Assisted Compression Ignition Research Articles

Computational insights into flame development and emission formation in an ammonia engine with hydrogen-assisted pre-chamber turbulent jet ignition

Ammonia, as a promising green fuel for achieving carbon neutrality and zero-carbon emissions, encounters obstacles in practical engine applications due to its intrinsic combustion properties like low burning velocity and high autoignition temperature. Hydrogen-assisted pre-chamber turbulent jet ignition (TJI) technology has recently demonstrated great potential in extending the lean limits and enhancing the stability of ammonia combustion. As such, this study conducts a comprehensive analysis on the flame development and emission formation processes under the active pre-chamber TJI mode fueled with ammonia and hydrogen using computational fluid dynamics (CFD) modeling integrated with detailed chemical kinetics, particularly focusing on the effects of hydrogen energy ratio and spark timing. It is found that the active ammonia-hydrogen TJI engine is typically characterized by four primary physical processes involving H2 mixture preparation in the pre-chamber, flame kernel formation and development within the pre-chamber, the formation and ejection of hot jet across the orifices, and the turbulent flame initiation and propagation in the main chamber. Hydrogen injection holds a scavenging effect on the residuals in the pre-chamber while forms fuel stratification near the spark plug, enhancing the ignition stability. Besides, the current combustion mode is commonly characterized by two-stage heat release in the main chamber, but exhibits a three-stage heat release as increasing the premixed H2 ratio. The first stage is dominated by a hot jet flame, the second stage is characterized by turbulent flame propagation or autoignition, and the third stage is induced by a secondary jet ejected from the pre-chamber, which becomes more prominent as the H2 ratio increases. Hydrogen blending could not only facilitate the turbulent flame propagation of NH3-air mixture in the main chamber but also enhance the initiation of flame kernel in the pre-chamber, thereby promoting the overall combustion process and potentially improving the thermal efficiency. The N2O emission is produced in two distinct stages, wherein N2O is generated and undergoes conversion during the main combustion phase, while unburned crevice gases mix with burned gases and convert into N2O during the expansion stroke. Increasing the premixed hydrogen ratio leads to an increased in-cylinder temperature, which in turn facilitates a reduction in N2O emissions and mitigates unburned ammonia. Furthermore, optimization of the spark timing is required after implementing the hydrogen addition into the intake manifold to achieve desirable combustion phasing and further improve the thermal efficiency. Finally, a novel combustion concept, denoted as TJI assisted compression ignition mode, is proposed for achieving high thermal efficiency in ammonia engines via adopting high compression ratio and premixed ammonia-hydrogen charge under the TJI strategy.

Experimental Study on Spark Assisted and Hot Surface Assisted Compression Ignition (SACI, HSACI) in a Naturally Aspirated Single-Cylinder Gas Engine

Abstract Spark assisted compression ignition (SACI) represents an efficacious technique to extend the operating range and control combustion timing in homogeneous charge compression ignition (HCCI) engines. Recently, a hot surface ignition system (HSI) was demonstrated to enable hot surface assisted compression ignition (HSACI) featuring similar combustion characteristics compared to SACI. This work compares both combustion processes with regard to control and combustion characteristics, the strength of the ignition systems, and cycle-by-cycle variations (CCV). Engine trials were conducted using a single-cylinder research engine fueled with natural gas. The engine operated naturally aspirated at an engine speed of 1400 1/min and steady-state conditions. Experimental conditions cover relative air-fuel ratios λ = 2.1–3.1, intake temperatures Tin = 140–170 °C and intake pressures pin = 993–995 mbar. Results show similar capabilities of SACI and HSACI to control combustion timing by means of spark timing in SACI and hot surface temperature in HSACI. Heat release analyses of individual combustion cycles point out the similarity of both combustion processes. The evaluation of the strength of the ignition systems reveals that HSACI extends the lean limit by Δλ = 0.05–0.10 and advances the earliest applicable combustion timing (MinCA50) by ΔMinCA50 = 1.0–4.5 °CA provided that ringing is not of concern. Comparison of CCV in HCCI, SACI, and HSACI shows highest combustion stability for HCCI, followed by SACI. HSACI evinces highest CCV due to a larger variation at the start of combustion.

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).

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.

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.

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.

Engine efficiency enhancement and operation range extension by argon power cycle using natural gas

Argon power cycle engine has been proved as a promising technology to increase engine efficiency and to reduce engine out emissions. In this research work, experiments were conducted in a port injection single-cylinder engine with air and argon/oxygen as the working fluids under spark ignition, homogeneous charge compression ignition, and spark assisted compression ignition conditions. Natural gas was used as fuel. Experimental results indicate that argon power cycle increases thermal efficiency, extends lean burn limit, suppresses oxides of nitrogen and unburned hydrocarbon emissions, compared to conventional air cycle. The maximum thermal efficiency of argon power cycle reaches 41.5% under low equivalence ratios and high compression ratio conditions. Given a fixed oxygen content, the thermal efficiency increases with decreasing equivalence ratio until the lean burn limit. With the decrease of oxygen content from 21% to 7.5%, the thermal efficiency of argon power cycle first increases and then drops with the peak occurring at ~15%. In practical applications, the engine load of argon power cycle can be controlled by changing equivalence ratio, oxygen content, or both. With argon power cycle, the use of spark assisted compression ignition can further increase the combustion stability, thermal efficiency, and decrease unburned hydrocarbon emission. Combined with an optimal spark timing, the engine cycle’s coefficient of variation of the indicated mean effective pressure decreases up to 84% and the thermal efficiency increases up to 2%, compared to homogeneous charge compression ignition.

Effects of miller cycle strategies on combustion characteristics and knock resistance in a spark assisted compression ignition (SACI) engine

Spark assisted compression ignition (SACI) is a proven strategy to extend the operation load range and control the combustion phase of homogeneous charge compression ignition (HCCI) with high thermal efficiency. However, the occurrence of abnormal combustion, such as engine knock, limits SACI to operate in a wider load range. In this paper, the effects of Miller cycle on knock suppression effect and combustion characteristics are experimentally studied using a single-cylinder gasoline engine. Two types of Miller cycle tests under different boundary conditions are conducted: one is Miller cycle with constant negative valve overlap (NVO), and the other is Miller cycle with variable NVO while maintaining intake charge. The results show that different Miller cycle strategies can be used to adjust the ratio of fuel consumed by flame propagation and auto-ignition, and it is effective to optimize the combustion process in SACI to suppress engine knock while maintaining high efficiency. The results show that thermal efficiency of the optimized condition increased by 2.75% compared to the baseline. Besides, split injection strategy is also studied and the results show that a proper second injection timing (180-100 CAD BTDC) could moderate engine knock.

Spark Assisted Compression Ignition Engine with Stratified Charge Combustion and Ozone Addition

<div class="section abstract"><div class="htmlview paragraph">Performance and emissions characteristics for stratified charge spark assisted compression ignition (SACI) with 30 ppm of added ozone (O<sub>3</sub>) were explored in a single-cylinder, optically accessible, spray-guided, research engine. For the present study, intake pressure and temperature were fixed at 1.0 bar and 42°C respectively, with a range of engine loads (1.5 – 5.5 bar indicated mean effective pressure) and speeds (800 – 1600 revolutions per minute) explored. Fuel stratification achieved by a late-cycle injection of ~ 10–25% of the total fuel was used to maintain stable operation at lower engine loads. For each condition spark timing, second injection SOI, and fuel split ratio between the main and second injection were optimized to maximize engine performance while maintaining nitrogen oxide emissions (NOx) below 5 g/kg-fuel.</div><div class="htmlview paragraph">Ozone addition was found to decrease specific fuel consumption by up to 9%, with across the board improvement in combustion stability relative to similar conditions without O<sub>3</sub>. The effect of O<sub>3</sub> addition was most substantial for the lowest loads. Moreover, because a higher fraction of the fuel burned was due to end-gas auto-ignition, specific NOx emissions likewise decreased by up to 30%. From complementary measurements of in-cylinder O<sub>3</sub> decomposition acquired via an ultraviolet light absorption diagnostic, it was observed that rapid decomposition of O<sub>3</sub> into molecular and atomic oxygen coincided with the onset of end-gas auto-ignition. The burst of resultant atomic oxygen was thought to accelerate low-temperature heat release (LTHR) reactions in the end gas. Optimal end-gas auto-ignition started between 20 and 30 crank angles before top dead center with temperatures at LTHR onset estimated to be between 575 and 700 K. An included analysis indicates that the spark deflagration was needed to add between 10 and 40 J of additional thermal energy to the end gas to achieve optimal auto-ignition.</div></div>

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.

Study on HCCI combustion improvement by using dual assisted compression ignition (DACI) on a hydraulic free piston engine fueled with methanol fuel

A dual assisted compression ignition combustion mode is researched on a hydraulic free piston engine. The dual assisted compression ignition combustion mode is realized by combining a glow plug with a spark plug in the ignition system to assist the homogeneous charge compression ignition combustion. The effects of control parameters and operating parameters on dual assisted compression ignition combustion mode are revealed by experiments. It is found that as the compression ratio increases, the combustion characteristics of dual assisted compression ignition change from spark ignition to two-stage heat release. The dual assisted compression ignition combustion has more controllability on the combustion phase and the heat release rate than glow plug assisted compression ignition due to the direct and stable spark ignition. When the temperature of the glow plug is lower, the heat release phase is dominated by the spark plug. With the temperature further increases, the heat release process is mainly controlled by the glow plug. The dual assisted compression ignition combustion has a wider compression ratio range than spark assisted compression ignition to guarantee the high indicated thermal efficiency. The suitable methanol/air equivalence ratio range for dual assisted compression ignition combustion mode is between 0.6 and 0.8.

Control and optimization of spark ignition–controlled auto-ignition hybrid combustion based on stratified flame ignition

Spark ignition–controlled auto-ignition hybrid combustion, also known as spark assisted compression ignition, is of considerable interest in gasoline engines because of its potential to enlarge the operating range of gasoline diluted combustion. However, it was found that the spark ignition–controlled auto-ignition hybrid combustion process was often characterized with large cycle-to-cycle variations. In this research, a new approach by combining the traditional second-order derivative method and Wiebe function fitting method was proposed to identify different heat release stages of spark ignition–controlled auto-ignition hybrid combustion. The heat release characteristics of the spark ignition–controlled auto-ignition hybrid combustion based on stratified flame ignition strategy and its control methods were investigated in detail. The effect of control parameters, including spark timing, direct injection ratio and dilution strategy, on improving the thermal efficiency and decreasing the variation of heat release trace in spark ignition–controlled auto-ignition hybrid combustion based on stratified flame ignition strategy was analysed. The advance of flame propagation ending point and the increase in the average heat release rate in flame propagation stage benefitted the fuel economy and reduced the variations of heat release in spark ignition–controlled auto-ignition hybrid combustion. Although the increase in direct injection ratio contributed to the stability of heat release in the spark ignition–controlled auto-ignition hybrid combustion based on the stratified flame ignition strategy, the thermal efficiency of spark ignition–controlled auto-ignition hybrid combustion cannot be effectively optimized due to the decrease in combustion efficiency. The application of exhaust gas recirculation and air dilution could decrease the variations of heat release process and increase the thermal efficiency of spark ignition–controlled auto-ignition hybrid combustion based on stratified flame ignition strategy.

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.

Knock characteristics and combustion regime diagrams of multiple combustion modes based on experimental investigations

Spark assisted compression ignition (SACI) is a kind of combustion mode that bridges the gap between homogeneous charge compression ignition (HCCI) under low load and spark ignition (SI) under high load. In a multi-mode combustion engine, different combustion modes are used under different engine loads and the transition between combustion modes is necessary. To realize transitions among the three combustion modes, this work studies combustion and knock characteristics of HCCI, SACI and SI, and proposes region diagrams that divide the operation regions of different combustion modes. The results show that knocking combustion of HCCI and SI is totally different, and SACI knock combines the features of both HCCI knock and SI knock. Further analysis indicates that compression ignition (CI) of SACI relies on both initial thermodynamic state and flame propagation. And the more auto-ignition is dependent on flame propagation, the more uncontrollable CI process becomes. Region diagrams show that HCCI works with high i-EGR ratios, and an HCCI knock region exists with moderate i-EGR ratios. Transition from normal SACI to HCCI knock can be completed even in two cycles. Therefore, air-fuel mixture and i-EGR should be carefully organized to avoid this region. In the region diagram of SACI-SI, there is an instable transition region between SACI and SI, which is caused by instability of flame propagation due to such amount of EGR and the reliability of CI process on flame propagation. Fuel stratification is helpful to suppress knock, advance combustion phase by earlier spark timing, and improve combustion stability. But it cannot eliminate the unstable region. Twin spark adds another flame kernel into combustion chamber and greatly improves combustion stability. With this strategy, stable transition between SACI and SI modes can be achieved.

Investigation of performance and combustion characteristics of a four-valve supercharged two-stroke DI engine fuelled with gasoline and ethanol

Worldwide emission legislations have promoted the development of more efficient internal combustion engines through engine downsizing and advanced lean-burn combustion concepts. However, these measures if solely employed in the next generation vehicles are still not enough to comply with future CO2 limits, so powertrain hybridisation has been already set up. In this circumstance, internal combustion engines play a less important role in the vehicle’s propulsion system serving mostly as a range extender or extra power supply unit. Power density (kW/dm3) and power per unit mass (kW/kg) are as important as fuel consumption, so not only four-stroke engines may serve to this purpose but also two-stroke engines. Therefore, the present research evaluates the performance and combustion characteristics of a two-stroke cycle engine embedded in the architecture of a contemporary four-stroke supercharged engine, with direct fuel injection, intake and exhaust valves, and a wet sump. Commercial gasoline and anhydrous ethanol (E100) were tested at loads from 0.2 MPa to 1.0 MPa indicated mean effective pressure and speeds varying from 800 rpm to 2400 rpm. The results demonstrated that ethanol increased the overall indicated efficiency by about 10% compared to gasoline, while improving the burning process at very light loads thanks to its higher tolerance to diluted combustion. At higher engine loads the calculated supercharger power consumption largely increased, which reduced the corrected indicated efficiency for both fuels tested. The maximum in-cylinder pressure of 6.5 MPa was obtained using ethanol at 160 Nm/dm3 of torque and 800 rpm. The short time available for air-fuel mixing at high loads and excessive combustion dilution at low loads deteriorated the combustion process. The lowest combustion efficiency of 0.80 was obtained at transitioning regions from spark ignition to spark assisted compression ignition combustion with gasoline. The 50% of mass fraction burned was found closer to top dead centre at several operating conditions due to a largely diluted combustion. Finally, the exhaust gas temperature presented a peculiar behaviour due to a competition between combustion rate and exhaust gas dilution by the intake air. This resulted in an exhaust gas temperature as low as 500 K at full load and 800 rpm, while the maximum gas temperature of 740 K was observed at mid-loads.

Numerical studies of the ignition characteristics of a high-pressure gas jet in compression ignition engines with glow plug ignition assist: Part 2-Effects of multi-opening glow plug shields

The results of a previous study, part 1, showed that use of a shield can improve the thermal performance of a glow plug, and thereby reduce ignition time. However, the part 1 study also found that use of a simple shield with only one circular opening can delay flame propagation out of the shield. The conclusions of that study suggested that there is scope for further improvements of the shield design, especially the shield opening geometry. Accordingly, this article presents the results of computational studies investigating the influence of multi-opening shield designs on natural gas ignition characteristics in glow plug–assisted compression–ignition engines. Two types of multi-opening glow plug shield, consisting of four small circular openings distributed in either diamond-pattern or square-pattern arrangements, were employed. The simulated results demonstrated that both multi-opening shields can not only increase glow plug surface temperature, but also increase the residence time of fuel mixture adjacent to the glow plug surface in the early injection stage, resulting in a faster ignition than the single-opening shield. Furthermore, the diamond-pattern multi-opening glow plug shield provides a faster or comparable flame propagation path back to combustion chamber, compared to single-opening glow plug shield, while the square-pattern multi-opening glow plug shield delays the flame propagation under several specific engine conditions. Compared to the single-opening glow plug shield, the overall natural gas ignition delays are further reduced by 6%–44% when using the diamond-pattern multi-opening glow plug shield, while the square-pattern multi-opening glow plug shield is only able to reduce the natural gas ignition delay under a few specific conditions.

Study of Assisted Compression Ignition in a Direct Injected Natural Gas Engine

Natural gas direct injection (DI) and glow plug ignition assist technologies were implemented in a single-cylinder, compression-ignition optical research engine. Initial experiments studied the effects of injector and glow plug shield geometry on ignition quality. Injector and shield geometric effects were found to be significant, with only two of 20 tested geometric combinations resulting in reproducible ignition. Of the two successful combinations, the combination with 0 deg injector angle and 60 deg shield angle was found to result in shorter ignition delay and was selected for further testing. Further experiments explored the effects of the overall equivalence ratio (controlled by injection duration) and intake pressure on ignition delay and combustion performance. Ignition delay was measured to be in the range of 1.6–2.0 ms. Equivalence ratio was found to have little to no effect on the ignition delay. Higher intake pressure was shown to increase ignition delay due to the effect of swirl momentum on fuel jet development, air entrainment, and jet deflection away from optimal contact with the glow plug ignition source. Analysis of combustion was carried out by examination of the rate of heat release (ROHR) profiles. ROHR profiles were consistent with two distinct modes of combustion: premixed mode at all test conditions, and a mixing-controlled mode that only appeared at higher equivalence ratios following premixed combustion.