state phase compared to the Bosch tube method. The effective duration of injection is overpredicted with the Bosch tube method. The momentum flux method is demonstrated to be more suitable for measuring the ROI of multiple injection strategies.

The Homogeneous Charge Compression Ignition (HCCI) concept shows great potential for improving engine efficiency and reducing pollutant emissions. However, the operation with this concept in Internal Combustion (IC) engines is still limited to low speed and load conditions, as excessive Pressure Rise Rates (PRR) are generated with its fast auto-ignition. To overcome this limitation, the use of moderate thermal and charge stratification has been promoted. This leads to multi-stage ignition, and thus a potentially acceptable PRR. Recently Sarathy et al. (2019), three-stage auto-ignition has been emphasized as a chemical phenomenon where the thermal runaway is inhibited during the main ignition event. The current paper demonstrates experimental evidence on this phenomenon observed during n-heptane and n-hexane auto-ignition at lean diluted conditions in a flat piston Rapid Compression Machine (RCM). Multi-stage ignition events caused by either chemical kinetics or by the well-known thermal stratification of this type of RCM are clearly identified and differentiated. The combination of these two factors seems to be a suitable solution to overcome PRR limitations.

The experimental work was supported by the French research agency (Association Nationale de la Recherche et de la Technologie ANRT), Renault S.A., and PPRIME Institute during the Ph.D. thesis of M. Ben Houidi (CIFRE N:384/2010). The simulation work was supported by King Abdullah University of Science and Technology (KAUST) Office of Sponsored Research (OSR) with funds given to the Clean Combustion Research Center. We acknowledge funding from the KAUST Clean Fuels Consortium and its member companies.

Hydrogen is a promising future fuel to enable the transition of transportation sector toward carbon neutrality. The direct utilization of H2 in internal combustion engines (ICEs) faces three major challenges: high NOx emissions, severe pressure rise rates, and pre-ignition at mid to high loads. In this study, the potential of H2 combustion in a truck-size engine operated in spark ignition (SI) and pre-chamber (PC) mode was investigated. To mitigate the high pressure rise rate with the SI configuration, the effects of three primary parameters on the engine combustion performance and NOx emissions were evaluated, including the compression ratio (CR), the air–fuel ratio, and the spark timing. In the simulations, the severity of the pressure rise was evaluated based on the maximum pressure rise rate (MPRR). Lower compression ratios were assessed as a means to mitigate the auto-ignition while enabling a wider range of engine operation. The study showed that by lowering CR from 16.5:1 to 12.5:1, an indicated thermal efficiency of 47.5% can be achieved at 9.4 bar indicated mean effective pressure (IMEP) conditions. Aiming to restrain the auto-ignition while maintaining good efficiency, growth in λ was examined under different CRs. The simulated data suggested that higher CRs require a higher λ, and due to practical limitations of the boosting system, λ at 4.0 was set as the limit. At a fixed spark timing, using a CR of 13.5 combined with λ at 3.33 resulted in an indicated thermal efficiency of 48.6%. It was found that under such lean conditions, the exhaust losses were high. Thus, advancing the spark time was assessed as a possible solution. The results demonstrated the advantages of advancing the spark time where an indicated thermal efficiency exceeding 50% was achieved while maintaining a very low NOx level. Finally, the optimized case in the SI mode was used to investigate the effect of using the PC. For the current design of the PC, the results indicated that even though the mixture is lean, the flame speed of H2 is sufficiently high to burn the lean charge without using a PC. In addition, the PC design used in the current work induced a high MPRR inside the PC and MC, leading to an increased tendency to engine knock. The operation with PC also increased the heat transfer losses in the MC, leading to lower thermal efficiency compared to the SI mode. Consequently, the PC combustion mode needs further optimizations to be employed in hydrogen engine applications.

International audience; Homogeneous Charge Compression Ignition (HCCI) and Spark Induced Compression Ignition (SICI) of a lean iso-octane air mixture are investigated through simultaneous measurements of planar laser-induced fluorescence at 355 nm and high-speed chemiluminescence in the parallelepipedic combustion vessel of a rapid compression machine (RCM). A radiofrequency igniter with a high energy deposit (305 mJ) is used to investigate the SICI combustion phenomena in lean conditions (Φ = 0.5), relatively close to the frontiers of the SICI regime. Fluorescence images enable to monitor both the development of the cool flame process and the topology and dynamics of reaction fronts during the second stage of ignition. The results are first analyzed from a phenomenological point of view, bringing insights into the understanding of the both HCCI and SICI combustion processes as they take place in the RCM. Additional data are gathered from double-pulse 355 nm PLIF imaging, with focus on the temporal evolution of the cool flame and on the reaction front propagation during hot ignition. From a more quantitative point of view, an analysis of apparent velocities of the reaction zones is then presented, and large variations of these values are observed depending on the experimental conditions. These local quantities are closely related to the global heat release rate which is a key parameter for practical applications of HCCI and SICI combustion modes. The proposed simultaneous diagnostics finally lead to a better understanding in the local reaction modes, namely deflagration, spontaneous ignition fronts and bulk auto-ignition -- e.g. volumetric auto-ignition -, which are implied in the combustion processes. The results highlight the complex aerothermal interactions taking place in the RCM vessel, in particular through the pre-ignition thermal stratification. The results suggest the latter strongly affects the HCCI combustion process, but also drives the heat release rate during the second stage of the SICI combustion mode. Furthermore, deflagration fronts are found to be significantly affected by cool flame chemistry, as well as by the large and small scale structures of the fluid flow.

The paper is based upon the work supported by Saudi Aramco Research and Development Center FUELCOM3 program under Master Research Agreement Number 6600024505/01. FUELCOM (Fuel Combustion for Advanced Engines) is a collaborative research undertaking between Saudi Aramco and KAUST intended to address the fundamental aspects of hydrocarbon fuel combustion in engines, and develop fuel/engine design tools suitable for advanced combustion modes. The authors would like to thank King Abdullah University of Science and Technology (KAUST) and the Clean Combustion Research Center (CCRC) for lab facilities and research support. Finally, the authors would like to convey gratitude towards the IC Engine Lab Safety Supervisor Adrian I. Ichim and the lab technician Riyad H. Jambi for their kind input and assistance in performing the experiments. The authors would like to express gratitude toward

The paper is based upon work supported by Saudi Aramco Research and Development Center FUELCOM3 program under Master Research Agreement Number 6600024505/01. FUELCOM (Fuel Combustion for Advanced Engines) is a collaborative research undertaking between Saudi Aramco and KAUST intended to address the fundamental aspects of hydrocarbon fuel combustion in engines, and develop fuel/engine design tools suitable for advanced combustion modes. The author would like to thank King Abdullah University of Science and Technology (KAUST) and the Clean Combustion Research Center (CCRC) for lab facilities and research support. Last but not least, the authors would like to convey gratitude towards the IC Engine Lab Safety Supervisor Adrian I. Ichim and the lab technician Riyad H. Jambi for their kind input and assistance in performing the experiments.

Isobaric combustion is more efficient than isochoric combustion when the peak cylinder pressure is restricted. Recently a double compression expansion engine concept using isobaric combustion was proposed to improve the engine performance. However, the knowledge about the in-cylinder isobaric combustion process is limited. This study investigated isobaric combustion in both metal and optical engines utilizing a centrally placed direct injector and a multiple injections strategy, and the performance and emissions of isobaric combustion and conventional diesel combustion (CDC) were compared. Mie scattering and fuel-tracer planar laser-induced fluorescence (PLIF) were used to visualize the liquid-phase fuel penetration length and fuel distribution under non-reactive conditions, respectively. The natural flame luminosity and spatial soot distribution were measured with high-speed imaging and laser-induced incandescence (LII), respectively. Results demonstrate an increased soot formation in the isobaric cases compared with the CDC case. The successive fuel injection into reacting zones, which induces a locally fuel-rich mixture and less fuel–air mixing, accounts for the high soot emissions in the isobaric combustion. This study further demonstrates that the late injections of the isobaric cases lead to more combustion in the squish zone and near the cylinder walls, which increases the local temperature gradient and may result in more heat losses. There is still room to reduce the heat losses in the isobaric combustion although the total heat loss of the isobaric combustion is lower than the CDC case due to a lower combustion temperature. Multiple injectors are suggested to reduce the soot emissions of the isobaric combustion by spreading the different injections in space.

Rapid compression machines (RCMs) have often been used to study auto-ignition phenomena and to validate chemical kinetic models at conditions relevant to engines. Many RCM designs were historically used for the latter purpose. First, the flat piston configuration was extensively used and contributed to valuable ignition delay data. Temperature heterogeneity in these devices was later addressed, and new designs were proposed. In particular, creviced piston configurations have been demonstrated to enhance the temperature homogeneity in the bulk volume of RCM chambers. Nevertheless, data from flat piston RCMs are still used in the community. Thus, additional understanding of experimental results from such devices is still useful, and it also helps to enhance the understanding of temperature stratification effects on auto-ignition phenomena. This paper addresses the interpretation of measured ignition delays from a flat piston RCM (without a circumferential crevice in the piston). First, we characterize local and temporal temperature evolution with a toluene planar laser induced fluorescence (PLIF) technique applied in oxygen-free gas under different test conditions. At a compression pressure ( P c ) of 11 bar and an adiabatic core temperature ( T c ) of 750 K, we find that the maximum measured temperature is very close to T c up to 30 ms after the compression stroke. In the second step, we qualitatively compare temperature fields with chemiluminescence images during the auto-ignition of n-hexane and an n-heptane/methyl-cyclohexane (MCH) blend for a fuel-air equivalence ratio of 0.4, P c values of 11 bar and 16 bar and T c values of 700–900 K. The chemiluminescence results of n-hexane show a fast sequential auto-ignition which strongly suggest that the combustion propagation regime is mainly a reaction fronts regime. In the n-heptane/MCH case, the tracking of ignition kernels demonstrates that the auto-ignition initiates in the cold vortex at T c values of 787 K and 834 K and in the adiabatically compressed region at T c values of 742 K and 900 K. In this study, flat piston RCM experiments demonstrate temperature heterogeneities with ranges up to 120 K and gradients of approximately 160 K/mm. This significantly influences the ignition delay measurement in the negative temperature coefficient (NTC) range. Through the example of n-heptane/MCH auto-ignition, it seems that despite temperature heterogeneities, the lower limit of the NTC range is well detected, whereas the higher limit is over-estimated. The determination of temperature thresholds in which ignition delay data may be confounded highly depends on the stratification level and on complex thermo-kinetic interactions. Outside these thresholds, when combustion is dominated by auto-ignition in regions at the adiabatic core temperature, the experimental data are easier to compare with simulation frameworks assuming a homogeneous hypothesis. In chemical kinetics-oriented studies, temperature heterogeneity should be considered when flat piston RCMs are used and when the piston crevice is not adequate.

The authors would like to thank Dr. Balaji Mohan for his considerable cooperation and valuable feedback throughout the progress of the experiment. Special thanks to the Engine group for their contribution during engine operation.

The canonical diesel spray A is characterized in an optical Rapid Compression Machine (RCM) at high temperature and density conditions (900 K and 850 K, ρ = 23 kg/m3) using simultaneous high-speed OH* chemiluminescence and two-pulse 355 nm Planar Laser Induced Fluorescence (PLIF). The focus is on the time evolution and the repeatability of the early stages of both cool flame and hot ignition phenomena, and on the time evolution of the fluorescing formaldehyde region in between. In particular, time resolved data related to the cool flame are provided. They show the development of several separated kernels on the spray sides at the onset of formaldehyde appearance. Shortly after this phase, the cool flame region expands at high velocity around the kernels and further downstream towards the richer region at the spray head, reaching finally most of the vapor phase region. The position of the first high temperature kernels and their growth are then characterized, with emphasis on the statistics of their location. These time-resolved data are new and they provide further insights into the dynamics of the spray A ignition. They bring some elements on the underlying mechanisms, which will be useful for the validation and improvement of numerical models devoted to diesel spray ignition.