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Gasoline compression ignition combustion has demonstrated the potential of getting high fuel efficiency. In this work, two engine models are established in the commercial software Converge 2.3 based on the experimental results of an optical engine and a modified metal engine. The effect mechanism of mixture stratification and the influence of compound control strategy on gasoline compression ignition (GCI) combustion and soot emissions under high load condition are investigated. Results show that synergistic effect of physics and chemistry is the dominant control mechanism of GCI combustion. Stronger mixture stratification can effectively reduce the maximum pressure rise rate (MPRR) and improve the indicated thermal efficiency (ITE) and emissions; applying high EGR will significantly reduce OH radical in the cylinder and subsequently weaken the soot oxidation process, resulting in high soot emission; under the premise of reasonable NOx emission and MPRR, the soot emission can be effectively reduced with a proper advanced main injection timing. The effect of temperature on the soot oxidation process is the primary mechanism for the ultimate soot emission at different main injection timings; properly increasing the interval between the pre‐ and main injection can reduce NOx and soot emissions under the premise of ensuring that the MPRR is within the upper limit. However, the effect is not significant because of the small proportion of preinjection fuel.

In this paper, high-speed imaging and spectrometry diagnostics were used to study the spray flame structures and emission spectra of polyoxymethylene dimethyl ethers (PODE) in an optical constant volume vessel. The effects of oxygen volume fraction (15% to 80%) on some typical combustion characteristics such as spatially integrated natural intensity, flame propagation speed, lift-off length and the distribution of flame emission spectra were investigated. The results show that the PODE spray flame mainly exhibits blue chemiluminescence, which is different from the diesel yellow diffusion flame dominated by soot radiation. As the oxygen concentration increases, the flame natural luminosity and propagation speed increases, the position of lift-off region and flame tip move towards the injector nozzle, and the flame width becomes narrower. According to the spectral results, the blue chemiluminescence generated by carbon monoxide oxidation dominates the PODE flame luminescence, suggesting that no soot is formed. The increase in oxygen concentration leads to enlarged intensity and gradient of PODE flame radiation. In summary, the combination of PODE and oxygen-enriched combustion shows no soot formation and promotes the oxidation of carbon monoxide and unburned hydrocarbon. This study can provide more insights into PODE spray combustion, and offer guidelines for achieving efficient and clean combustion.

Octane sensitivity (OS), as one of the fuel anti-knock quality indexes, is critical with respect to the effective design of next-generation spark ignition (SI) engines. This simulation study focuses on the effects of OS on knock behavior as a function of spark timing (ST) and compression ratio (CR) under boosted high load condition. Eight fuels with identical Research Octane Number (RON) and varying OS were selected to specify the relationship between OS and fuel-specific properties, including a primary reference fuel (PRF), Ethanol Reference Fuels (ERFs), Toluene Reference Fuels (TRFs). It was found that increasing OS to decrease end-gas reactivity is conditional. The end-gas reactivity becomes less sensitive to OS with advancing ST. Analysis for the relationship between OS and fuel-specific properties illustrate that fuel-specific variations beyond OS play an important role in knock tendency when increasing CR, where OS value is insufficient to describe the fuel anti-knock performance. ERF yields better knock resistance than the corresponding OS TRF at high CR conditions. The cause for this behavior is that the decrease in the autoignition temperature moves the end-gas of high OS fuel into a long-ignition-delay region. Comparable chemical and charge cooling effects are effective to retard auto-ignition more dramatically for ERFs.

Low-temperature combustion (LTC) is a new combustion mode, and low cetane number fuels may be more appropriate for the LTC. Fuel properties play important roles both in the physical process...

The operation range of some new compression ignition (CI) combustion modes was extended compared to that of HCCI because of fuel stratification. Few researches tried to analyze the mechanism by comparison of flame development patterns and flame speed under different stratified conditions. In this work, high-speed imaging of natural flame luminosity was used to study the combustion process from homogeneous charge to stratified charge with a higher frame rate. Different stratification conditions were formed by adjusting the injection timings. Results show that the proportion of flame propagation increases and combustion reaction rate decreases as fuel distribution in cylinder changes from homogeneous charge to stratified charge. Flame propagation of auto-ignition kernel exists although the combustion is dominated by multipoint auto-ignition in HCCI combustion. The flame spreading speed is much higher than the flame speed because the fictitious reaction front is shorter than the actual reaction front for flame spreading speed calculation. Four principles are proposed for equivalent radius method to get more reasonable results of flame speed. For conditions that do not satisfy these principles, effective front method can be used. Flame speed in CI combustion modes is in the range of 10–50 m/s depending on different stratification conditions in current study. There is a good correlation between the flame speed and the peak heat release rate as SOI is retarded, i.e., high flame speed corresponds to high peak heat release rate. It can be concluded that controlling fuel stratification is an effective method to regulate the ratio between auto-ignition and flame propagation and achieve effective control on combustion reaction rate.

The effects of PODE and multiple fuel injection strategy on combustion and emission characteristics of a multiple-cylinder gasoline compression ignition (GCI) engine are investigated at high load (BMEP = 16 bar) operation condition with a fixed engine speed of 1660 r/min. Experimental results indicate that the sensitivity of soot emission to the variation of injection parameters is decreased by blending PODE with gasoline, mainly due to the enhanced soot oxidation through PODE’s high oxygen content. The main heat release of gasoline is more strongly affected by its pilot heat release compared to that of gasoline/PODE blends. Low PRRmax (maximum pressure rise rate) of about 4.5 bar/°CA can be obtained for gasoline with multiple-injection strategy, under which condition the advantage of gasoline/PODE blends only embodies in reducing soot emission, which is different from that with single injection strategy. The PHRRMI,max (maximum premixed heat release rate of main injection) and Kmain (heat release acceleration ratio of main injection) of gasoline show more sensitivity to the variation of EGR compared to that of PODE20 with pilot-main injection strategy, and the PHRRMI,max and PRRmax of gasoline increase more rapidly in contrast to that of gasoline/PODE blends as injection pressure increases. At higher injection pressure, the soot of all the fuels can be significantly decreased with penalty in PRRmax, especially for gasoline. It is seen that when PODE20 with triple-injection strategy, 1400 bar injection pressure and 30% EGR were employed under the operation condition with the engine speed of 1660 r/min and BMEP of 16 bar, the NOx of 1.3 g/kWh, soot of 0.007 g/kWh and BSFC of 199.67 g/kWh can be obtained while maintaining PRRmax of about 4.5 bar/°CA.

The ignition delay prediction model of three-component surrogates was established based on the back propagation (BP) neural network. The ambient temperature, ambient pressure, molar fraction of n-heptane, iso-octane and toluene were utilized as the input parameters. The ignition delay was utilized as the output parameter. The training and validation set only contained the 0-D simulation ignition delay of single- and two-component surrogates. But the trained BP neural network also showed a strong predictive ability towards the ignition delay of three component toluene primary reference fuel (TPRF) surrogates. Results show that the BP neural network with two hidden layers performs better than that with single hidden layer. With the optimization of genetic algorithm (GA) and particle swarm optimization (PSO) algorithm, the correlation coefficient is higher than 0.9996. The mean relative error (MRE) and the mean square error (MSE) are also maintained at a low level. The computational costs of 0-D simulation and BP neural network methods are compared. In 0-D simulation, the computational time of one case is 28 s. When the BP neural network is utilized, the computational time of 176 cases is just 3.2 s, which shows a significant improvement in the computation time. Through the mean impact value (MIV) analysis, the significance of each input variable to the output results is investigated. The input parameters of ambient temperature and molar fraction of iso-octane obtain the highest and lowest absolute value of MIV, respectively, indicating the major and minor effects on the ignition delay. Based on the predicted ignition delay of three-component TPRF surrogates, the research octane number (RON) and motor octane number (MON) can also be accurately predicted with the maximum deviance no more than 3 units. For real fuels of fuels for advanced combustion engines (FACE) gasolines, such as FACE A and FACE C gasolines, the surrogate fuel which has the same ignition delay at the specific pressure and temperature can be determined through the construction of the ignition delay look-up table of TPRF surrogates by the BP neural network. Following this method, the ignition delay of the real fuels can be matched accurately and the molar fraction of each component of the corresponding TPRF surrogates can also be acquired.

Based on the experimental ignition delay results of n-butane/hydrogen mixtures in a rapid compression machine, a Genetic Algorithm (GA) optimized Back Propagation (BP) neural network model is originally developed for ignition delay prediction. In the BP model, the activation function, learning rate and the neurons number in the hidden layer are optimized, respectively. The prediction ability of the BP model is validated in wide operating ranges, i.e., compression pressures from 20 to 25 bar, compression temperatures from 722 to 987 K, equivalence ratios from 0.5 to 1.5 and molar ratios of hydrogen ( X H 2 ) from 0 to 75%. Compared with the BP model, the GA optimized BP model could increase the average correlation coefficient from 0.9745 to 0.9890, in the opposite, the average Mean Square Error (MSE) decreased from 2.21 to 1.06. On the other hand, to assess the BP-GA model prediction ability in the never-seen-before cases, a limited BP-GA model is fostered in the X H 2 range from 0 to 50% to predict the ignition delays at the cases of X H 2 =75%. It is found that the predicted ignition delays are underestimated due to the training dataset lacking of “acceleration feature” that happened at X H 2 =75%. However, three possible options are reported to improve the prediction accuracy in such never-seen-before cases.

Both concentration stratification and reactivity stratification are beneficial for reducing the combustion rate and controlling the combustion phasing in HCCI (homogeneous charge compression ignition) operation. However, the different effects on combustion and emissions of concentration and reactivity stratification have not been fully explored and the interaction between the two stratification modes also needs to be clarified. Therefore, the effects of concentration stratification and reactivity stratification on combustion and emission characteristics were investigated in a single-cylinder diesel engine. The different concentration stratifications were designed by five port injection ratios to study the influence of concentration stratification. Cases with and without reactivity stratification were compared at the same overall reactivity and the same concentration stratification in the cylinder to roughly separate the impact of reactivity stratification. Results show that with the decrease of the premixed ratio, the peak heat release rate decreases, combustion phasing delays, and the thermal efficiency show the tendency of first decreasing and then increasing. For cases with same concentration stratification, the introduction of iso-octane/n-heptane reactivity stratification decreases combustion efficiency but increases indicated thermal efficiency, NOx (nitrogen oxides) emissions can be improved while CO (carbon monoxide) and HC (hydrocarbon) emissions are increased. The introduction of n-heptane/iso-octane reactivity stratification presents a positive effect on both thermal efficiency and combustion efficiency and contributes to the improvement in NOx, CO and HC emissions.

In recent years, thermal barrier coating (TBC) has been used as an effective way to reduce the heat transfer losses and to improve the thermal efficiency of internal combustion (IC) engine. In this study, a mathematic model has been proposed by taking the TBC material parameters into account, which is applied to explore the effects of TBC on engine combustion and emissions. Details of the combustion process is analyzed for the coated and uncoated engines under both low and high load conditions. The result shows that TBC has the ability of reducing wall heat transfer losses, thus improving the indicated thermal efficiency. A redistribution of different temperature regions is found with TBC and the coating shows better performance at rich mixture region due to higher temperature increase rate and at squish region due to higher surface/volume ratio. Both the soot oxidation process and NOx formation process are accelerated with TBC. However, TBC also enhances the overlap region between the high soot region and high temperature region, which accelerates the soot oxidation rate and greatly improves the soot emission.

Fuel properties play important roles both in the physical process of fuel air mixing and chemical process of combustion in the cylinder of diesel engines. There are many parameters to represent physical and chemical properties of a diesel fuel, which may affect combustion of diesel engine to different degrees. It is valuable to deeply understand the effects of fuel properties and the relations between some major properties as well. Therefore, the effects of diesel fuel properties on combustion and emissions have been experimentally investigated on a heavy-duty diesel engine in this study. In addition, the relationships among some major properties have been discussed. Twelve fuels with different fuel properties, which were produced by different refining processes from different refineries in China, were selected to ensure that the tested fuels have a wide representative. The results show that there is a strong correlation between fuel density and other fuel properties such as cetane number, aromatic hydrocarbon fraction, heat value, etc. Especially, the linear regression model between density and cetane number shows a certain reference significance. The cetane number of fuel with high density is low, which results in the delay of combustion and the rise of peak heat release rate at low load. There is no significant difference in brake thermal efficiency (BTE) for fuels with different fuel properties in the current study. However, the brake specific fuel consumption (BSFC) increases with the increase of fuel density because of the decrease in heat value. As the fuel density increases, the NOx emissions increase accordingly and the soot emissions roughly show an increasing trend as well. At low load, both CO and HC emissions obviously increase with the increase of fuel density. For example, at low load and low speed, CO and HC increase by 256%, 158% respectively as the fuel density increases from 800 kg/m3 to 920 kg/m3. Nonetheless, CO emissions remain a low level and show no obvious changes at medium and high loads, which is different from HC emissions that increase gradually especially for high speed conditions. The results of the European Steady-state Cycle (ESC) show that weighted BSFC and weighted emissions exhibit strong correlations with fuel density. When fuel density is bigger than a certain value (about 845 kg/m3), a rapid increasing trend is presented in the emissions of NOx, soot, CO and HC.

Oxygenated biofuels have become one of the research focuses of engines due to their renewability and improvement in combustion. There has been some studies concentrating on the dual fuel RCCI mode or blended combustion mode using biofuels in engines and valuable progress has been obtained. However, the comparison between biofuel RCCI and blended combustion modes has been rarely reported. Therefore, in current work, experimental study was conducted on a single-cylinder engine to investigate the differences between the two combustion modes fueled with biodiesel/n-butanol at different EGR rates (0%, 30%, 50%), n-butanol ratios (20%, 50%, 80%), injection timings and engine loads (low, medium, high). Results show that the ignition delay of blended mode is longer than that of RCCI mode, and more sensitive to n-butanol ratio and EGR rate. The optimum EGR rate is 30% considering efficiency and emissions for both combustion modes. Blended fuel mode can maintain high efficiency at all test loads and n-butanol ratios, the maximum indicated thermal efficiency (ITE) is up to 47.5%, while RCCI only shows comparable efficiency at high load. The problem of high maximum pressure rise rate (MPRR) that blended fuel mode faces can be addressed by retarded combustion phasing. Under current research conditions, blended fuel mode usually presents lower soot, HC, CO emissions and slight higher NOx compared with RCCI mode. Generally, blended mode has better performance when MPRR problem is addressed while RCCI mode shows the potential in load extension due to the low MPRR and flexible split ratio and injection timing.

The present study focuses on the experimental investigation on the optimal pilot injection strategy under GCI combustion mode in a multi-cylinder heavy-duty diesel engine. Three experiments were conducted at a high-speed high-load operating point with different operating parameters and emission targets, namely engine-out NOx target, fuel injection pressure, and main injection timing. The engine-out NOx targets were set to 5.0 g/kWh and 1.5 g/kWh, and the gasoline injection pressures were set to 100 MPa and 140 MPa. These high and low values represent different requirements of SCR efficiency and practical capability of fuel supply system when addressing the hypothetical future tailpipe NOx limit of 0.02 g/hp-hr (0.027 g/kWh). The results show that the use of optimized pilot injection always achieves lower pressure rise rate and soot emissions than the single injection baseline. The pilot gasoline fuel with low injection pressure is more ignitable than that with high injection pressures, hence a distinct heat release spike usually occurs for a pilot injection. The optimal pilot mass should be increased for a higher fuel injection pressure because the pilot fuel stratification level decreases. A relatively late pilot timing is preferable for the early main injection timing. For the late main injection timing, however, a relatively early pilot timing with large pilot mass is preferable and brings about a distinct two stage high temperature heat release, which can reduce the fuel consumption and soot emissions simultaneously. The two stage split combustion process obtained by double injection with retard combustion phasing can be considered to be an important way to alleviate the requirement of GCI fuel system.

To investigate the effect of biofuel properties on RCCI combustion and emissions, the experimental study was conducted on a single-cylinder diesel engine. Three low reactivity oxygenated biofuels, i.e. n-butanol, 2,5-dimethylfuran (DMF) and ethanol were injected in the intake port and biodiesel was directly injected into the cylinder to realize RCCI operation. Results show that the heat releases are changed from two-stage to single-peak with the increase of EGR rates. Biodiesel/ethanol presents 1 °CA longer ignition delay than the others, which indicates latent heat has significant effect on ignition delay. Under different injection timings, the trends of combustion and emissions are similar for three RCCI modes, meantime, biodiesel/ethanol shows greater potential on reducing NOx and soot emissions simultaneously (soot

Gasoline compression ignition (GCI) is a promising combustion concept with high thermal efficiency, low emissions, and minimal modification of standard engine hardware. With a relaxed constraint on the engine-out NOx emissions, different GCI operating parameters such as exhaust gas recirculation (EGR), injection timing, injection pressure, pilot-main injection interval, and pilot mass were swept to find their optimal calibrations. The entire operating map of a heavy duty diesel engine using GCI combustion with multi-injection strategies was also investigated. Results show that the use of pilot injection is effective in controlling the premixing heat release rate, reducing the combustion noise and emissions, and improving controllability, and allows for advancing combustion timing within the imposed mechanical constrains. With the engine-out NOx calibration of around 4.5 g/kW-h for typical Euro 6 compliant engines, the double injection strategy is applied over the entire operating map in GCI mode, and similar engine performance and emissions can be achieved by GCI combustion compared to conventional diesel combustion (CDC) mode, just using lower injection pressures. The peak brake thermal efficiency (BTE) of 44% over the entire operating map is demonstrated with minimal pumping and friction losses while keeping the peak cylinder pressure (PCP) within 16 MPa.