The effect of motion on fuel diffusion and mixture preparation of a free-piston linear hydrogen engine

The most notable difference between linear hydrogen engine (LHE) and conventional crank hydrogen engine (CHE) is that the former supports variable motion operation for combustion optimizing. This article presents a study to reveal the effect of variable motion on hydrogen fuel diffusion and mixing of a direct injection LHE. A multi-dimensional model of coupled dynamics is proposed to describe the motion effect on mixture formation. The fuel mixing characteristic of the LHE is evaluated by comparing a corresponding CHE. Result indicates that compared with the CHE, the LHE behaves a slight lower uniformity of the hydrogen-air mixture during injection stage due to its slower motion, but the slower motion also provides longer time for hydrogen diffusion to form more homogeneous mixture before sparking ignition. Moreover, the effect of variable motion stroke on the hydrogen-air mixing of the LHE is also analyzed. Study reveals that the variable stroke has little effect on the hydrogen-air mixing during the injection stage, and the mixture uniformity varies in positive correlation with the stroke length at the occurrence of spark ignition. Suggesting that in order to achieve homogeneous combustion mode, the long motion stroke operation is a good choice for the LHE.     

A coupling dynamics and thermodynamics study of diesel pilot-ignition injection effect on hydrogen combustion of a linear engine

Linear hydrogen engine is a new type of energy conversion device to supports variable compression ratio operation for clean emission. However, the new hydrogen engine using conventional spark ignition shows slow combustion speed and low thermal efficiency. This study makes a preliminary assessment to discuss the application of diesel pilot-ignition technology in linear hydrogen engine aiming to accelerate combustion and improve efficiency. A new coupling model between dynamics and thermodynamics is proposed and then iteratively calculated to give insight the interrelationship of combustion and motion in a diesel pilot-ignited linear hydrogen engine, while the effect of injection position on the hydrogen engine combustion is also investigated to make clear the feasibility of combustion optimization. The results indicate that the linear hydrogen engine is speeded by properly advancing the injection to promote combustion, and it has a positive effect on in-cylinder gas temperature, pressure and pressure rise rate, unless the injection is too early which results in higher NO emissions and aggravate the working intensity of the engine. In addition, the closer the fuel injection is to the top dead center, the incomplete combustion of hydrogen and diesel in the cylinder, the decrease of engine fuel economy and the increase of soot emissions. There is an optimal thermal efficiency of 40.7% for the LHE when it operates in the 0.8 mm injection position condition.     

Modelling of hydrogen-blended dual-fuel combustion using flamelet-generated manifold and preferential diffusion effects

In the present study, Reynolds-Averaged Navier-Stokes simulations together with a novel flamelet generated manifold (FGM) hybrid combustion model incorporating preferential diffusion effects is utilised for the investigation of a hydrogen-blended diesel-hydrogen dual-fuel engine combustion process with high hydrogen energy share. The FGM hybrid combustion model was developed by coupling laminar flamelet databases obtained from diffusion flamelets and premixed flamelets. The model employed three control variables, namely, mixture fraction, reaction progress variable and enthalpy. The preferential diffusion effects were included in the laminar flamelet calculations and in the diffusion terms in the transport equations of the control variables. The resulting model is then validated against an experimental diesel-hydrogen dual-fuel combustion engine. The results show that the FGM hybrid combustion model incorporating preferential diffusion effects in the flame chemistry and transport equations yields better predictions with good accuracy for the in-cylinder characteristics. The inclusion of preferential diffusion effects in the flame chemistry and transport equations was found to predict well several characteristics of the diesel-hydrogen dual-fuel combustion process: 1) ignition delay, 2) start and end of combustion, 3) faster flame propagation and quicker burning rate of hydrogen, 4) high temperature combustion due to highly reactive nature of hydrogen radicals, 5) peak values of the heat release rate due to high temperature combustion of the partially premixed pilot fuel spray with entrained hydrogen/air and then background hydrogen-air premixed mixture. The comparison between diesel-hydrogen dual-fuel combustion and diesel only combustion shows early start of combustion, longer ignition delay time, higher flame temperature and NOx emissions for dual-fuel combustion compared to diesel only combustion.     

Effect of variable stroke on fuel combustion and heat release of a free piston linear hydrogen engine

Variable stroke (or compression ratio) has been expected as a potential technology for optimizing engine combustion all the time. The free piston motion of linear engines introduced by its unconstrained dynamics is well suited for this expectation. This paper introduces a numerical analysis to explore the effects of variable stroke operation on the combustion and heat release of a linear hydrogen engine (LHE). A system model which couples zero-dimensional dynamics, multi-dimensional combustion, and one-dimensional gas exchange is established and verified experimentally to predict the combustion of the LHE, and then a series of simulations are performed over a range of motion stroke from 62 mm to 72 mm in 2 mm interval to evaluate its effect on LHE combustion. Results indicates that short stroke operation of the LHE shows obvious advantages in thermal efficiency and high peak combustion pressure, although the completion level of hydrogen combustion is slightly poor. Fast combustion, large heat release, and low level of post-combustion effect can be obtained by neither lengthening nor shortening from the certain stroke length of 68 mm, while serious NO emission is indicated. Long-stroke operation makes the LHE clean, although it induces slow engine speed, low thermal efficiency and output power.     

The effect of hydrogen injection parameters on the quality of hydrogen–air mixture formation for a PFI hydrogen internal combustion engine

To research the quality of the hydrogen–air mixture formation and the combustion characteristics of the hydrogen fueled engine under different hydrogen injection timings, nozzle hole positions and nozzle hole diameter, a three-dimensional simulation model for a PFI hydrogen internal combustion engine with the inlet, outlet, valves and cylinder was established using AVL Fire software. In the maximum torque condition, research focused on the variation law of the total hydrogen mass in the cylinder and inlet and the space distribution characteristics and variation law of velocity field, concentration field and turbulent kinetic energy under different hydrogen injection parameters (injection timings, nozzle hole positions and nozzle hole area) in order to reveal the influence of these parameters on hydrogen–air mixture formation process. Then the formation quality of hydrogen–air mixture was comprehensively evaluated according to the mixture uniformity coefficient, the remnant hydrogen percentage in the inlet and restraining abnormal combustion (such as preignition and backfire). The results showed that the three hydrogen injection parameters have important influence on the forming quality of hydrogen–air mixture and combustion state. The reasonable choice of the nozzle hole position of hydrogen, nozzle hole diameter and the hydrogen injection time can improve the uniformity of the hydrogen–air mixing in the cylinder of the hydrogen internal combustion engine, and the combustion heat release reaction is more reasonable. At the end of the compression stroke, the equivalence ratio uniform coefficient increased at first and then decreased with the beginning of the hydrogen injection. When hydrogen injection starting point was with 410–430°CA, equivalence ratio uniform coefficient was larger, and ignition delay period was shorter so that the combustion performance index was also good. And remnant hydrogen percentage in the inlet was less, high concentration of mixed gas in the vicinity of the inlet valve also gathered less, thus suppressing the preignition and backfire. With the increase of the distance between the nozzle and the inlet valve, the selection of the hydrogen injection period is narrowed, and the optimum hydrogen injection time was also ahead of time. The results also showed that it was favorable for the formation of uniform mixing gas when the nozzle hole diameter was 4 mm.     

A computer simulation of hydrogen fueled spark ignition engine

The work reported here pertains to some of the computer simulation models developed for hydrogen fueled spark ignition (SI) engines. The engine combustion process is modeled by using a semi-empirical turbulent flame speed expression. This combustion model has been employed to account for the hydrogen-air combustion process over a wide range of stoichiometric variables for the Varimax engine operating at various speeds and compression ratios. Based on the computed results, graphs showing the variation of combustion crank angle and flame speed with fuel-air equivalence ratio, engine speed, compression ratio etc., have been plotted.     

Study of the cycle-to-cycle variations of an internal combustion engine fuelled with natural gas/hydrogen blends from the diagnosis of combustion pressure

A methodology is presented for studying the influence of using alternative fuels on the cycle-to-cycle variations of a spark ignition engine which has been fuelled with mixtures of natural gas and hydrogen in different proportions (0–100%). The experimental facility consists of a single-cylindrical spark ignition engine coupled to an asynchronous machine with a constant engine rotation speed of 1500 rpm. A thermodynamic combustion diagnostic model based on genetic algorithms is used to evaluate the combustion chamber pressure data experimentally obtained in the mentioned engine. The model is used to make the pressure diagnosis of series of 830 consecutive engine cycles automatically, with a high grade of objectivity of the combustion analysis, since the relevant adjustment parameters (i.e. pressure offset, effective compression ratio, top dead center angular position, heat transfer coefficients) are calculated by the genetic algorithm. Results indicate that the combustion process is dominated by the turbulence inside the combustion chamber (generated during intake and compression), showing little dependency of combustion variation on the mixture composition. This becomes more evident when relevant combustion variables are plotted versus the Mass Fraction Burned of each mixture. The only exception is the case of 100% hydrogen, due to the inherent higher laminar speed of hydrogen that causes combustion acceleration and thus turbulence generation.     

Research on the hydrogen injection strategy of a turbulent jet ignition (TJI) rotary engine fueled with natural gas/hydrogen blends

Natural gas/hydrogen blends (NGHB) fuel is considered as one of the ideal alternative fuels for the rotary engine (RE), which can effectively reduce the carbon emissions of RE. Additionally, applying turbulent jet ignition (TJI) mode to RE can significantly increase the combustion rate. The purpose of this study is to numerically investigate the influence of hydrogen injection position (HIP) and hydrogen injection timing (HIT) on the in-cylinder mixture formation, flame propagation and NOx emission of a TJI hydrogen direct injection plus natural gas port injection RE. Therefore, in this paper, a test bench and a 3D dynamic simulation model of the turbulent jet ignition rotary engine (TJI-RE) fueled with NGHB were respectively established. Moreover, the reliability of the 3D simulation model was verified by experimental data. Furthermore, based on the established 3D model, the fuel distribution and flame propagation in the cylinder under different HIPs and HITs were calculated. The results indicated that the HIP and HIT could change the hydrogen distribution by altering the impact position, impact angle, and the strength of vortexes in the cylinder. To improve the flame propagation speed, more hydrogen should be distributed in the pre-chamber. Additionally, a higher concentration of hydrogen in the cylinder should be maintained above the jet orifice. This was not only conducive to the rapid formation of the initial fire core in the pre-chamber, but also significantly improved the combustion rate of the in-cylinder mixture. Compared with other hydrogen injection strategies, the hydrogen injection strategy by using the HIP at the middle of the cylinder block and the HIT of 190^oCA(BTDC) could obtained the highest peak value of in-cylinder pressure and the highest NOx emission.     

Numerical investigation on the effects of load conditions and hydrogen-air ratio on the combustion processes of a HSDI engine

Owing to high specific energy and low emissions production, hydrogen is a desirable alternative fuel. The combustion and emission performance can be improved by hydrogen addition injected in-cylinder, intake manifold and aspirated with air. However, engine loads and hydrogen-air ration have a significant effect on the performance, combustion and emission of the diesel-hydrogen (high speed direct injection) HSDI engine. In this paper, the CFD method is used to calculate the combustion process of a diesel-hydrogen dual HSDI engine working at constant speed of 4000 rpm, at different hydrogen added from intake port (hydrogen volume fraction of 0%–10%) and five engine loads (equivalent to 20%, 40%, 60%, 80% and 100% of its maximum output power), respectively. The modelling results showed that the in-cylinder pressure and temperature under low engine load were more affected by hydrogen addition. With increasing hydrogen volume fraction, the indicated expansion work and in-cylinder peak pressure increased, and combustion duration decreased due to faster fuel-air mixing and spray flame speed.     

汽油加氢改善发动机性能的试验研究

在汽油机的燃烧过程中加入部分的氢气进行燃烧，可对发动机的性能改善和废气排放物的降低起到良好的作用。本文就加氢后的汽油过量空气系数，点火提前角的变化，燃烧过程的循环压力变动，发动机的经济性及ＨＣ和ＣＯ排放问题进行了研究。试验结果表明：随着汽油燃烧过程中加氢比例的增大，混合气着火界线加宽。另外，由于氢气的火焰传播速度快，加氢后混合气的着火延迟期缩短，最佳点火提前角减小，燃烧过程的循环压力变动减小，而且     

氢空气混合气的燃烧及其NOx的形成

氢是可再生洁净能源，氢空气混合燃烧产生的唯一的有害排放物是ＮＯｘ，本文从燃烧理论和化学动力学角度分析了氢喷射火花点火氢发动机中氢空气混合气的燃烧及其ＮＯｘ的形成，并着重描述了低ＮＯｘ燃烧。     

Effect of distributions of fuel concentration and temperature on ignition processes in diesel PCCI combustion

The distributions of fuel concentration and temperature have significant effect on the ignition processes of diesel premixed charge compression ignition (PCCI) combustion. It was found in this study that the ignition process of PCCI combustion organized by multi-pulse injection was strongly inuenced by conditions of fuel stratification. The start of low temperature reactions occurred in the leaner area of the combustion chamber in the test engine because the temperature here first reached the point of low temperature reactions. Ignition always occurred in the position where the mixture featured with equivalence ratios close to the mean equivalence ratio of the overall mixture, while the neighboring area of the initial ignition area accumulate heat with a finite speed until finally autoigniting. Moreover, the appearance of highest combustion temperature occurred in the same area at the combustion chamber. For more homogeneous mixture, a higher amount of mixture reached ignition simultaneously, resulting in a larger initial ignition area and a higher temperature at the ignition area. Furthermore, V-type distribution of equivalence ratio was found to be beneficial to retarding high temperature reaction.     

Effect of distributions of fuel concentration and temperature on ignition processes in a diesel PCCI combustion

The distributions of fuel concentration and temperature have significant effect on the ignition processes of diesel premixed charge compression ignition (PCCI) combustion. It was found in this study that the ignition process of PCCI combustion organized by multi-pulse injection was strongly influenced by conditions of fuel stratification. The start of low temperature reactions occurred in the leaner area of the combustion chamber in the test engine because the temperature here first reached the point of low temperature reactions. Ignition always occurred in the position where the mixture featured with equivalence ratios close to the mean equivalence ratio of the overall mixture, while the neighboring area of the initial ignition area accumulate heat with a finite speed until finally autoigniting. Moreover, the appearance of highest combustion temperature occurred in the same area at the combustion chamber. For more homogeneous mixture, a higher amount of mixture reached ignition simultaneously, resulting in a larger initial ignition area and a higher temperature at the ignition area. Furthermore, V-type distribution of equivalence ratio was found to be beneficial to retarding high temperature reaction.     

高速直喷柴油机燃烧室形状与喷孔结构匹配规律的三维仿真研究

对某高速直喷柴油机的燃烧过程进行了多维仿真计算,研究了燃烧室形状和喷孔结构参数之间的匹配规律.通过变参数研究确定了燃烧室形状和喷孔结构参数之间的匹配规律.为了定量描述燃烧过程中燃油雾化、液体蒸发、油气混合的特性,建立了缸内平均湍流混合速率、燃油蒸气质量分数方差的中间特征参数,同时详细分析了不同当量比油气混合气在燃烧过程中的贡献率.从燃烧的宏观和微观角度综合分析了柴油机燃烧室形状与喷孔结构间的耦合作用机制.结果表明,对于所研究的机型,0.64口径比燃烧室匹配10孔喷油器的方案最优:预混过程好,燃烧速度快,后期扩散燃烧阶段过稀和过浓混合气参与燃烧的比例较小且预混与扩散燃烧放热情况差别小,放热情况更均匀.     

Basic tuning of hydrogen powered car and artificial intelligent prediction of hydrogen engine characteristics

Many studies of renewable energy have shown hydrogen is one of the major green energy in the future. This has lead to the development of many automotive application of using hydrogen as a fuel especially in internal combustion engine. Nonetheless, there has been a slow growth and less knowledge details in building up the prototype and control methodology of the hydrogen internal combustion engine [1]. In this paper, The Toyota Corolla 4 cylinder, 1.8l engine running on petrol was systematically modified in such a way that it could be operated on either gasoline or hydrogen at the choice of the driver. Within the scope of this project, several ancillary instruments such as a new inlet manifold, hydrogen fuel injection, storage system and leak detection safety system were implemented. Attention is directed towards special characteristics related to the basic tuning of hydrogen engine such as: air to fuel ratio operating conditions, ignition timing and injection timing in terms of different engine speed and throttle position. Based on the experimental data, a suite of neural network models were tested to accurately predict the effect of different engine operating conditions (speed and throttle position) on the hydrogen powered car engine characteristics. Predictions were found to be ±3% to the experimental values for all of case studies. This work provided better understanding of the effect of hydrogen engine characteristic parameters on different engine operating conditions.     

A simulation of the combustion of hydrogen in HCCI engines using a 3D model with detailed chemical kinetics

The influence of changes in the swirl velocity of the intake mixture on the combustion processes within a homogeneous charge compression ignition (HCCI) engine fueled with hydrogen were investigated analytically. A turbulent transient 3D predictive computational model which was developed and applied to the HCCI engine combustion system, incorporated detailed chemical kinetics for the oxidation of hydrogen. The effects of changes in the initial intake swirl, temperature and pressure, engine speed and compression and equivalence ratios on the combustion characteristics of a hydrogen fuelled HCCI engine were also examined. It is shown that an increase in the initial flow swirl ratio or speed lengthens the delay period for autoignition and extends the combustion period while reducing NO_x emissions. There are optimum values of the initial swirl ratio and engine speed for a certain mixture intake temperature, pressure, compression and equivalence ratios operational conditions that can achieve high thermal efficiencies and low NO_x emissions while reducing the tendency to knock     

丙烷发动机燃烧变动研究

测量了火花点火丙烷气体发动机在不同转速、涡流强度及混合比下的气缸压力,并对测量的气缸压力及由气缸压力求出的用曲轴转角表示的初期燃烧期间等进行了统计分析。试验结果表明,随着混合气变稀平均指示压力的变动迅速增大;转速相同时,平均指示压力的变动随着涡流比的增大而减小;在稀薄混合气条件下,随着初期燃烧期间平均值的增加平均指示压力的变动急剧上升。     

Water induction in hydrogen-powered IC engines

Addition of water to the hydrogen-air mixture in the intake manifold is an effective means of both suppressing the tendency to backflash and reducing the production of oxides of nitrogen. Tests are run on a Dodge 440 CID V8 engine having a compression ratio of 12:1. Dramatic reduction in oxides of nitrogen is observed as the water flow is increased, yet essentially no change is observed in either power or efficiency. Exhaust temperature, NO_x, and equivalence ratio is measured at each exhaust valve. It is found that a large cylinder to cylinder variation in NO_x production is caused by slight non-uniformity in mixing of the hydrogen-air streams. It is further shown that NO_x production is an exponential function of equivalence ratio, water to hydrogen mass ratio, and engine speed.     

均质混合气压燃汽油机燃烧特性和碳烟的生成

作者将一台单缸四冲程试验用汽油机改造成均质混合气压燃点火发动机。在大量试验的基础上,证明柴油均质混合气压燃点火在技术上可以实现。利用该燃烧模型进一步研究柴油机的燃烧和碳烟生成中的一些问题:如点火延迟时间与混合气浓度的关系,排气中的碳烟浓度与混合气油气当量比的关系。     

Local heat flux measurements in a hydrogen and methane spark ignition engine with a thermopile sensor

This paper describes an experimental investigation of heat transfer inside a CFR spark ignition engine operated at a constant engine speed of 600 rpm. The heat flux is directly measured under motored and fired conditions with a commercially available thermopile sensor. The heat transfer during hydrogen and methane combustion is compared examining the effects of the compression ratio, ignition timing and mixture richness. Less cyclic and spatial variation in the heat flux traces are observed when burning hydrogen, which can be correlated to the faster burn rate. The peak heat flux increases with the compression ratio, but the total cycle heat loss can decrease due to less heat transfer at the end of the expansion stroke. An advanced spark timing and increased mixture richness cause an increased and advanced peak in the heat flux trace. Hydrogen combustion gives a heat flux peak which is three times as high as the one of methane for the same engine power output.