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The purpose of this study was to gain a better understanding of the effects of port fuel injection strategies and thermal stratification on the HCCI combustion processes. Experiments were conducted in a single-cylinder HCCI engine modified with windows in the combustion chamber for optical access. Two-dimensional images of the chemiluminescence were captured using an intensified CCD camera in order to understand the spatial distribution of the combustion. N-heptane was used as the test fuel. The experimental data consisting of the in-cylinder pressure, heat release rate, chemiluminescence images all indicate that the different port fuel injection strategies result in different charge distributions in the combustion chamber, and thus affect the auto-ignition timing, chemiluminescence intensity, and combustion processes. Under higher intake temperature conditions, the injection strategies have less effect on the combustion processes due to improved mixing.

A new type direct-injection stratified-charge combustion system for gasoline engines is developed by the authors. In the system, gasoline is directly injected into a cylinder near the end of compression stroke by a nozzle with the injection holes unequally spaced on its tip. The angles among sprays in the vicinity of spark plug are small, and become larger downstream along the direction of air swirl motion. Therefore the circularly concentration stratification form rich to lean of air-fuel mixture is mechanically realized to ensure the reliable ignition and smooth flame propagation in the inhomogeneous mixture after sparking. The selection of main parameters of the system, the performance and the combustion characteristics of the engine after optimization of those parameters are introduced in detail in this paper.

Biobutanol, i.e. n-butanol, as a second generation bio-derived alternative fuel of internal combustion engines, can facilitate the energy diversification in transportation and reduce carbon dioxide (CO2) emissions from engines and vehicles. However, the majority of research was conducted on spark-ignition engines fuelled with n-butanol and its blend with gasoline. A few investigations were focused on the combustion and exhaust emission characteristics of homogeneous charge compression ignition (HCCI) engines fuelled with n-butanol-gasoline blends. In this study, experiments were conducted in a single cylinder four stroke port fuel injection HCCI engine with fully variable valve lift and timing mechanisms on both the intake and exhaust valves. HCCI combustion was achieved by employing the negative valve overlap (NVO) strategy while being fueled with gasoline (Bu0), n-butanol (Bu100) and their blends containing 30% n-butanol by volume (Bu30).

Homogeneous charge compression ignition (HCCI) technology is promising to reduce engine exhaust emissions and fuel consumption. However, it is still confronted with the problem of its narrow operation range that covers only the light and medium loads. Therefore, to expand the operation range of HCCI, mode switching between HCCI combustion and transition SI combustion is necessary, which may bring additional problems to be resolved, including load fluctuation and increasing the complexity of control strategy, etc. In this paper, a continuously adjustable load strategy is proposed for gasoline engines. With the application of the strategy, engine load can be adjusted continuously by the in-cylinder residual gas fraction in the whole operation range. In this research, hybrid combustion is employed to bridge the gaps between HCCI and traditional SI and thus realize smooth transition between different load points.

Exhaust Gas Recirculation (EGR) is an important parameter for control of diesel engine combustion, especially to achieve ultra low NOx emissions. In this paper, the effects of EGR on engine emissions and engine efficiency have been investigated in a heavy-duty diesel engine while changing combustion control parameters, such as injection pressure, injection timing, boost, compression ratio, oxygenated fuel, etc. The engine was operated at 1400 rpm for a cycle fuel rate of 50mg. The results show that NOx emissions strongly depend on the EGR rate. The effects of conventional combustion parameters, such as injection pressure, injection timing, and boost, on NOx emissions become small as the EGR rate is increased. Soot emissions depend strongly on the ignition delay and EGR rate (oxygen concentration). Soot emissions can be reduced by decreasing the compression ratio, increasing the injection pressure, or burning oxygenated fuel.

Chemical reaction kinetics plays an important role in homogeneous charge compression ignition (HCCI) combustion. In order to control the combustion process, the underlying mechanism of auto-ignition must be explored, especially for the HCCI combustion using negative valve overlap (NVO) strategy, in which the residual gas affects the auto-ignition of next cycle remarkably. In this research, experimental research was carried out in a single cylinder gasoline engine equipped with an in-cylinder sampling system which mainly consists of a special spark plug, a sampling tube and a high-speed electromagnetic valve. In-cylinder charge was sampled at compression stroke and analyzed by FTIR with two types of fuel injection strategy, such as port fuel injection (PFI) solely and port fuel injection combined with injection during negative valve overlap (PFI & NVO-Injection).

Approximately 50% energy is consumed during the acceleration of a city bus. Fuel consumption during acceleration is significantly affected by driving behavior. In this study, 13 characteristic parameters were selected to describe driving style based on analysis of how driving influences fuel consumption during acceleration. The 100,000 km real-world vehicle running data of six drivers on three city buses in a particular bus line in Tianjin, China were sampled using a vehicle-on-line data logger. Based on the selected characteristic parameters and collected driving data, an evaluation model of the fuel consumption level of a driver was established by adopting the method of decision tree C4.5. For two-level classification, the model has over 85% prediction accuracy. The model also has the advantages of having a few training samples and strong generalization. As an example of the model application, the fuel-saving potential of a driver under optimal operations was analyzed.

To investigate the effects of fuel volatility on combustion and emissions in a diesel engine, a high-volatility fuel of n-heptane was blended into diesel fuel with different volumetric fractions (0%, 40%, 70%, 100%). A wide range of EGR rates from 0% to 65% were investigated, which covered both the conventional diesel combustion and low temperature combustion. Experiments under two engine load conditions, ∼5.2 bar and ∼10.5 bar gross IMEP were performed at 1500 rpm. The injection timing was fixed at 8°CA BTDC for all test cases. Results show that even if the ignition delay and combustion duration are nearly the same for all tested fuels, the premixed combustion fractions are increased for higher volatility fuels due to the improvement on mixing process during the ignition delay period. The indicated specific fuel consumption is decreased as using high-volatility fuels. The effect of fuel volatility on soot emissions depends on engine loads.

It is found that a thin and rich mixture layer on a wall is formed after impingement of a gas jet of fuel on the wall. The measured thickness of the mixture layer is about 2 mm. and its dispersion rate after the end of injection is much lower, compared to that of a space gas jet. This phenomenon in a small D.I. diesel engine is known as “wall wetting” or “wall fuel accumulation” which has an important influence on engine fuel consumption and emissions. This paper presents a technique for enhancing the near wall mixing of an impinging jet by means of a bump on the wall. The development of a wall jet formed after an impingement of a gas jet has been investigated by simultaneously measuring the near wall velocity and concentration distribution. It has been found that a wall jet is stripped off the wall and ejected as a secondary jet when the wall jet encounters a bump of the wall.

Homogeneous charge compression ignition (HCCI) technology is promising to reduce engine exhaust emissions and fuel consumption in gasoline engine. However, it is still confronted with the problem of its limited operation range. High load is limited by the tradeoff between the quantity of working charge and dilution charge. Low load is limited by the high residual gas fraction and low temperature in the cylinder. One of the highlights of HCCI combustion research at present is to expand the low load limit of HCCI combustion by developing HCCI idle operation. The main obstacle in developing HCCI idle combustion is too high residual gas fraction and low temperature to misfire in cylinder. This paper relates to a method for achieving the appropriate environment for auto-ignition at idle and the optimal tradeoff between the combustion stability and fuel consumption by employing EIVO valve strategy with an equivalent air-fuel ratio.

The dual-fuel Reactivity Controlled Compression Ignition (RCCI) combustion could achieve high efficiency and low emissions over a wide range of operating conditions. However, further high load extension is limited by the excessive pressure rise rate and soot emission. Polyoxymethylene dimethyl ethers (PODE), a novel diesel alternative fuel, has the capability to achieve stoichiometric smoke-free RCCI combustion due to its high oxygen content and unique molecule structure. In this study, experimental investigations on high load extension of gasoline/PODE RCCI operation were conducted using late intake valve closing (LIVC) strategy and intake boosting in a single-cylinder, heavy-duty diesel engine. The experimental results show that the upper load can be effectively extended through boosting and LIVC with gasoline/PODE stoichiometric operation.

The spark-assisted compression ignition (SACI) is widely used to expend the high load limit of homogeneous charge compression ignition (HCCI), as it can reduce the high heat release rate effectively while partially maintain the advantage of high thermal efficiency and low NOx emission. But as engine load increases, the SACI combustion traditionally using negative valve overlap strategy (NVO) faces the drawback of higher pumping loss and limited intake charge availability, which lead to a restricted load expansion and a finite improvement of fuel economy. In this paper, research is focused on the SACI combustion using positive valve overlap (PVO) strategy. The characteristics of SACI combustion employing PVO strategy with external exhaust gas recirculation (eEGR) are investigated. Two types of PVO strategies are analyzed and compared to explore their advantages and defects, and the rules of adjusting SACI combustion with positive valve overlap are concluded.

Natural gas is regarded as one of the most potential alternative fuels of engines because of its fruitful storage, good obtainable resources, and high octane value, etc. Compared with port fuel injection (PFI), Gasoline direct injection (GDI) has many advantages on volumetric efficiency improvement, lean-burning, emissions control, etc. A set of measuring and data acquisition system on CNG direct injection spark-ignited engines and the CNG multi-direct injection system are developed in this paper. Based on different injection modes of CNG single DI and compound DI (main injection and auxiliary-Injection), the investigations on combustion process of CNG DI engines has been conducted by the factors of in-cylinder mixture formation, air/fuel ratio, and injection timing, etc. Meanwhile, the three-dimension simulation on mixture formation and combustion principle of low-pressure CNG compound direct injection engines has also been researched.

Droplet velocities in a diesel fuel spray before and after impinging on a wall as well as air movement around the spray are measured at room temperature and pressure. The range of fuel injection pressure is from 101 MPa to 139 MPa. The diagnostic equipment is a Laser Doppler Velocimetry with Burst Spectrum Analyzer (LDV-BSA).The results show that the droplet velocities of such a high pressure diesel fuel spray spread in a wide range (about 0-250m/s), so it is necessary to use the ensemble average for describing the velocity variation with time and space. After injection, the velocity reaches its peak value rapidly then attenuates gradually. When the spray impinges on the wall, the average velocity of the rebounded droplets is obviously reduced and the rebounded angle of most droplets is smaller than 30 degree when the incident angle is 70 degree. In the near field zone, the air entrainment in spray jet appears to be lower than that in gaseous one.

Spray behaviors of pure biodiesel and its blend with conventional diesel have been substantially studied in the last decade. However, the studies on the spray behaviors of pure fatty acid methyl esters (FAMEs) are scarce. The primary components of most biodiesel fuels are methyl palmitate (C16:0), methyl stearate (C18:0), methyl oleate (C18:1), methyl linoleate (C18:2) and methyl linolenate (C18:3), and methyl laurate (C12:0) is also the dominant component of some biodiesels. In this study, the spray behaviors of the aforementioned six FAMEs in biodiesel fuels under engine-relevant conditions were numerically studied using the KIVA-3V code. The physical properties needed for spray modeling were predicted with most recently developed property prediction models and added into the fuel library of KIVA-3V. The transient behaviors of liquid penetrations and vaporization characteristics of these FAMEs were numerically studied under various engine-relevant conditions.

A numerical simulation was performed to investigate the pilot ignited natural gas combustion process in a direct injection natural gas engine. Various mixture distribution characteristics were compared in terms of the evolution of mixture equivalent ratio distributions and mixture concentration stratifications around top dead center (TDC). Based on above, the pilot injections were specially designed to investigate ignition core formation and its effects on natural gas combustion process. The result shows that pilot ignition sites have great impacts on pilot fuel ignition process and natural gas combustion process. The pilot ignition site on the region with rich NG/Air mixture is disadvantageous to the pilot fuel ignition due to a lack of oxygen, which is not beneficial to ignition core formation.

Reactivity Controlled Compression Ignition (RCCI) has been shown to be an attractive concept to achieve clean and high efficiency combustion. RCCI can be realized by applying two fuels with different reactivities, e.g., diesel and gasoline. This motivates the idea of using a single low reactivity fuel and direct injection (DI) of the same fuel blended with a small amount of cetane improver to achieve RCCI combustion. In the current study, numerical investigation was conducted to simulate RCCI and HCCI combustion and emissions with various fuels, including gasoline/diesel, iso-butanol/diesel and iso-butanol/iso-butanol+di-tert-butyl peroxide (DTBP) cetane improver. A reduced Primary Reference Fuel (PRF)-iso-butanol-DTBP mechanism was formulated and coupled with the KIVA computational fluid dynamic (CFD) code to predict the combustion and emissions of these fuels under different operating conditions in a heavy duty diesel engine.

SI-CAI hybrid combustion, also known as spark-assisted compression ignition (SACI), is a promising concept to extend the operating range of CAI (Controlled Auto-Ignition) and achieve the smooth transition between spark ignition (SI) and CAI in the gasoline engine. In order to stabilize the hybrid combustion process, the port fuel injection (PFI) combined with gasoline direct injection (GDI) strategy is proposed in this study to form the in-cylinder fuel stratification to enhance the early flame propagation process and control the auto-ignition combustion process. The effect of bowl piston shapes and fuel injection strategies on the fuel stratification characteristics is investigated in detail using three-dimensional computational fluid dynamics (3-D CFD) simulations. Three bowl piston shapes with different bowl diameters and depths were designed and analyzed as well as the original flat piston in a single cylinder PFI/GDI gasoline engine.

The spark ignition-controlled auto-ignition (SI-CAI) hybrid combustion is promising in achieving smooth transition between SI and CAI combustion but, it is limited by the combustion cyclic-variation at late combustion phasing to avoid too high pressure rise rate (PRR). In this paper, to stabilize the combustion and reduce PRR, the in-cylinder fuel-stratification strategy is investigated in a gasoline engine, equipped with port fuel injection combined with single pulse direct injection (PFI-DI). Experimental results confirm the benefits of employing PFI-DI in comparison with PFI and single-pulse DI strategy. The influence of DI timing (Start of injection, SOI) on the combustion process is found to be quite complicated, in terms of combustion phasing, combustion stability, PRR and thermal efficiency. It makes the optimal-SOI calibration time-intensive, since complex trade-off between PRR and thermal efficiency is needed.

A pressure accumulative injection rate controllable unit injector-PAIRCUI is proposed and developed. This unit injector is powered by fuel pressure accumulation controlled by an electronic control unit and its injection rate is shaped by inner valves of the injector. Inherent advantages of an accumulator type unit injector have been carried out in this new design, including sructural simplicity, totally flexible injection timing, medium common rail pressure, tolerable pump size and flow requirement. A number of decisive features have also been realized that are significant for high efficiency and low emissions of engine combustion, including higher mean effective injection pressure(MEIP), pilot injection capability and rapid end of injection. The injection pressure is independent of engine speed, but regulated upon engine load. These characteristics are beneficial in improving engine performance and fuel consumption.