Spark Assisted Compression Ignition Combustion Research Articles

Research on the performance of methanol/PODE dual fuel spark-assisted compression ignition combustion with different intake air adjustment

There is room for continued research and optimization on the exothermal processes and performance of methanol/polyoxymethylene dimethyl ether (PODE) dual fuel spark-assisted compression ignition (DF-SACI) with different air adjustment. The exothermal processes of methanol/PODE DF-SACI were investigated and optimized at varying methanol premix ratios (RM) and intake adjustment methods to maximize the promoting effect of spark ignition (SI). The results show that burning intensity progressively falls, while the ignition delay, CA50 and combustion duration progressively rise as excess air coefficient (λ) climbs. The resistance of high RM to lean combustion is weaker than low RM. As λ grows, the high-intensity exotherm is progressively transformed to the softer exotherm. Consequently, the exothermal energy from premixed compression ignition is progressively converted into the exothermal energy from DF-SACI. Subsequently, the exothermal energy from DF-SACI is progressively replaced by the exothermal energy of diffusion and SI combustion. By reducing air through intake variable valve timing (VVT), the optimal brake thermal efficiency (BTE) and RM under brake mean effective pressure (BMEP) of 0.4 MPa can be elevated to 40.67 % and 60 %, respectively. The optimal BTE for BMEP of 0.6 and 0.4 MPa are further enlarged from 42.82 % and 40.67 % to 43.52 % and 41.41 % by adjusting exhaust VVT, respectively.

Experimental study of a dual-fuel spark-assisted compression ignition engine with polyoxymethylene dimethyl ether and methanol as fuels

Polyoxymethylene dimethyl ethers (PODE) and methanol are promising alternative fuels for internal combustion engines, offering emission reduction and compatibility with existing engine combustion technologies. They can be chemically synthesized, providing a renewable and clean energy source for sustainable transportation. Additionally, dual-fuel spark-assisted compression ignition (SACI) is an advanced combustion mode that offers precise control of the combustion. However, there is limited research on dual-fuel SACI based on PODE and methanol, and the understanding of its multi-stage combustion, the cross-effects of multiple parameters, and the transition between ignition and compression ignition are still unclear. In this study, a modified gasoline engine was used to investigate the combustion characteristics of a dual-fuel SACI engine. The engine was fueled with direct injection PODE and port injection methanol. The combustion process was examined with a focus on the cooperative control of multiple parameters. The effects of the mass ratio of PODE, spark ignition timing, and start of injection timing were analyzed. The results demonstrated the presence of multiple heat-release stages during dual-fuel SACI combustion, involving both compression ignition and flame propagation ignition. An optimal interval between the start of injection timing and spark ignition timing was observed, leading to high brake thermal efficiency. This interval facilitated the formation of a highly reactive mixture during ignition, enabling efficient ignition of methanol by PODE. At a brake mean effective pressure of 0.8 MPa, a brake thermal efficiency of 44% was achieved with a 75% methanol ratio. Additionally, by appropriately increasing the mass ratio of PODE and advancing the start of injection timing, a significant reduction in hydrocarbon emissions was observed, albeit with a slight increase in nitrogen oxide emissions. These findings provide support for low-carbon operation of engines.

An experimental study on combustion and emissions characteristics in a dual-injection spark-assisted compression ignition engine fueled with PODE/gasoline

Spark-assisted compression ignition (SACI) is a potential way to improve thermal efficiency for gasoline engine with a relatively low compression ratio. The dual-injection system and spark strategy are considered to be an effective approach to control the combustion of SACI engine. Polyoxymethylene dimethyl ethers (PODE) is a potential fuel for carbon neutral with high oxygen content and unique molecule structure. In this study, the transition of combustion modes with different equivalence ratio and effects of direct injection ratio on SACI combustion and emissions fueled with PODE/gasoline under different loads were investigated. The results showed that SACI combustion could be achieved with the compression ratio of 13 and the brake thermal efficiencies at 2 bar, 3 bar, and 4 bar under the dual-fuel SACI were increased by 49%, 29%, and 27%, respectively, compared with the gasoline spark ignition mode. The increase in direct injection ratio first shortened the combustion duration and then prolonged. An appropriate direct injection ratio was shown to control the combustion process to achieve high efficiency combustion, at which low total HC, CO, and PM emissions were achieved while the NOx emissions remained at a low level.

A Quasi-Dimensional Burn Rate Model for Spark-Assisted Compression Ignition (SACI) Combustion

<div class="section abstract"><div class="htmlview paragraph">Future combustion engine applications require highest possible energy conversion efficiencies to reduce their environmental impact and be economically competitive. So far, spark-ignition (SI) engine combustion development mostly consisted of optimizing the hemispherical flame propagation combustion method. Thereby, a significant efficiency increase is only achievable in combination with high excess air dilution or increased combustion speed. However, with increasing excess air dilution, this is difficult due to decreasing flame speeds and flammability limits. Simultaneously, researchers have been investigating homogeneous charge compression ignition (HCCI) that achieves higher efficiencies due to its rapid volume reaction combustion and also enables high excess air dilution. However, the combustion is complex to control as it is initiated by auto-ignition (AI) processes. In-cylinder conditions reliably need to be reproduced to prevent damaging pre-ignitions. Consequently, HCCI has only been applied for low load operation. The spark-assisted compression ignition (SACI) represents a compromise between SI combustion and HCCI. Thereby, a flame propagation is initiated that triggers a controlled volume reaction in the remaining charge. By now, there is only one (mainly stoichiometric) application of SACI combustion in the market. In combination with lean mixtures, SACI allows overall efficiencies >40%. A market launch of a lean mixture SACI engine is therefore desirable. This requires a fast-running and predictive physical model to conduct robust concept studies in the early development process. This paper addresses the development of a quasi-dimensional burn rate model for the SACI combustion method. The modeling approach combines the well-established two-zone entrainment model (for flame propagation) with a multi pseudo-zone volume reaction model based on a distributed AI integral, which is linked to a detailed two-stage AI model. The model is integrated into the so-called cylinder module developed at IFS (Institute of Automotive Engineering Stuttgart). A validation versus measurement data shows a satisfactory prediction of the burn rate and pressure curve.</div></div>

Fuel property impacts on gaseous and PM emissions from a multi-mode single-cylinder engine

The U.S. Department of Energy’s Co-Optima initiative has focused on improving fuel economy and vehicle performance while reducing emissions through the simultaneous development of emerging sustainable fuels with beneficial properties and advanced combustion strategies. A major thrust has been the development of advanced compression ignition (ACI) combustion strategies of gasoline range fuels in combination with spark-ignited (SI) combustion in a single engine capable of multi-mode operation to achieve high power density with enhanced part load efficiency. The aim of this study was to further the understanding of how emissions from both ACI and SI strategies operating on the same fuels in the same engine are impacted by different fuel properties. This investigation focused on particulate matter (PM) and gaseous hydrocarbon emissions from 6 different fuels across 3 different combustion modes on the same single-cylinder engine designed for multi-mode operation: SI combustion, partial fuel stratification (PFS), and spark-assisted compression ignition (SACI). In each of these modes, 3 different CA50 phasings were studied such that all 6 fuels could be studied at the same phasings. Three of the six different fuels used were specially formulated in a previous investigation to study the impact of fuel distillation and aromatic content while maintaining the research octane number (RON) and octane sensitivity. Additionally, neat isooctane and two ethanol containing fuels (RD5-87 and Co-Optima E30) were studied. Different fuel and phasing impacts on emissions were observed across the three combustion modes. Fuel properties were found to impact soot PM and particle number more than the CA50 phasing, while the phasing had more impact on NOx emissions. The NOx emissions were reduced in the PFS mode for all fuels compared to SI combustion, but the SACI combustion mode did not reduce NOx emissions. Although PFS produced low soot PM emissions like SI, total PM mass emissions were significantly higher due to large organic carbon (OC) PM mass contribution. Both PFS and SACI had greater particle number emission than SI operation with small nuclei mode particles dominating in PFS compared to large agglomeration particles in SACI.

Characteristics of SACI combustion in a diesel rotary engine based on combined 1D and 3D comprehensive transient simulations

Wankel rotary engines (WREs) are widely used in aviation equipment, vehicles, micropower systems, etc. Diesel WREs are well suited for military and aerospace applications due to great safety features of the diesel fuel. In this paper, the transient in-chamber flow and combustion processes of a spark-assisted compression-ignition (SACI) direct injection diesel WRE were numerically simulated. Firstly, an effective method of using two rotating vectors is proposed to calculate the crank-angle-based instantaneous position of each location on the rotor surface in the rotor housing, which defines the law of dynamic boundary changes, to realize transient simulation. Secondly, the one-dimensional (1D) simulation results were used as the initial boundary conditions of the three-dimensional (3D) simulation, which improves the accuracy of the simulation and reduces the number of computational cycles during the validation of the 3D simulation model. Thirdly, model reduction and dynamic mesh control were used in the numerical simulation. The time of one calculation cycle was reduced from 223 h to 125 h, which is a 43.95% reduction. Finally, the 1D simulation revealed that ignoring the leakage between two adjacent combustion chambers was one of the main reasons for the error between the 3D simulated in-cylinder pressure and the experimental result. Based on the simulation methodology developed in this paper, the SACI combustion characteristics of a direct-injection diesel WRE were revealed. This paper provides an efficient simulation method for optimizing the design of novel WREs.

Compression ignition and spark assisted ignition of direct injected PRF65 spray

In this study the spark assisted compression ignition combustion (SACI) developments were investigated using PRF65 (low octane fuel), a mixture of 65% isooctane (by volume) and 35% n-heptane (by volume) with a RON of 65. Characteristics like the cumulative heat release (CHR) and the peak heat release rates (HRR) were studied pressure data from experiments conducted in a constant volume combustion chamber (CVCC) for more precise control of the tested conditions. Spray flame images were also studied using high speed imaging systems to understand the effect of the conditions tested in the luminosity of the flame. Experiments were performed to understand the effects of oxygen concentration and ambient temperatures. Results show that the heat release rate increases initially and then decreases with the increase in the ambient temperature and the peak heat release rate appears around 650 K to 700 K. The peak heat release rate timing is advanced with the increase of the ambient temperature or oxygen level. Flame luminosity was also found to increases with the increase in ambient temperature. Under a low ambient temperature, the oxygen level plays a major role in affecting the peak heat release rate. Under lower oxygen levels, the flame becomes darker, the ignition delay becomes longer, and the combustion process takes more time to complete. A well timed spark timing was found to advance the peak HRR and shorten ignition delay, but this effect becomes minor when the temperature increases.

The mitigation of mode-switching challenges of spark-assisted compression ignition engines through powertrain electrification

Spark-assisted compression ignition (SACI) is a novel combustion strategy that combines the fuel efficiency benefits of high EGR dilution, high compression ratio, and lean operation. Though the efficiency benefit is significant, SACI has several control and calibration challenges. To resolve this limitation, switching combustion modes between SACI and SI-only combustion is implemented. This work demonstrates the synergistic benefit of mild hybridisation on improving torque tracking compared to a non-hybrid SACI vehicle. A tip-in performed on a one-dimensional engine model shows a significantly reduced mode-switching challenge as the electric machine can adapt torque during mode changes, offsetting transport delays and actuator constraints. Furthermore, a vehicle model demonstrates that an electric machine can reduce the number of required mode transitions over four drive cycles from 15-40%. In addition to control benefits, the SACI powertrain also demonstrated 4-5% higher fuel economy compared to an SI-only powertrain in each vehicle configuration.

Research on ethanol and toluene's synergistic effects on auto-ignition and pressure dependences of flame speed for gasoline surrogates

Spark-assisted compression ignition (SACI) has a promising potential to substantially improve engine's fuel efficiency. To this end, two exothermic stages in SACI combustion, flame propagation and auto-ignition, need to be well organized to increase control authority of bulk ignition timing especially in lean burn. In this study, three gasoline surrogates, namely EPRF, ETPRF and TPRF, formulated through blending ethanol/toluene with primary reference fuel (PRF) and having the same research octane number (RON) and octane sensitivity (S), were used to conduct experiments in a rapid compression machine (RCM) under lean engine-relevant conditions (10–30 bar and 722–862 K). Under different ethanol blending ratios, both ethanol's synergistic effect during auto-ignition and its stronger pressure dependence of flame speed (SFlame) than toluene were observed. The ethanol's synergistic effect is mainly attributed to its more HO2 production and then faster consumption by benzyl which results in more OH radical production. As for the stronger pressure dependence of SFlame of ethanol, at 722 K, it is primarily determined by the stronger pressure dependence of H radical in EPRF's flame structure rather than the promotion effect from critical reactions on SFlame; while at 862 K, these two factors influence the pressure dependence of SFlame simultaneously. Whatever the temperature is, third-body reactions have larger impacts on ethanol's SFlame than on toluene's. In this study, the relative magnitude of SFlame’s pressure dependence between ethanol and toluene shows rationality at lower φ and higher T, which is in line with the pressure exponents extracted from the existing high-p laminar burning velocities of ethanol and toluene. Further verification was made in a spark-ignition engine, which showed that low-carbon alcohols, exhibited stronger pressure dependence of SFlame than monophenyl aromatics in commercial gasoline, represented by toluene. The aforementioned characteristics of ethanol can be utilized under different engine loads and provide a reference in fuel design for lean SACI combustion.

Evaluation of autoignition models for production control of a spark-assisted compression ignition engine

The drive to improve performance and efficiency of internal combustion engines has greatly expanded the degrees of freedom of engine systems. As efficiency objectives exceed the capability of traditional combustion strategies, advanced combustion modes are more attractive for production. These advanced combustion strategies typically add sensors, actuators, and degrees of freedom to the combustion process itself. Spark-assisted compression ignition is an efficient production-viable advanced combustion mode characterized by a spark-ignited flame propagation that triggers autoignition in the remaining unburned gas. This research focuses on autoignition modeling for spark-assisted compression ignition combustion phasing control. This work comprehensively evaluates several autoignition model structures and identifies the real-time production control implications of each. The candidate models include four ignition delay correlations, an ignition delay lookup, three polynomial regressions, and an artificial neural network. All are computationally feasible using production controllers, but the artificial neural network model represents autoignition phasing significantly better than the other options evaluated. The polynomial regressions were similar in error and exceeded the accuracy of ignition delay models. The low performance of the induction time integral–based models stems primarily from the exclusion of low-temperature heat release. The regression models are also exercised on an experimental engine dataset to identify the impact of engine phenomenon such as charge stratification on the performance of each model structure. The trends in the model performance as well as the magnitude of the error were similar when evaluated on both spark-assisted compression ignition simulation data and homogeneous charge compression ignition experimental data.

Homogeneous charge compression ignition combustion stability improvement using a rapid ignition system

When compared to traditional engines, homogeneous charge compression ignition has the potential to significantly reduce NO x raw emissions, while maintaining a high fuel efficiency. Homogeneous charge compression ignition is characterized by compression-induced autoignition of a lean homogeneous air–fuel mixture. Since homogeneous charge compression ignition does not utilize direct ignition control, it is strongly dependent on the state of the cylinder charge and can suffer from high cyclic variability. With spark-assisted compression ignition, it has been demonstrated that misfires can be reduced, while preserving the high thermal efficiency of homogeneous charge compression ignition as a result of the more favorable physical mixture properties due to dilution. However, spark-assisted compression ignition reduces the NO x benefits of homogeneous charge compression ignition, as it increases the local combustion temperatures. To merge the advantages of the homogeneous charge compression ignition and the spark-assisted compression ignition combustion processes, a field-programmable gate array for detailed simulation of the physical gas exchange is combined with a rapid spark system. The low latency and computational speed of the field-programmable gate array allows the simulation process to be implemented in real time. When combined with the rapid reaction time of the high-frequency current-based rapid ignition system, a feedforward controller based on the cylinder pressure or heat release is realized. The developed model-based controller determines if a spark is required to assist the homogeneous charge compression ignition combustion process. The use of the field-programmable gate array and rapid ignition system allows for the calculation of combustion properties and controller output within 0.1 °CA. This article presents the development and experimental validation of the developed controller on a single-cylinder research engine. The combustion stability has been significantly improved as reflected in an improved standard deviation of the indicated mean effective pressure and a reduction of the combustion phasing variations. Furthermore, compared to a traditional homogeneous charge compression ignition system, the hydrocarbon emissions can be reduced, while maintaining low NO x emissions.

Effects of partitioned fuel distribution on auto-ignition and knocking under spark assisted compression ignition conditions

Spark-assisted compression ignition (SACI) is a potential way to enhance ignition-control robustness and extend the load range of homogeneous charge compression ignition (HCCI) engines. However, the mechanism underlying combustion mode transition without knocking is not completely elucidated. Using direct high-speed photography and simultaneous pressure acquisition, the proposed study examines different auto-ignition and engine knock scenarios under SACI conditions. A small amount of heptane was directly injected into premixed methane-air mixture to modify local mixture reactivity. Additionally, late side injection was adopted to achieve stable SACI combustion whilst suppressing knocking combustion. Results obtained demonstrate that SACI combustion is essentially determined by the stochastic auto-ignition of the unburned end-gas mixture. End-gas mixture auto-ignition becomes stable thus normal SACI under conditions of high mixture reactivity, but a further rise in mixture reactivity causes engine knock prevail. To suppress SACI knocking, partitioned distribution of heptane via side injection has been found to be significantly effective. Results demonstrate that partitioned fuel distribution via late injection can greatly reduce the knock intensity, thereby implying a transition from SACI knocking to normal SACI combustion. Further analysis demonstrates that knock intensity is determined by the peak heat release rate of auto-ignition and there exists distinct boundaries between different combustion modes. The corresponding threshold value characterizing knocking combustion is approximately 202.30 J/CAD under current conditions. Besides, the peak heat release rate is related to the auto-ignition propagation speed and can be adjusted by the partitioned fuel distribution. The proposed study provides great insight into realizing and controlling SACI combustion to improve engine efficiency.

Effects of applying EGR with split injection strategy on combustion performance and knock resistance in a spark assisted compression ignition (SACI) engine

Spark assisted compression ignition (SACI) is a proven method for extending the load range and controlling the combustion phase of homogeneous charge compression ignition (HCCI) while maintaining high thermal efficiency. However, the occurrence of abnormal combustion, such as knock, limits the improvement of efficiency in SACI combustion. In this study, the effects of a coupling strategy, which combines internal/external exhaust gas recirculation (i & e-EGR) and split injection, on knock suppression in SACI mode were investigated in a high-compression-ratio, single-cylinder gasoline engine with a fully variable valve system. During the experiment, the mass of intake air remained constant while e-EGR was added. The results show that the coupling strategy combines the advantages of e-EGR and split injection, providing an effective method for resisting knock and improving engine efficiency. The results also demonstrate that applying e-EGR to SACI combustion significantly decreases the knock intensity by effectively reducing the in-cylinder temperature. In addition, the effect of split injection on knock suppression is related to the initial in-cylinder temperature and fuel stratification. With high initial in-cylinder temperature, the relationship between knock probability and split injection timing is non-monotonic. However, with low initial in-cylinder temperature, the capacity of resisting knock monotonically increases with the delay of secondary injection timing.

Thermodynamic efficiency assessment of gasoline spark ignition and compression ignition operating strategies using a new multi-mode combustion model for engine system simulations

Advanced combustion strategies for gasoline engines employing highly dilute and low-temperature combustion modes, such as homogeneous charge compression ignition and spark-assisted compression ignition, promise significant improvements in efficiency and emissions. This article presents a novel, reduced-order, physics-based model to capture advanced multi-mode combustion involving spark ignition, homogeneous charge compression ignition and spark-assisted compression ignition operating strategies. The purpose of such a model, which until now was unavailable, was to enhance existing capabilities of engine system simulations and facilitate large-scale parametric studies related to these advanced combustion modes. The model assumes two distinct thermodynamic zones divided by an infinitely thin flame interface, where turbulent flame propagation is captured using a new zero-dimensional formulation of the coherent flame model, and end-gas auto-ignition is simulated using a hybrid approach employing chemical kinetics and a semi-empirical burn rate model. The integrated model was calibrated using three distinct experimental data sets for spark ignition, homogeneous charge compression ignition and spark-assisted compression ignition combustion. The results demonstrated overall good trend-wise agreement with the experimental data, including the ability to replicate heat release characteristics related to flame propagation and auto-ignition during spark-assisted compression ignition combustion. The calibrated model was assessed using a large parametric study, where the predicted homogeneous charge compression ignition and spark-assisted compression ignition operating regions at naturally aspirated conditions were representative of those determined during engine testing. Practical advanced combustion strategies were assessed relative to idealized engine simulations, which showed that efficiency improvements up to 30% compared with conventional spark-ignition operation are possible. The study revealed that poor combustion efficiency and pumping work are the primary mechanisms for efficiency losses for the advanced combustion strategies evaluated.

Study on ignition and flame development in gasoline partially premixed combustion using multiple optical diagnostics

Gasoline partially premixed combustion (PPC) is a potential strategy to achieve high engine efficiency, as well as low NOx and soot emissions. But the in-cylinder combustion process of PPC is not well understood. In this paper, multiple optical diagnostics are applied to investigate the PPC ignition and flame development in a light-duty optical engine under single-injection condition. For the injection timing of −25 CA after top dead center (ATDC), the results indicate that the combustion process of gasoline PPC can be basically divided into four stages: 1) multiple auto-ignition kernels emerging in fuel-rich regions; 2) flame front propagation of the ignition kernels towards fuel-lean regions; 3) auto-ignition in the end-gas of fuel-lean regions; 4) a “burnout” stage in the whole combustion chamber after the main heat release process ends. The natural flame emission spectra from these four stages in PPC are analyzed. Distinct flame front propagation is verified during the early stages of the flame development process by both formaldehyde and OH planar laser-induced fluorescence (PLIF) imaging. The wide spread and late persistence of OH radicals after the main heat release process may account for the low soot emissions of gasoline PPC. The flame expansion speeds, determined by monitoring the flame fronts extracted from the combustion images, are much higher than that in SI (spark ignition) or SACI (spark-assisted compression ignition) combustion. With earlier fuel injection timing of −90 CA ATDC, the flame propagation process is less pronounced, and the sequential auto-ignition process prevails. Variation of the fuel stratification degree caused by the different fuel injection timings is responsible for this transformation in the flame development pattern for gasoline PPC.

Effect of Ambient Temperature and Humidity on Combustion and Emissions of a Spark-Assisted Compression Ignition Engine

Spark-assisted compression ignition (SACI) offers more practical combustion phasing control and a lower pressure rise rate than homogeneous charge compression ignition (HCCI) combustion and improved thermal efficiency and lower NOx emissions than spark ignition (SI) combustion. Any practical passenger car engine, including one that uses SACI in part of its operating range, must be robust to changes in ambient conditions. This study investigates the effects of ambient temperature and humidity on stoichiometric SACI combustion and emissions. It is shown that at the medium speed and load SACI test point selected for this study, increasing ambient air temperature from 20 °C to 41 °C advances combustion phasing, increases maximum pressure rise rate, causes a larger fraction of the charge to be consumed by auto-ignition (and a smaller fraction by flame propagation), and increases NOx. Increasing ambient humidity from 32% to 60% retards combustion phasing, reduces maximum pressure rise rate, increases coefficient of variation (COV) of indicated mean effective pressure (IMEP), reduces NOx, and increases brake-specific fuel consumption (BSFC). These results show that successful implementation of SACI combustion in real-world driving requires a control strategy that compensates for changes in ambient temperature and humidity.

Optical study of spray-wall impingement impact on early-injection gasoline partially premixed combustion at low engine load

Spray-wall impingement caused by early fuel injection for gasoline partially premixed combustion (PPC) can lead to low combustion efficiency and a significant rise of UHC emissions. But the influence of spray-wall impingement on the in-cylinder combustion process is not well understood. In this study, multiple optical diagnostics were applied to investigate the ignition, flame development and UHC formation of gasoline PPC with early single fuel-injection in a light-duty optical engine under low engine load. Natural combustion luminosity images and emission spectra were obtained. Planar laser-induced fluorescence (PLIF) of the fuel-tracer and formaldehyde were used to explore the fuel/air mixing and UHC formation in PPC, respectively. The results indicated that there was a fuel-injection time window (about −30° to −60° ATDC in the present study), during which the spray-impingement led to a decrease in combustion efficiency. The fuel-trapping effect in the squish region and piston crevice was shown to be the main reason because it prevented the fuel/air mixture from entering the combustion chamber. Two typical fuel injection timings of −35° (PPC-35) and −60° (PPC-60) were chosen for further study. For both cases, ignition sites first emerged in the fuel-rich regions and then the flames developed to the fuel-lean regions. The formaldehyde PLIF images revealed distinct flame front in the flame development process. For the PPC-35 case, residual formaldehyde persisted in the fuel-lean regions late during the power stroke and might become a source of UHC emissions. When misfire happened, the combustion chamber was filled with formaldehyde. For the PPC-60 case, the flame development was composed of initial flame front propagation and following sequential auto-ignition, and the flame expansion speed of the initial flame front propagation was much higher than that in SI (spark ignition) or SACI (spark assisted compression ignition) combustion. When the injection timing was further advanced (earlier than −60°), the impact of spray-wall impingement on PPC was reduced because of more time being available for fuel premixing.

On the sensitivity of low temperature combustion to spark assist near flame limit conditions

Abstract Spark assisted compression ignition (SACI) is a practical mode for controlling the heat release rate of low temperature combustion (LTC). While flames are key phenomena in the SACI combustion process, they may not always be effective or viable under the mildly stratified and highly dilute low burned gas temperature conditions of LTC. To better understand the limits of flammability or flame effectiveness, this work explores combustion within a single cylinder direct injection engine near the high load limit of the HCCI combustion regime, where spark induced flame propagation has been seen to affect combustion phasing and heat release rate. Flame limiting conditions were identified using progressively more advanced spark timing, up to 120° before top dead center, for differing levels of air and EGR dilution while holding the chemical energy content of the charge constant. Under air dilute conditions, the measured combustion phasing advanced from 8° to 0° after top dead center with spark advance, while almost no effect was seen under EGR dilute conditions. Estimates of the experimental global and local state conditions were made at the time of spark using heat release analysis and a KIVA-3V engine model, respectively, and the flammability for each case was evaluated using the Karlovitz criterion. The results show that fuel rich stratification near the spark plug was likely responsible for the observed variations in the SACI flame behavior.

Enhanced heat release analysis for advanced multi-mode combustion engine experiments

Advanced combustion strategies, such as Homogeneous-Charge Compression-Ignition (HCCI) and Spark-Assisted HCCI or Spark-Assisted Compression-Ignition (SACI) hold considerable promise for improving engine efficiencies while maintaining low pollutant emissions, yet few models exist that accurately include the important chemical and physical mechanisms of these advanced combustion strategies. Further, experimental data from advanced combustion engine experiments are not well represented using conventional spark ignited analytical tools. This paper presents new methods for advanced combustion analysis that integrate previous analytical methods with new algorithms to capture the unique features of advanced combustion strategies like SACI.The new analytical capabilities were incorporated into a program which was designated the Advanced Combustion Engine Heat Release Analysis (ACE-HRA) tool. The models developed and applied in ACE-HRA were assessed by comparison with high fidelity engine simulations of HCCI and SACI. The high fidelity simulations provided data sets with detailed predictions of heat release rates, temperatures, auto-ignition timing, flame speeds and other key parameters not resolved or measured in engine experiments. The sensitivity of ACE-HRA estimates to model input data was quantified for important engine performance parameters. The sensitivity analysis showed that estimates for in-cylinder masses have the largest overall impact on the ACE-HRA results (e.g. ±10% variation led to changes on the order of ±5–10% in peak rate of heat release, burn duration and peak temperature). Noticeable differences in peak heat release rate and ringing intensity were also observed when comparing cycle-by-cycle analysis against ensemble-average analysis, which has implications on how the results are interpreted and applied in modeling work.After validating ACE-HRA with the high-fidelity simulations, ACE-HRA was applied to interpret the data from a recent experimental study of SACI combustion. The ACE-HRA methods were used to infer the effects of flame propagation on in-cylinder gradients and cycle-to-cycle variability, and to provide quantitative estimates for the associated changes in the end-gas burn rate. The trends observed, such as the decrease in burn rate for later auto-ignition and higher burn fraction by flame, provide more in-depth understanding of SACI combustion and demonstrate the insight that can be revealed by the new ACE-HRA tool.

The effect of spark timing and negative valve overlap on Spark Assisted Compression Ignition combustion heat release rate

Spark Assisted Compression Ignition (SACI) combustion is capable of partially decoupling combustion phasing from peak heat release rate, overcoming one of the major challenges associated with low temperature combustion. Experimental SACI studies have shown that for a given combustion phasing, peak heat release rate can be modulated by trading the fraction of the fresh charge consumed by flame propagation with the fraction consumed by auto-ignition. The chemical and physical mechanisms controlling the changes in heat release rate are not well known. The current work uses computational simulation to explore these modes through the combined control of spark timing and charge temperature via changes in negative valve overlap. Open cycle CFD simulation results are compared with experimental data to validate a newly developed SACI model, and used to gain insight into the processes governing combustion behavior. The simulations predict a 43% reduction in the peak rate of heat release with increasing spark advance, while the unburned charge mass at the time of auto-ignition decreases by only 23%. Detailed analysis of the end-gas thermodynamic state prior to auto-ignition shows as spark is advanced, the end-gas thermal and compositional distributions vary with the fraction of the charge consumed by the flame. The reduction in the rate of peak rate of heat release during the auto-ignition process is therefore a function of both the mass of the end-gas and the end-gas reactivity.