缸内直接喷射式汽油机点火稳定性的数值分析

缸内直接喷射式汽油机的一个显著特点是依靠火花塞点燃喷入缸内的汽油油束。由于缸内混合气浓度极不均匀，所以其点火及火焰传播过程与普通均质燃烧式发动机有很大的不同。火焰核心的稳定形成及初始火焰发展对缸内的整个燃烧过程有极其重要的影响。本文利用二维两相混合模型模拟喷雾过程，利用一个详细的准维模型模拟火花塞的点火过程，并采用特殊处理方法使两个子模型相匹配，计算了缸内直接喷射式汽油机从喷雾到形成稳定火核的全过程，分析了多种因素对点火稳定性的影响，尤其是对涡流比、点火时刻和喷油定时之间的适当配合进行了模拟分析。计算结果对优化实验有明显的指导作用。     

Research on the inducing factors and characteristics of knock combustion in a DI hydrogen internal combustion engine in the process of improving performance and thermal efficiency

Hydrogen energy is gaining greater attention because of the energy crisis and CO2 emissions, and knock combustion has become the main obstacle to improving thermal efficiency and power performance of hydrogen engines, which is an important method of hydrogen energy application. In this paper, the knock characteristic parameters and the factors affecting knock tendency of a 2.0 L DI hydrogen engine are investigated experimentally. The results reveal the variation in knock intensity is not linear with the retarding of SOI, which is related to the cylinder mixture distribution. Furthermore, methods such as increasing injection pressures can be useful in reducing the knock intensity. Equivalence ratio has a greater impact on knock compared with other parameters. The conclusions can be used in the further exploration of the knock combustion mechanism in DI hydrogen engines.     

Effects of hydrogen injection strategy on the hydrogen mixture distribution and combustion of a gasoline/hydrogen SI engine under lean burn condition

Hydrogen is a carbon free energy carrier with high diffusivity and reactivity, it has been proved to be a kind of suitable blending fuel of spark ignition (SI) engine to achieve better efficiency and emissions. Hydrogen injection strategy affects the engine performance obviously. To optimize the combustion and emissions, a comparative study on the effects of the hydrogen injection strategy on the hydrogen mixture distribution, combustion and emission was investigated at a SI engine with gasoline intake port injection and four hydrogen injection strategies, hydrogen direct injection (HDI) with stratified hydrogen mixture distribution (SHMD), hydrogen intake port injection with premixed hydrogen mixture distribution (PHMD), split hydrogen direct injection (SHDI) with partially premixed hydrogen mixture distribution (PPHMD) and no hydrogen addition. Results showed that different hydrogen injection strategy formed different kinds of hydrogen mixture distribution (HMD). The ignition and combustion rate played an important role on engine efficiency. Since the SHDI could use two hydrogen injection to organize the HMD, the ignition and combustion rate with the PPHMD was the fastest. With the PPHMD, the brake thermal efficiency of the engine was the highest and the emissions were slight more than that with the PHMD. PHMD achieve the optimum emission performance by its homogeneous hydrogen. The engine combustion and emission performance can be optimized by adjusting the hydrogen injection strategy.     

Numerical study on effects of hydrogen direct injection on hydrogen mixture distribution,combustion and emissions of a gasoline/hydrogen SI engine under lean burn condition

A numerical study on effects of hydrogen direct injection on hydrogen mixture distribution, combustion and emissions was presented for a gasoline/hydrogen SI engine. Under lean burn conditions, five different direct hydrogen injection timings were applied at low speeds and low loads on SI engines with direct hydrogen injection (HDI) and gasoline port injection. The results were showed as following: firstly, with the increase of hydrogen direct injection timing, the hydrogen concentration near the sparking plug first increases and then decreases, reaching the highest when hydrogen direct injection timing is 120°CA BTDC: Secondly, hydrogen can speed up the combustion rate. The main factor affecting the combustion rate and efficiency is the hydrogen concentration near the sparking plug: Thirdly, in comparing with gasoline, the NO_X emissions with hydrogen addition increase by an average of 115%. For different hydrogen direct injection timings, the NO_X emissions of 120°CA BTDC is the highest, which is 29.9% higher than the 75°CA BTDC. The hydrogen addition make the NO_X emissions increase in two ways. On the one hand, the average temperature with hydrogen addition is higher. On the other hand, the temperature with hydrogen addition is not homogeneous, which makes the peak of temperature much higher. In a word, the main factor of NO_X emissions is the size of high temperature zone in the cylinder: Finally, because the combustion is more complete, in comparing with gasoline, hydrogen addition can reduce the CO and HC emissions by 32.2% and 80.4% respectively. Since a more homogeneous hydrogen mixture distribution can influence a lager zone in the cylinder and reduce the wall quenching distance, these emissions decrease with the increase of hydrogen direct injection timing. The CO and HC emissions of 135°CA BTDC decrease by 41.5% and 71.4%, respectively, compared to 75°CA BTDC.     

Hydrogen engine: research and development (R&D) programmes in Indian Institute of Technology (IIT), Delhi

Active research in the development of hydrogen-fuelled low-emission engines is being pursued at the Engines and Unconventional Fuels Laboratory of the Indian Institute of Technology (IIT), for a period of close to two decades. This paper highlights the significant pursuits and attainments of the research and development (R&D) activities carried out in IIT, Delhi on hydrogen-operated engines. Both spark ignition (SI) and compression ignition engine test rigs have been developed and instrumented for the use of hydrogen fuel. Several existing petroleum-fuelled engine configurations have been modified by taking care to observe that the converted system does not need substantial hardware modifications. Various fuel induction techniques have been experimentally evaluated keeping in view the temperamental combustion characteristic of this fuel. Curative and preventive steps have been adopted and suitable retrofits and subsystems have been installed at the appropriate locations to preclude the possibility of any undesirable combustion phenomena such as backfire, knocking and rapid rate of pressure rise. Performance, emission and combustion characteristics of the systems have been determined. It has been observed that an appropriately designed timed manifold injection system can overcome the problem of backfire in a hydrogen engine. NO_x emission level from a hydrogen-operated SI engine can be drastically reduced by way of lean engine operation.     

Development of a pneumatic actuated low-pressure direct injection gas injector for hydrogen-fueled internal combustion engines

Mixture formation is one of the greatest challenges for the development of robust and efficient hydrogen-fueled internal combustion engines. In many reviews and research papers, authors pointed out that direct injection (DI) has noteworthy advantages over a port fuel injection (PFI), such as higher power output, higher efficiency, the possibility of mixture stratification to control NO_x-formation and reduce heat losses and above all to mitigate combustion abnormalities such as back-firing and pre-ignitions. When considering pressurized gas tanks for on-vehicle hydrogen storage, a low-pressure (LP) injection system is advantageous since the tank capacity can be better exploited accordingly. The low gas density upstream of the injector requires cross-sectional areas far larger than any other injectors for direct injection in today's gasoline or diesel engines. The injector design proposed in this work consists of a flat valve seat to enable the achievement of lifetime requirements in heavy-duty applications. The gas supply pressure is used as the energy source for the actuation of the valve plate by means of a pneumatic actuator. This article describes the design and the performed tests carried out to prove the concept readiness of the new LP-DI-injector.     

Engine speed and air-fuel ratio effect on the combustion of methane augmented hydrogen rich syngas in DI SI engine

The main challenge on the fueling of pure hydrogen in the automotive vehicles is the limitation in the hydrogen separation from the product of steam reforming and gasification plants and the storage issues. On the other hand, hydrogen fueling in automotive engines has resulted in uncontrolled combustion. These are some of the factors which motivated for the fueling of raw syngas instead of further chemical or physical processes. However, fueling of syngas alone in the combustion chamber has resulted in decreased power output and increased in brake specific fuel consumption. Methane augmented hydrogen rich syngas was investigated experimentally to observe the behavior of the combustion with the variation of the fuel-air mixture and engine speed of a direct-injection spark-ignition (DI SI) engine. The molar ratio of the high hydrogen syngas is 50% H₂ and 50% CO composition. The amount of methane used for augmentation was 20% (V/V). The compression ratio of 14:1 gas engine operating at full throttle position (the throttle is fully opened) with the start of the injection selected to simulate the partial DI (180° before top dead center (BTDC)). The relative air-fuel ratio (λ) was set at lean mixture condition and the engine speed ranging from 1500 to 2400 revolutions per minute (rpm) with an interval of 300 rpm. The result indicated that coefficient of variation of the indicate mean effective pressure (COV of IMEP) was observed to increase with an increase with λ in all speeds. The durations of the flame development and rapid burning stages of the combustion has increased with an increase in λ. Besides, all the combustion durations are shown to be more sensitive to λ at the lowest speed as compared to the two engine speeds.     

Optimization of injection system for a medium-speed four-stroke spark-ignition marine hydrogen engine

In order to avoid the abnormal combustion in high-power hydrogen engine, a 3D CFD numerical model of a direct-injection spark-ignition hydrogen engine was built up based on a large-bore medium-speed four-stroke marine diesel engine using CONVERGE software. To obtain the influence of injection parameters on mixture homogeneity, a dimension reduction optimization method was proposed. The results revealed that the turbulence intensity and the penetration distance varied with the injection parameters, determined the level of mixture homogeneity. The performance comparison between the hydrogen engine and prototype diesel engine showed a great potential of hydrogen in internal combustion (IC) engines.     

天然气直喷燃烧的可视化研究

利用高速摄影装置开展了天然气直喷燃烧的可视化研究。研究结果表明:缸内形成的分层充量分布与喷射方式有关,双点平行喷射和单点喷射将比双点对向喷射形成的充量分层强。双点平行喷射和单点喷射时火焰将向燃料喷流的下流区发展,而双点对向喷射时火焰显示出由燃烧室中心向外围方向发展的特征。天然气直喷燃烧呈现出湍流燃烧特征的皱褶火焰前锋面。通过优化喷射正时形成的可燃分层充量可实现直喷天然气超稀燃烧。     

Effect of pre-chamber geometrical parameters and operating conditions on the combustion characteristics of the hydrogen-air mixtures in a pre-chamber spark ignition system

The pre-chamber spark ignition system is a promising advanced ignition system adopted for lean burn spark ignition engines as it enables stable combustion and enhances engine efficiency. The performance of the PCSI system is governed by the turbulent flame jet ejected from the pre-chamber, which is influenced by the pre-chamber geometrical parameters and the operating conditions. Hence, the current study aims to understand the effects of pre-chamber volume, nozzle hole diameter, equivalence ratio, and initial chamber pressure on the combustion and flame jet characteristics of hydrogen-air mixture in a passive PCSI system. Pre-chamber with different nozzle hole diameters (1 mm, 2 mm, 3 mm, and 4 mm) and volumes (2%, 4%, and 6% of the engine clearance volume) were selected and manufactured in-house. The experimental investigation of these pre-chamber configurations was carried out in a constant-volume combustion chamber with optical access. The flame development process was captured using a high-speed camera at a rate of 20000 fps, and the images were processed in MATLAB to obtain quantitative data. The combustion characteristics of hydrogen-air mixtures with the PCSI system improved when compared to the conventional SI system; however, the improvement was more significant for ultra-lean mixtures. Early start of combustion and shorter combustion duration were observed for PCSI – D2 and PCSI – D3 configurations, respectively and improved combustion and flame jet characteristics were also noted for these configurations. With the increase in pre-chamber volume, ignition energy associated with the flame jet increases, which reduces the combustion duration and the ignition lag.     

Numerical study on mixture formation characteristics in a direct-injection hydrogen engine

Numerical modeling of direct hydrogen injection and in-cylinder mixture formation is performed in this paper. Numerical studies on direct-injection hydrogen engines are very limited due mainly to the complexity in modeling the physical phenomena associated with the high-velocity gas jet. The high injection pressure will result in a choked flow and develop an underexpanded jet at the nozzle exit, which consists of oblique and normal shock waves. A robust numerical model and a very fine computational mesh are required to model these phenomena. However, a very fine mesh may not be feasible in the practical engine application. Therefore, in this study a gas jet injection model is implemented into a multidimensional engine simulation code to simulate the hydrogen injection process, starting from the downstream of the nozzle. The fuel jet is modeled on a coarse mesh using an adaptive mesh refinement algorithm in order to accurately capture the gas jet structure. The model is validated using experimental and theoretical results on the penetrations of single and multiple jets. The model is able to successfully predict the gas jet penetration and structure using a coarse mesh with reasonable computer time. The model is further applied to simulate a direct-injection hydrogen engine to study the effects of injection parameters on the in-cylinder mixture characteristics. The effects of the start of fuel injection, orientation of the jets, and the injector location on the mixture quality are determined. Results show that the hydrogen jets impinge on the walls soon after injection due to the high velocity of the gas jet. The mixing of hydrogen and air takes place mainly after wall impingement. The optimal injection parameters are selected based on the homogeneity of the in-cylinder mixture. It is found that early injection can result in more homogeneous mixture at the time of ignition. Results also indicate that it is more favorable to position the injector near the intake valve to take advantage of the interaction of hydrogen jets and the intake flow to create a more homogeneous mixture.     

One-dimensional turbulence simulations of hydrogen-fueled HCCI combustion

Homogeneous charge compression ignition (HCCI) engines fueled by hydrogen have the potential to provide cost-effective power with high efficiencies and very low emissions. This paper investigates the ability of two of the most commonly used injection methods, port fuel injection (PFI) and single-pulse direct injection (DI), to prepare an ideal in-cylinder hydrogen-air mixture and control the autoignition process. Computations are performed using the one-dimensional turbulence (ODT) model formulated for engine simulations. It is found that direct injection is able to prepare a more uniformly lean mixture and control the autoignition more effectively than port fuel injection. A combination of ignition modes are found to be operating when PFI is used as compared to mainly volumetric autoignition in the case of DI. Also, DI is able to maintain comparatively lower temperatures than PFI.     

A numerical study on the effects of constant volume combustion phase on performance and emissions characteristics of a diesel-hydrogen dual-fuel engine

A detailed numerical study is carried out to investigate the performance of a diesel-hydrogen dual fuel (DF) compression ignition engine operating under a novel combustion strategy in which diesel injection and most of the combustion occur at a constant volume. A detailed validation of the numerical model for diesel-hydrogen DF engine operation has been carried out. Then a parametric study has been performed to investigate the effects of the constant volume combustion phase (CVCP) at up to 90% hydrogen energy share (HES) on engine performance and emissions at low and high load with comparisons to the conventional engine. The results demonstrate that the CVCP strategy can improve thermal efficiency at all HESs and load conditions with far lower carbon-based emissions. Conventional DF engines struggle at low load high HESs due to the reduced diesel injection failing to ignite the leaner premixed charge. Through use of a CVCP thermal efficiency at low load 90% HES increased from 11% to 38% with considerably reduced hydrogen emission due to the increased temperatures and pressures allowing for the wholesale ignition of the hydrogen-air mix. It was also found that increasing the time allowed for combustion within the CVCP, by advancing the diesel injection, can lead to even further thermal efficiency gains while not negatively impacting emissions.     

The lean burn direct injection jet ignition gas engine

This paper presents a new in-cylinder mixture preparation and ignition system for various fuels including hydrogen, methane and propane. The system comprises a centrally located direct injection (DI) injector and a jet ignition (JI) device for combustion of the main chamber (MC) mixture. The fuel is injected in the MC with a new generation, fast actuating, high pressure, high flow rate DI injector capable of injection shaping and multiple events. This injector produces a bulk, lean stratified mixture. The JI system uses a second DI injector to inject a small amount of fuel in a small pre-chamber (PC). In the spark ignition (SI) version, a spark plug then ignites a slightly rich mixture. In the auto ignition version, a DI injector injects a small amount of higher pressure fuel in the small PC having a hot glow plug (GP) surface, and the fuel auto ignites in the hot air or when in contact with the hot surface. Either way the MC mixture is then bulk ignited through multiple jets of hot reacting gases. Bulk ignition of the lean, jet controlled, stratified MC mixture resulting from coupling DI with JI makes it possible to burn MC mixtures with fuel to air equivalence ratios reducing almost to zero for a throttle-less control of load diesel-like and high efficiencies over almost the full range of loads.     

Mixture distribution measurement using laser induced breakdown spectroscopy in hydrogen direct injection stratified charge

Reduction in cooling loss due to the heat transfer from burning gas to the combustion chamber wall is very important for improving the thermal efficiency in hydrogen engines. The previous research has shown that the direct injection stratified charge can be a technique to reduce the cooling loss and improve thermal efficiency in hydrogen combustion. For effective reductions in cooling loss by the stratified charge, it is very important to know the relation between the fuel injection conditions and mixture distribution. The current research employs the laser induced breakdown spectroscopy as a method to measure the hydrogen concentration distribution in the direct injection stratified charge. Measurement of instantaneous local equivalence ratio by the method clears the characteristics of mixture formation in hydrogen direct injection stratified charge. This research also tries to actively control the mixture distribution using a split fuel injection.     

Hydrogen combustion under diesel engine conditions

The autoignition and combustion of hydrogen were investigated in a constant-volume combustion vessel under simulated direct-injection (DI) diesel engine conditions. The parameters varied in the investigation included: the injection pressure and temperature, the orifice diameter, and the ambient gas pressure, temperature and composition. The results show that the ignition delay of hydrogen under DI diesel conditions has a strong, Arrhenius dependence on temperature; however, the dependence on the other parameters examined is small. For gas densities typical of top-dead-center (TDC) in diesel engines, ignition delays of less than 1.0 ms were obtained for gas temperatures greater than 1120 K with oxygen concentrations as low as 5% (by volume). These data confirm that compression ignition of hydrogen is possible in a diesel engine at reasonable TDC conditions. In addition, the results show that DI hydrogen combustion rates are insensitive to reduced oxygen concentrations. The insensitivity of ignition delay and combustion rate to reduced oxygen concentration is significant because it offers the potential for a dramatic reduction in the emission of nitric oxides from a compression-ignited DI hydrogen engine through use of exhaust-gas-recirculation.     

Starting a spark-ignited engine with the gasoline-hydrogen mixture

Because of the increased fuel-film effect and dropped combustion temperature, spark-ignited (SI) gasoline engines always expel large amounts of HC and CO emissions during the cold start period. This paper experimentally investigated the effect of hydrogen addition on improving the cold start performance of a gasoline engine. The test was carried out on a 1.6-L, four-cylinder, SI engine equipped with an electronically controlled hydrogen injection system. A hybrid electronic control unit (HECU) was applied to control the opening and closing of hydrogen and gasoline injectors. Under the same environmental condition, the engine was started with the pure gasoline and gasoline-hydrogen mixture, respectively. After the addition of hydrogen, gasoline injection duration was adjusted to ensure the engine to be started successfully. All cold start experiments were performed at the same ambient, coolant and oil temperatures of 17 °C. The test results showed that cylinder and indicated mean effective pressures in the first cycle were effectively improved with the increase of hydrogen addition fraction. Engine speed in the first 20 start cycles increased with hydrogen blending ratio. However, in later cycles, engine speed varied only a little with and without hydrogen addition due to the adoption of close loop control on engine speed. Because of the low ignition energy and high flame speed of hydrogen, both flame development and propagation durations were shortened after hydrogen addition. HC and CO emissions were dropped markedly after hydrogen addition due to the enhanced combustion process. When the hydrogen flow rate increased from 0 to 2.5 and 4.3 L/min, the instantaneous peak HC emissions were sharply reduced from 57083 to 17850 and 15738 ppm, respectively. NOx emissions were increased in the first 5 s and then reduced later after hydrogen addition.     

Scalar predictors of premixed gas ignition by a suddenly-starting hot jet

Hot-jet ignition is usually designed to reduce emissions in lean-burn combustion engines, and has potential in enabling novel pressure-gain combustion. Inspired by our experimental observations related to wave-rotor combustion chamber ignition, this work employs a numerical method to examine the sudden injection of a hot jet into a quiescent mixture of CH₄–H₂-air and the subsequent ignition. The goal is to provide the range of thermo-physical scalars that are supportive of successful ignition. The evolution of scalar fields is evaluated using large-eddy simulation (LES). The temporal evolution of mixture fraction, the squared gradient of mixture fraction (as indicative of scalar dissipation rate), strain rate, and intermediate species are investigated in order to find the appropriate physical conditions which support ignition. Independent distribution of stain rate and squared gradient of mixture fraction, especially in the leading head vortex, shows the necessity of correlated scalar analysis of the ignition process. Experimental and numerical methods are then employed to provide the qualitative and quantitative understanding of ignition process for fuels with two distinct hydrogen contents. Results show the meaningful difference in spatial distribution of local ignition as hydrogen content of the fuel increases.     

Backfire control and power enhancement of a hydrogen internal combustion engine

Frequent backfire can occur in inlet port fuel injection hydrogen internal combustion engines (HICEs) when the equivalence fuel–air ratio is larger than 0.56, thus limiting further enhancement of engine power. Thus, to control backfire, an inlet port fuel injection HICE test system and a computational fluid dynamics model are established to explore the factors that cause backfire under high loads. The temperature and the concentration of the gas mixture near the intake valves are among the essential factors that result in backfire. Optimizing the timing and pressure of hydrogen injection reduces the concentration distribution of the intake mixture and the temperature of the high-concentration mixture through the inlet valve, thus allowing control of backfire. Controlling backfire enables a HICE to work normally at high equivalence fuel–air ratio (even beyond 1.0). A HICE with optimized hydrogen injection timing and pressure demonstrates significant enhancement of the power output.     

湍流射流点火对天然气发动机燃烧特性影响的试验研究

湍流射流点火（Turbulent Jet Ignition,TJI）是一种有效的燃烧增强技术,可提供更高的点火能量,使发动机稳定着火,且可以提高燃烧压力和燃烧速率,缩短燃烧持续期,是实现发动机稀薄燃烧的有效手段。基于一台带有预燃室的点燃式单缸试验机,开展了TJI模式下天然气发动机性能的试验研究。首先,研究了不同过量空气系数下TJI对天然气发动机动力性能、排放性能及燃烧特性的影响,并与火花塞点火（Spark Ignition,SI）模式进行对比;其次,在稀燃条件下分别探究了进气增压和预燃室喷氢对天然气发动机动力性、经济性及燃烧过程的优化作用。结果表明：TJI的使用可有效拓展天然气发动机的稀燃极限,且燃烧滞燃期和燃烧持续期均更短,放热率更高;过量空气系数1.5为甲烷TJI最佳稀燃工况,此时燃油消耗率最低,且可实现氮氧化物近零排放;此外,采用进气增压的方式可以提高TJI发动机在高负荷下的经济性;TJI模式下,相较于预燃室喷甲烷,预燃室喷氢气可进一步缩短滞燃期和燃烧持续期,提高放热率,达到提升TJI性能的效果。