Spark Assisted Compression Ignition Research Articles

A review on ammonia-hydrogen fueled internal combustion engines

In the face of the electrification trend in transportation, the internal combustion engine (ICE) is expected to continue playing a vital role in generating electricity for power systems or directly propelling vehicles in certain sectors. However, ICEs are also under significant pressure to achieve carbon neutrality, with the key lying in carbon-free fuels. Ammonia, compared to hydrogen, offers advantages in terms of hydrogen-carrying capacity, storage and transportation convenience, and safety, making it a promising carbon-free fuel for large-scale use in ICEs. Nonetheless, ammonia's combustion inertness poses challenges for its application, requiring efforts to enhance its combustion. Hydrogen, as a carbon-free and highly reactive fuel, serves as a powerful combustion promoter, maximizing the carbon-free effect of ammonia. Furthermore, on-board ammonia decomposition can produce hydrogen, ensuring a stable hydrogen supply and enabling ammonia-hydrogen synergy combustion while carrying only ammonia. This ammonia-hydrogen synergy combustion, based on on-board hydrogen production, presents a highly promising development direction for ammonia engines. When combined with hybridization, it further enhances the overall energy efficiency of ammonia. The objective of this paper is to review recent advancements in ammonia-hydrogen engines, covering topics such as ignition methods and combustion strategies, fuel supply, pollutants, and after-treatment. Based on this review, a conceptual ammonia-hydrogen engine for hybrid power systems is proposed. This engine ignites the ammonia-hydrogen mixture in the main chamber using hydrogen active jet ignition, achieving spark-assisted compression ignition. Technical measures for efficient engine combustion, synergistic utilization of exhaust heat for hydrogen production, and effective after-treatment of NOx, unburned NH3, and N2O are discussed. At last, some perspectives on the development of ammonia-hydrogen engines are also presented.

Effects of intake conditions on a dual-fuel spark-assisted compression ignition engine with polyoxymethylene dimethyl ether and methanol as fuels

Dual-fuel spark-assisted compression ignition (DF-SACI) is an advanced engine combustion mode. The use of polyoxymethylene dimethyl ether (PODE) and methanol as fuel can reduce the carbon emissions of the DF-SACI engine. To improve the combustion continuity and minimize the knocking and emissions of DF-SACI engines, the effects of intake heating and boosting were investigated. The results showed that the combination of intake heating and delayed spark ignition promoted fuel diffusion and prevented multi-peak heat release. Intake boosting promoted the transformation of compression ignition into flame propagation ignition. Intake heating and boosting allowed engine combustion optimization. The brake thermal efficiency was 41.4 % and 44 % when the brake mean effective pressure was 0.6 and 0.8 MPa, respectively, and the maximum pressure rise rate, coefficient of cyclic variation in the indicated mean effective pressure, and emissions were within reasonable ranges. A high intake air temperature combined with delayed spark ignition reduced the enrichment of PODE, thereby reducing the self-ignition probability of the fuel mixture, which can reduce knocking. When intake boosting was applied, the adoption of an excess air coefficient (λ) of 1.8 eliminated knocking under most conditions. Intake heating improved fuel reactivity and increased the ratio of compression ignition, to allow for the simultaneous reduction of hydrocarbon (HC) and NOx emissions. The effects of intake boosting on NOx and HC emissions were related to the energy ratio of PODE. As the energy ratio of PODE is between 47 % and 68 %, the appropriate increase in λ reduced both HC and NOx emissions. The results are conducive to the development of low-carbon engines and can provide a new direction for engine research.

Experimental Investigation of Spark-Assisted Compression-Ignition with Ammonia-Hydrogen Blends

Carbon-free emissions and stable operation of ammonia-fueled internal combustion engines have been demonstrated in recent studies through combustion strategies suited to the fuel’s unique properties. High ignition energy and slow flame speed of ammonia are typically compensated for by hydrogen addition and/or enhanced ignition systems while the high octane rating of ammonia allows high compression ratio. The experimental study in this work investigates the performance of ammonia in spark-assisted compression-ignition (SACI) mode, where the cylinder charge is spark ignited and a subsequent auto-ignition event results in a faster burn and more ideal combustion phasing. The high-compression ratio engine was fueled by 100% anhydrous ammonia or blends with small quantities of hydrogen (2.5% and 5% by volume). Fuels were port-injected for a homogeneous intake charge mixture at stoichiometric equivalence ratio. Two different engine speeds were tested at several intake temperatures and spark timings. All cases showed an inflection point in the apparent heat release rate (AHRR) followed by more rapid rate of heat release, signaling the onset of auto-ignition. Blending hydrogen in the fuel benefited gross indicated mean effective pressure (gIMEP) at maximum brake torque (MBT) timing, but pure ammonia fuel was more stable across a wide range of spark timings and intake temperatures. Burn durations in the present study were similar to those of conventional SI engines fueled by gasoline indicating that the combustion enhancement of SACI compensated for the lower flame speed of ammonia. The control of SI percent, defined as the fraction of heat release before auto-ignition compared to the total heat release, is shown to be strongly related to the sensitivity of auto-ignition timing to spark timing. Unburned ammonia emissions are hypothesized to be primarily driven by unburned gases trapped in the crevices and are mitigated slightly through blending hydrogen in the fuel or by increasing intake temperature. However, the addition of hydrogen and increasing intake temperature led to increased nitric oxide (NO) emissions. Of primary concern because of its potent global warming potential, nitrous oxide (N2O) did not show a clear trend with fuel hydrogen content or intake temperature, but definitively increased for the higher engine speed.

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.

Prospects of Controlled Auto-Ignition Based Thermal Propulsion Units for Modern Gasoline Vehicles

Gasoline engines employing the spatially distributed auto-ignition combustion mode, known as controlled auto-ignition (CAI), are a prospective technology for significantly improving engine efficiency and reducing emissions. This review paper provides an overview of developments in various gasoline CAI technologies and discusses their attendant strengths and weaknesses. Hybrid propulsion systems powered by high-efficiency gasoline CAI engines can provide a low-carbon pathway for mobility sector decarbonisation. Therefore, this paper focuses on the challenges and opportunities of CAI implementation, especially for electrified powertrains. Different control actuators that can extend the CAI operating range are discussed, and opportunities for synergistic operation between thermal and electric components of hybridised powertrains are identified. Such synergies can remove impediments in the way of CAI system adoption and can, thus, support CAI adoption and maximise efficiency gains from its implementation. The prospects of supporting CAI combustion for different powertrain electrification levels, hybrid architectures, engine size, and energy management systems are discussed. Load levelling offered by electrified powertrains through CAI-favouring energy management strategies has the potential to substantially relax the operating point requirements for CAI-based thermal propulsion units and to remove the need for expensive actuators. The highly flexible spark-assisted partially premixed compression ignition hybrid mode (SACI-PPCI) emerges as a promising CAI strategy for conventional powertrains, and the moderately flexible spark-assisted compression ignition (SACI) configuration can be a cost-effective thermal propulsion mode for electrified powertrains.

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.

Experimental study on controlled hot surface assisted compression ignition (HSACI) in a naturally aspirated single cylinder gas engine

Homogeneous charge compression ignition (HCCI) promises low [Formula: see text] emissions and high efficiency due to fast combustion at low temperatures. However, the control of combustion timing represents a serious challenge due to the lack of dedicated ignition control parameters. This challenge is addressed in this work by means of a hot surface ignition (HSI) system, whose core element consists of a shielded, electrically heated ceramic glow plug. In this approach, termed as hot surface assisted compression ignition (HSACI), a small portion of mixture is thought to ignite in the vicinity of the shielded glow plug and to subsequently propagate to the main combustion chamber in order to initiate bulk-gas auto-ignition. Adjusting the hot surface temperature enables to either advance or retard the onset of combustion and thus, allows to control combustion timing. This paper presents experimental results of initial engine trials, using the HSACI concept in a naturally aspirated single cylinder natural gas engine. Measurements were conducted at a constant engine speed of 1400 l/min and include intake air temperatures in the range of 150–175°C and relative air-fuel ratios ([Formula: see text]) from 2.0 to 2.8. Results show that the HSI system enables combustion under conditions, which do not allow for pure HCCI operation. Moreover, the combustion timing can be actively controlled within certain limits by changing the HSI temperature. Increasing cycle-to-cycle variations limit stable operation at lower temperatures, while a transition to pure HCCI is found at intake temperatures beyond 170°C. The applicable [Formula: see text] range is limited by knocking or uncontrolled combustion toward the rich side and instable operation toward the lean side. Loss analysis points out that wall heat flow and imperfect combustion represent the dominant loss mechanisms. Heat release analysis reveals two pronounced phases, indicating initial flame propagation and subsequent auto-ignition similar to spark assisted compression ignition (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.

Cycle-to-cycle variability in spark-assisted compression ignition engines near optimal mean combustion phasing

Stoichiometric spark-assisted compression ignition (SACI) combustion with exhaust gas recirculation (EGR) dilution has demonstrated higher part-load thermal efficiencies compared to spark-ignited engines, while maintaining ultra-low tailpipe emissions. However, SACI is often characterized by high cyclic variability in heat release or torque output, which poses a challenge to its implementation in light-duty vehicles. This paper presents an experimental investigation of cyclic variability in stoichiometric SACI combustion under EGR-dilute conditions, while maintaining mean combustion phasing (θ50) near optimal timing for thermal efficiency. The present work focuses on SACI conditions where the flame-based heat release fraction is between approximately 10% and 40% of the overall heat release, and therefore contributes significantly to the combustion process. For the SACI conditions examined, the variability in θ50was driven by variability in autoignition timing, which in turn correlated with the start of measurable heat release (θ02). High variability in θ50caused unstable work output due to very late combustion with poor or no end-gas autoignition. The use of a high ignition energy dual-coil offset ignition system had negligible impact in reducing θ50variability. Analysis of experimental data from close to 1000 operating conditions showed that the magnitude of θ50variability correlates with the flame-based heat release fraction ([Formula: see text]), for a large range of intake pressures, spark timings and exhaust gas recirculation levels. For the SACI conditions examined, combustion phasing variability was largely determined by flame-based combustion and particularly the initial flame formation (θIGN-02), and minimally by the end-gas autoignition heat release. The analysis also demonstrated that θ50variability is amplified as the contribution of the flame to the overall heat release increases.

Fuel Effects on Advanced Compression Ignition Load Limits

<div class="section abstract"><div class="htmlview paragraph">In order to maximize the efficiency of light-duty gasoline engines, the Co-Optimization of Fuels and Engines (Co-Optima) initiative from the U.S. Department of Energy is investigating multi-mode combustion strategies. Multi-mode combustion can be describe as using conventional spark-ignited combustion at high loads, and at the part-load operating conditions, various advanced compression ignition (ACI) strategies are being investigated to increase efficiency. Of particular interest to the Co-Optima initiative is the extent to which optimal fuel properties and compositions can enable higher efficiency ACI combustion over larger portions of the operating map. Extending the speed-load range of these ACI modes can enable greater part-load efficiency improvements for multi-mode combustion strategies. In this manuscript, we investigate fuel effects for six different fuels, including four with a research octane number (RON) of 98 and differing fuel chemistries, iso-octane, and a market representative E10 fuel, on the load limits for two different ACI strategies: spark-assisted compression ignition (SACI) and partial fuel stratification-gasoline compression ignition (PFS-GCI) operation. Experimental results show that limits to intake boosting limit high load operation for most fuels, but high smoke emissions for high particulate matter index (PMI) fuels under SACI conditions could also be a limitation. Contrastingly, low load is limited by combustion efficiency, but these effects have more pronounced variation with fuel chemistry for PFS-GCI than with SACI. Additional, distinct effects affecting autoignition timing and peak heat release at higher speeds were identified for fuels having different low temperature heat release (LTHR) propensities for both ACI modes.</div></div>

Flame propagation and auto-ignition behavior of iso-octane across the negative temperature coefficient (NTC) region on a rapid compression machine

Strict regulations on fuel economy are driving modern gasoline engines to adopt more advanced technologies to improve thermal efficiency. Some of the technologies, for example ultra-high compression ratio and spark assisted compression ignition, will elevate the thermodynamic condition near top dead center (TDC) to a considerably high level, even up to or beyond the negative temperature coefficient (NTC) region. This will definitely increase knock tendency when the combustion is not well controlled. Previous knock-related research mainly focused on temperature ranges in/below the NTC region, while the knock combustion beyond the NTC region has rarely been studied. To understand the knock behavior beyond the NTC region, in this study the flame propagation process and end-gas auto-ignition of iso-octane under wide thermodynamic conditions across the NTC region were optically studied using dual-camera photography. The results showed that the flame propagation speed increased with increasing initial temperature and decreasing initial pressure, exhibiting no NTC characteristic. With the intervention of flame propagation, the residence time of the end-gas was shortened as the initial thermodynamic conditions were promoted, indicating no NTC behavior in the overall ignition delay time of the end-gas. Two kinds of detonation initiation processes were identified. In the cases strongly affected by low temperature chemistry (LTC), the auto-ignition showed a two-stage characteristic during which a widespread but relatively weak auto-ignition (first-stage) was observed prior to the final detonation initiation. In contrast, when the LTC was absent, the detonation was initiated directly in a single auto-ignition event. Lower initial energy densities were needed to initiate detonation in the cases less affected by LTC. Thermodynamic analyses based on Bradley's ε-ξ diagram showed that, for the LTC-affected cases, the pressure rise which resulted from the widespread weak first-stage auto-ignition had vital impacts on the final detonation initiation by shifting the ε-ξ location into or away from the detonation region. Finally, thermal diffusivity was demonstrated to be capable of distinguishing detonation from other combustion modes as detonation tended to occur with lower thermal diffusivities of the mixture.

Performance of ammonia fuel in a spark assisted compression Ignition engine

Recent studies concluded that the use of ammonia in SI engines is possible thanks to an ignition booster or promoter. In this paper, the improvement of premixed ammonia/air combustion for internal combustion engines is studied as a function of performance and exhaust pollutants in a Spark-Assisted Compression Ignition single-cylinder engine, which supports a higher compression ratio (CR). For the first time, pure NH3 combustion was performed over a large range of engine operating conditions. The study concludes that neat ammonia can be used over a large operating range, here driven by the intake pressure, using a classical ignition device with a CR of 14–17 at 1000 rpm. The comparison with previous data obtained in a current single-cylinder SI engine clearly shows the potential of this engine mode, even for very low loads and various engine speeds (650, 1000, 2000 rpm), in spite of an initial aerodynamic that is not optimized to enhance flame-turbulence interaction. Kinetic simulations provide some explanations about exhaust emission behaviour, especially unburnt NH3, H2, NOx and N2O.

Machine Learning-Based Identification Strategy of Fuel Surrogates for the CFD Simulation of Stratified Operations in Low Temperature Combustion Modes

Many researchers in industry and academia are showing an increasing interest in the definition of fuel surrogates for Computational Fluid Dynamics simulation applications. This need is mainly driven by the necessity of the engine research community to anticipate the effects of new gasoline formulations and combustion modes (e.g., Homogeneous Charge Compression Ignition, Spark Assisted Compression Ignition) to meet future emission regulations. Since those solutions strongly rely on the tailored mixture distribution, the simulation and accurate prediction of the mixture formation will be mandatory. Focusing purely on the definition of surrogates to emulate liquid phase and liquid-vapor equilibrium of gasolines, the following target properties are considered in this work: density, Reid vapor pressure, chemical macro-composition and volatility. A set of robust algorithms has been developed for the prediction of volatility and Reid vapor pressure. A Bayesian optimization algorithm based on a customized merit function has been developed to allow for the efficient definition of surrogate formulations from a palette of 15 pure compounds. The developed methodology has been applied on different real gasolines from literature in order to identify their optima surrogates. Furthermore, the ‘unicity’ of the surrogate composition is discussed by comparing the optimum solution with the most different one available in the pool of equivalent-valuable solutions. The proposed methodology has proven the potential to formulate surrogates characterized by an overall good agreement with the target properties of the experimental gasolines (max relative error below 10%, average relative error around 3%). In particular, the shape and the end-tails of the distillation curve are well captured. Furthermore, an accurate prediction of key chemical macro-components such as ethanol and aromatics and their influence on evaporative behavior is achieved. The study of the ‘unicity’ of the surrogate composition has revealed that (i) the unicity is strongly correlated with the accuracy and that (ii) both ‘unicity’ and accuracy of the prediction are very sensitive to the high presence of aromatics.

Spark effects on compression ignition of PRF95 direct injection spray in a constant volume combustion chamber

In this study, spark assisted compression ignition (SACI) combustion was investigated using PRF95 (a low reactivity high octane reference fuel of common commercial gasoline), a mixture of 95% isooctane (by volume) and 5% n-heptane (by volume) with a RON of 95 and compared with pure compression-ignition without spark. Characteristics like the cumulative heat release (CHR) and the heat release rates (HRR) were studied using pressure data from experiments conducted in a constant volume combustion chamber (CVCC). The combustion process was visualized with a high speed imaging technique. Tests were conducted to understand the effects of ambient oxygen concentration and temperatures on the heat release and combustion flame developments while controlling other factors. From the experiments it is evident that the peak heat release rate increases initially and then decreases with the increase in the ambient temperature and the highest peak heat release rate appears around 650–700 K. The peak heat release rate timing decreases with the increase of the ambient temperature or oxygen level. 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 weaker, the ignition delay becomes longer, and the combustion process takes more time to complete. Proper spark timing can help advance the peak HRR and shorten ignition delay, but this effect becomes minor when the ambient temperature increases. For the current high octane low reactivity fuel, auto-ignition can be achieved for all the investigated ambient temperature and oxygen levels. But it is noticed that a spark is necessary for low ambient oxygen and low ambient temperature to achieve proper combustion.

Evaluation of control-oriented flame propagation models for production control of a spark-assisted compression ignition engine

The drive to improve internal combustion engines has led to efficiency objectives that exceed the capability of conventional combustion strategies. As a result, advanced combustion modes are more attractive for production. These advanced combustion strategies typically add sensors, actuators, and degrees of freedom to the combustion process. Spark-assisted compression ignition (SACI) is an efficient production-viable advanced combustion strategy characterized by spark-ignited flame propagation that triggers autoignition in the remaining unburned gas. Modeling this complex combustion process for control demands a careful selection of model structure to maximize predictive accuracy within computational constraints. This work comprehensively evaluates a physics-based and a data-driven model. The physics-based model produces a burn duration by computing laminar flame speed as a function of test point conditions. The crank-angle domain is intentionally excluded to reduce computational expense. The data-driven model is an artificial neural network (ANN). The candidate models are compared to a one-dimensional engine model validated to experimental SACI engine data. Though both models capture the trends in burn rates, the ANN model has a root-mean square error (RMSE) of 1.4 CAD, significantly lower than the 10.4 CAD RMSE of the physics-based model. The exclusion of the crank-angle domain results in insufficient detail for the physics-based model, while the ANN can tolerate this exclusion.

Impact of fuel chemical function characteristics on spark assisted and kinetically controlled compression ignition performance focused on multi-mode operation

The DOE Co-Optima initiative has a focus on investigating the ability of fuel properties to work in tandem with advanced combustion engines to increase fuel economy. Advanced compression ignition strategies like spark assisted compression ignition (SACI) and partial fuel stratification (PFS) have been shown to achieve better efficiency and emissions performance than traditional combustion processes (i.e., conventional diesel combustion, spark ignited combustion). These strategies rely on a high degree of fuel mixing and a globally dilute environment to achieve lower temperature combustion. The avoidance of fuel rich regions and the reduction in peak flame temperatures result in low soot and NOx formation. Despite their clear benefits, operating range limitations have been identified for all combustion strategies. The limitations stem from the fundamental characteristics of each combustion process, hence they can't be entirely avoided. These limitations are also geometry, fuel type, dilution level and mixture preparation dependent. Metal engine experiments have been conducted on a single-cylinder research engine equipped with variable valve actuation and a 12.5:1 compression ratio, more appropriate for high load boosted spark ignition operation. Five fuels with different chemical class compositions but matched research octane number (RON) were tested under both SACI and PFS at mid load conditions. Combustion phasing was changed over the whole range of operability to show trade-offs between the fuels. Specific chemical, mixture preparation and thermodynamic effects are discussed for performance and emission results. SACI and PFS are then compared to a baseline spark ignition (SI) condition to estimate potential benefits of operating under advanced combustion modes. The results show that there are significant fuel specific effects even at matched RON and octane sensitivity that affect emissions, engine efficiency and range of operability of the different advanced combustion modes.

A Quasi-Dimensional Fuel Distribution Model for a Radially Stratified Engine

Abstract Several combustion strategies leverage radial fuel stratification to adapt cylinder composition between the center of the chamber and the outer regions independently. Spark-assisted compression ignition (SACI) relies on careful tuning of this radial stratification to maximize the combined performance of flame propagation and auto-ignition. Established techniques for determining in-cylinder fuel stratification are computationally intensive, limiting their feasibility for control strategy development and real-time control. A fast running model for radial fuel stratification is developed for control-oriented objectives. The model consists of three submodels: spray penetration, fuel distribution along the spray axis, and postinjection mixing. The spray penetration model is adapted from fuel spray models presented in the literature. The fuel distribution and mixing submodels are validated against injection spray results from an large eddy simulation (LES) three-dimensional (3D) computational fluid dynamics (CFD) reference model for three test points as a function of crank angle. The quasi-dimensional model matches the CFD results with a root-mean-square error (RMSE) for equivalence ratio of 0.08–0.11. This is a 50% reduction from the 0.16–0.20 RMSE for a model that assumes a uniform fuel distribution immediately after injection. The computation time is 230 ms on an Intel (Santa Clara, CA) Xeon E5-1620 v3 to solve each case without optimizing the code for execution speed.

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.