Application of exhaust gas fuel reforming in diesel and homogeneous charge compression ignition (HCCI) engines fuelled with biofuels

This paper documents the application of exhaust gas fuel reforming of two alternative fuels, biodiesel and bioethanol, in internal combustion engines. The exhaust gas fuel reforming process is a method of on-board production of hydrogen-rich gas by catalytic reaction of fuel and engine exhaust gas. The benefits of exhaust gas fuel reforming have been demonstrated by adding simulated reformed gas to a diesel engine fuelled by a mixture of 50% ultra low sulphur diesel (ULSD) and 50% rapeseed methyl ester (RME) as well as to a homogeneous charge compression ignition (HCCI) engine fuelled by bioethanol. In the case of the biodiesel fuelled engine, a reduction of NO_x emissions was achieved without considerable smoke increase. In the case of the bioethanol fuelled HCCI engine, the engine tolerance to exhaust gas recirculation (EGR) was extended and hence the typically high pressure rise rates of HCCI engines, associated with intense combustion noise, were reduced.     

催化燃烧对均质压燃发动机燃烧特性影响的数值模拟

通过运用DETCHEM软件包,对甲烷在催化剂Rh表面的详细反应机理进行了分析,结果表明数值模拟结果与实验数据相当吻合;通过耦合DETCHEM软件包及CHEMKIN软件包中的SENKIN模块,对活塞顶涂有催化剂铑的均质压燃(HCCI)发动机的燃烧过程进行了数值计算,建立了单区和多区模型．利用单区模型分析了催化燃烧对HCCI发动机着火时刻的影响,同时讨论了催化燃烧对燃烧过程中主要化学组分浓度变化的影响,结果表明催化燃烧会使HCCI发动机着火时刻提前;利用多区模型分析了催化燃烧对HCCI发动机的未燃碳氢化合物(UHC)、氮氧化合物(NOx)排放的影响,结果表明催化燃烧能降低UHC的排放,但会提高NOx的排放．     

均质压燃式(HCCI)燃烧的研究

均质压燃式（HCCI）燃烧方式是目前内燃机燃烧领域的研究热点。HCCI燃烧是以预混合燃烧和低温反应为特征的燃烧方式。采用HCCI燃烧方式可以同时有效降低柴油机的NOX和碳烟排放，并提高柴油机的循环热效率。HCCI发动机通常工作在高空燃比和较低的压缩比条件下，工作范围较小，高负荷时功率输出不足。“双模式”HCCI发动机是解决上述问题的有效途径，并成为近期HCCI发动机研究中的热点。     

双燃料发动机放热率计算和试验分析

提出了一个描述双燃料发动机燃烧特性的多区模型,模型将气体燃料的燃烧和引燃柴油的燃烧分别进行考虑。建立了由实测示功图求解双燃料发动机放热率的微分方程式,开发了计算双燃料发动机燃烧放热规律的软件,并在一台生物质气-柴油双燃料发动机上与传统柴油机放热率计算模型进行了试验验证和对比。研究和试验结果表明,用传统柴油机分析方法计算双燃料发动机的放热率峰值偏大,所计算的缸内工质平均温度偏高,新模型计算的结果与实际情况更吻合。     

Numerical analysis of two-stroke free piston engine operating on HCCI combustion

An opposed-piston hydraulic free piston engine operating with homogenous charge compression ignition (HCCI) combustion, has been proposed by State Key Laboratory of Engines as a means of significantly improving the IC engine’s cycle thermal efficiency and lowering exhaust emissions. Single and multi-zone Chemkin model with detailed chemical kinetics, and unique piston dynamics extracted from one dimensional gas dynamic model, have been used to analyze the combustion characteristics and engine performance. Intake heating, variable compression ratio and internal EGR are utilized to control the combustion phasing and duration in the cycle simulations, revealing the critical factors and possible limits of performance improvement relative to conventional crank engines. Furthermore, real engine effects such as heat transfer with air swirl, residual mass fraction, thermal stratification, and heat loss fraction between zones are considered in the sequential CFD/multi-zone method to approach the realistic engine performance at an acceptable knock level.     

Progress and recent trends in homogeneous charge compression ignition (HCCI) engines

HCCI combustion has been drawing the considerable attention due to high efficiency and lower nitrogen oxide (NO_x) and particulate matter (PM) emissions. However, there are still tough challenges in the successful operation of HCCI engines, such as controlling the combustion phasing, extending the operating range, and high unburned hydrocarbon and CO emissions. Massive research throughout the world has led to great progress in the control of HCCI combustion. The first thing paid attention to is that a great deal of fundamental theoretical research has been carried out. First, numerical simulation has become a good observation and a powerful tool to investigate HCCI and to develop control strategies for HCCI because of its greater flexibility and lower cost compared with engine experiments. Five types of models applied to HCCI engine modelling are discussed in the present paper. Second, HCCI can be applied to a variety of fuel types. Combustion phasing and operation range can be controlled by the modification of fuel characteristics. Third, it has been realized that advanced control strategies of fuel/air mixture are more important than simple homogeneous charge in the process of the controlling of HCCI combustion processes. The stratification strategy has the potential to extend the HCCI operation range to higher loads, and low temperature combustion (LTC) diluted by exhaust gas recirculation (EGR) has the potential to extend the operation range to high loads; even to full loads, for diesel engines. Fourth, optical diagnostics has been applied widely to reveal in-cylinder combustion processes. In addition, the key to diesel-fuelled HCCI combustion control is mixture preparation, while EGR is the main path to achieve gasoline-fuelled HCCI combustion. Specific strategies for diesel-fuelled, gasoline-fuelled and other alternative fuelled HCCI combustion are also discussed in the present paper.     

Applying the multi-zone model in predicting the operating range of HCCI engines

In this paper, a multi-zone model is developed to predict the operating range of homogeneous charge compression ignition (HCCI) engines. The boundaries of the operating range were determined by knock (presented by ringing intensity), partial burn (presented by combustion efficiency), and cycle-to-cycle variations (presented by the sensitivity of indicated mean effective pressure to initial temperature). By simulating an HCCI engine fueled with iso-octane, the knock and cycle-to-cycle variations predicted by the model showed satisfactory agreement with measurements made under different initial temperatures and equivalence ratios; the operating range was also well reproduced by the model. Furthermore, the model was applied to predict the operating range of the HCCI engine under different engine speeds by varying the intake temperatures and equivalence ratios. The potential to extend the operating range of the HCCI engine through two strategies, i.e., variable compression ratio and intake pressure boosting, was then investigated. Results indicate that the ignition point can be efficiently controlled by varying the compression ratio. A low load range can be extended by increasing the intake temperature while reducing the compression ratio. Higher intake temperatures and lower compression ratios can also extend the high load range. Boosting intake pressure is helpful in controlling the combustion of the HCCI engine, resulting in an extended high load range.     

催化燃烧对HCCI发动机着火点、燃烧性能及排放的影响

对甲烷在催化剂铑(Rh)表面的反应机理进行了分析。通过修改CHEMKIN软件包中的SENKIN模块,对活塞顶涂有催化剂的HCCI发动机的燃烧过程进行了数值计算,建立了单区、多区模型。利用单区模型分析了催化燃烧对HCCI发动机着火时刻的影响,结果表明在控制HCCI发动机着火时刻方面催化燃烧有其他方式所没有的优势;利用多区模型分析了催化燃烧对HCCI发动机的燃烧性能及HC、CO、NOx排放的影响,结果表明催化燃烧对燃烧效率、着火持续期有较大的影响,同时能降低HC、CO的排放,但会提高NOx的排放。     

Applying the multi-zone model in predicting the operating range of HCCI engines

In this paper, a multi-zone model is developed to predict the operating range of homogeneous charge compression ignition (HCCI) engines. The boundaries of the operating range were determined by knock (presented by ringing intensity), partial burn (presented by combustion efficiency), and cycle-to-cycle variations (presented by the sensitivity of indicated mean effective pressure to initial temperature). By simulating an HCCI engine fueled with iso-octane, the knock and cycle-to-cycle variations predicted by the model showed satisfactory agreement with measurements made under different initial temperatures and equivalence ratios; the operating range was also well reproduced by the model. Furthermore, the model was applied to predict the operating range of the HCCI engine under different engine speeds by varying the intake temperatures and equivalence ratios. The potential to extend the operating range of the HCCI engine through two strategies, i.e., variable compression ratio and intake pressure boosting, was then investigated. Results indicate that the ignition point can be efficiently controlled by varying the compression ratio. A low load range can be extended by increasing the intake temperature while reducing the compression ratio. Higher intake temperatures and lower compression ratios can also extend the high load range. Boosting intake pressure is helpful in controlling the combustion of the HCCI engine, resulting in an extended high load range.     

An extended multi-regime flamelet model for IC engines

Modeling internal combustion engines is challenging due to the various coupled multi-physics phenomena. With the advent of modern supercomputing and advanced modeling techniques, studying and designing these engines through detailed simulations is becoming tractable. Since the combustion process is the primary controlling feature in these engines, a high fidelity combustion model is essential. This model must be efficient and valid across different combustion regimes, since modern engines might operate in hybrid modes. The Representative Interactive Flamelet (RIF) combustion model is a possible choice. This model has been developed to describe ignition, combustion, and pollutant formation in direct-injected diesel engines. However, it has recently been shown that the model has the correct asymptotic behavior for both diesel and Homogeneous Charge Compression Ignition (HCCI) regimes, and the model has been applied successfully for HCCI type combustion. In this study, the model is validated against two-dimensional direct numerical simulation data with multi-step finite rate chemistry to evaluate model performance in the diesel, the HCCI, and hybrid regimes. A wide range of temperature and mixture fraction stratification cases are simulated to evaluate the model performance across different modes. The model performs well for all cases considered, even when high levels of concurrent thermal and charge stratification are present.     

Fundamental phenomena affecting low temperature combustion and HCCI engines,high load limits and strategies for extending these limits

Low temperature combustion (LTC) engines are an emerging engine technology that offers an alternative to spark-ignited and diesel engines. One type of LTC engine, the homogeneous charge compression ignition (HCCI) engine, uses a well-mixed fuel–air charge like spark-ignited engines and relies on compression ignition like diesel engines. Similar to diesel engines, the use of high compression ratios and removal of the throttling valve in HCCI allow for high efficiency operation, thereby allowing lower CO₂ emissions per unit of work delivered by the engine. The use of a highly diluted well-mixed fuel–air charge allows for low emissions of nitrogen oxides, soot and particulate matters, and the use of oxidation catalysts can allow low emissions of unburned hydrocarbons and carbon monoxide. As a result, HCCI offers the ability to achieve high efficiencies comparable with diesel while also allowing clean emissions while using relatively inexpensive aftertreatment technologies.     

Comprehensive study of biodiesel fuel for HSDI engines in conventional and low temperature combustion conditions

In this research, an experimental investigation has been performed to give insight into the potential of biodiesel as an alternative fuel for High Speed Direct Injection (HSDI) diesel engines. The scope of this work has been broadened by comparing the combustion characteristics of diesel and biodiesel fuels in a wide range of engine loads and EGR conditions, including the high EGR rates expected for future diesel engines operating in the low temperature combustion (LTC) regime.The experimental work has been carried out in a single-cylinder engine running alternatively with diesel and biodiesel fuels. Conventional diesel fuel and neat biodiesel have been compared in terms of their combustion performance through a new methodology designed for isolating the actual effects of each fuel on diesel combustion, aside from their intrinsic differences in chemical composition.The analysis of the results has been sequentially divided into two progressive and complementary steps. Initially, the overall combustion performance of each fuel has been critically evaluated based on a set of parameters used as tracers of the combustion quality, such as the combustion duration or the indicated efficiency. With the knowledge obtained from this previous overview, the analysis focuses on the detailed influence of biodiesel on the different diesel combustion stages known ignition delay, premixed combustion and mixing controlled combustion, considering also the impact on CO and UHC pollutant emissions.The results of this research explain why the biodiesel fuel accelerates the diesel combustion process in all engine loads and EGR rates, even in those corresponding with LTC conditions, increasing its possibilities as alternative fuel for future DI diesel engines.     

催化燃烧对均质压燃发动机排放影响的数值模拟

通过耦合DETCHEM软件包及CHEMKIN软件包中的SENKIN模块,对活塞顶涂有催化剂的均质压燃(HCCI)发动机的燃烧过程进行了数值计算,建立了多区模型.利用此模型分析了催化燃烧对HCCI发动机缸内温度、热释放速率以及未燃碳氢化合物(UHC)、氮氧化合物(NOx)、一氧化碳(CO)排放的影响,结果表明催化燃烧能降低UHC、CO的排放,但NOx的排放会有所升高.对不同催化剂及混合催化剂对HCCI发动机缸内温度、热释放速率以及UHC、NOx、CO排放的影响进行了探索,结果表明,和金属铂相比,以铑作催化剂时UHC的排放降低,但NOx、CO排放会有所升高;采用500/0Pt-500/0Rh的混合催化剂时,UHC、NOx的排放介于1000/0Pt与1000/0Rh之间,但CO的排放却比采用1000/0Pt与1000/0Rh时都要低.     

代理汽油在均质压燃内燃机中燃烧特性的模拟研究

在均质压燃（HCCI）内燃机中,燃烧主要由化学动力学控制。研究燃料的化学动力学反应机理对了解和控制HCCI具有重要意义。本文利用CHEMKIN多区模型,研究了由正庚烷、异辛烷和甲苯混合而成的代理汽油的燃烧特性。计算结果显示,多区模型弥补了单区模型中出现的温度压强陡升缺点,能更好地反映缸内真实燃烧过程。多区温度分布区间越广,则燃烧提前,燃烧持续期长。各区NOx排放和温度分布趋势类似。HC和CO排放主要集中在燃烧不完全的第1区。     

均质压燃发动机燃烧与排放的多区模型模拟

应用一个有质量交换的6区模型模拟正庚烷在HCCI发动机中的燃烧和排放特性．通过把缸内划分为缝隙区、边界层区、外核心区和内核心区,加入Woschni传热模型计入了缸内的温度和浓度的不均匀分布．全部计算基于正庚烷燃烧的包含57种组分290个反应的详细机理,结果表明,该多区模型合理地模拟了HCCI发动机的燃烧过程,并可满意地预测出HC、CO和NO的排放．最后采用此多区模型分别讨论了缝隙容积、边界层厚度和壁面温度对HCCI发动机的燃烧和排放的影响．     

在汽油机上实施HCCI的技术策略

均质混合气压燃(HCCI)燃烧方式，是一种克服常规柴油机和汽油机缺点、集常规汽油机和柴油机优点于一体的新概念燃烧。本文分析了汽油机实施HCCI的可行性，介绍了HCCI发动机实用化所面临的问题，提出了双工作模式的折衷方案：在中低负荷工况实施HCCI，而在大负荷工况和冷起动工况恢复常规发动机工作方式。推荐可变压缩比(VCR)方案、可变废气再循环率(EGR)方案、可变排气门关闭时刻方案，以及废气再循环滚流分层充气方案等。为尽快在汽油机上实施HCCI燃烧方式指出了技术方向。     

HCCI发动机多区燃烧模型的比较研究

多区模型作为现阶段均质压燃（HCCI）发动机高效准确的数值模型得到了世界范围的广泛关注。讨论了不同子模型对多区模型预测性能的影响。以实验为基准,比较了多区模型中区间划分、缸壁传热模型、区间热量交换模型、区间质量交换模型和边界层模型对HCCI发动机燃烧和排放模拟结果的影响,全部计算均基于异辛烷的详细化学动力学机理。结果表明：在区间划分时对温度较低的区域细化可以提高排放的计算效果,而对高温区域的细化对计算结果影响不大;改进的Woschni传热模型更准确地模拟了缸壁的传热过程;区间的质量和热量交换对计算结果影响显著,特别是质量交换模型的加入使CO排放的预测与实验值更为接近;而边界层厚度模型对整个结果影响不大。     

柴油均质压燃发动机的循环模拟研究

建立了一个柴油均质压燃发动机的零维循环计算模型,用基元反应机理和感应时间（EMIT）模型预测着火正时,用拟合的对数正态分布函数计算不同工况的放热率。与试验的对比表明,模型能够比较准确地预测不同工况柴油均质压燃发动机的工作性能。用建立的模型分析了配气定时对柴油均质压燃燃烧过程的影响,计算表明配气定时可以控制均质压燃燃烧的运行工况范围。     

A cycle simulation model for predicting the performance of a diesel engine fuelled by diesel and biodiesel blends

Among the alternative fuels, biodiesel and its blends are considered suitable and the most promising fuel for diesel engine. The properties of biodiesel are found similar to that of diesel. Many researchers have experimentally evaluated the performance characteristics of conventional diesel engines fuelled by biodiesel and its blends. However, experiments require enormous effort, money and time. Hence, a cycle simulation model incorporating a thermodynamic based single zone combustion model is developed to predict the performance of diesel engine. The effect of engine speed and compression ratio on brake power and brake thermal efficiency is analysed through the model. The fuel considered for the analysis are diesel, 20%, 40%, 60% blending of diesel and biodiesel derived from Karanja oil (Pongamia Glabra). The model predicts similar performance with diesel, 20% and 40% blending. However, with 60% blending, it reveals better performance in terms of brake power and brake thermal efficiency.     

Heat transfer in HCCI multi-zone modeling: Validation of a new wall heat flux correlation under motoring conditions

The present study focuses on the development and a preliminary validation of a heat transfer model for the estimation of wall heat flux in HCCI engines via multi-zone modeling. The multi-zone model describes heat flow between zones and to the combustion chamber wall. Mass, species and enthalpy transfer, which affect the temperature field within the combustion chamber, are also considered between zones, accounting for the convective heat transfer terms. The multi-zone heat transfer model presented herein has been developed for HCCI combustion simulation and although it has been used in the past, its validation was based on cylinder pressure data under firing conditions. In the present study a more accurate validation of the model is conducted. This is achieved by comparing the multi-zone model heat loss rate predictions to the corresponding predictions of a validated CFD code. The cases examined correspond to actual motoring cases, against which the CFD code has been validated in a previous work. Moreover, a sensitivity analysis is presented, to assess the effect of the zone configuration, i.e. zone thickness and number, on the predicted heat loss rate and temperature profiles. In addition, a comparison is made between the results obtained from the proposed heat flux correlation and one in which the temperature gradient at the wall is approximated via finite differences.