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The influence of boost pressure (Pin) and fuel chemistry on combustion characteristics and performance of homogeneous charge compression ignition (HCCI) engine was experimentally investigated. The tests were carried out in a modified four-cylinder direct injection diesel engine. Four fuels were used during the experiments: 90-octane, 93-octane and 97-octane primary reference fuel (PRF) blend and a commercial gasoline. The boost pressure conditions were set to give 0.1, 0.15 and 0.2MPa of absolute pressure. The results indicate that, with the increase of boost pressure, the start of combustion (SOC) advances, and the cylinder pressure increases. The effects of PRF octane number on SOC are weakened as the boost pressure increased. But the difference of SOC between gasoline and PRF is enlarged with the increase of boost pressure. The successful HCCI operating range is extended to the upper and lower load as the boost pressure increased.

By mixing iso-octane with octane number 100 and normal heptane with octane number 0, it was possible to obtain a PRF fuel with octane rating between 0 and 100. The influence of PRF fuel’s octane number on the combustion characteristics, performance and emissions character of homogeneous charge compression ignition (HCCI) engine was investigated. The experiments were carried out in a single cylinder direct injection diesel engine. The test results show that, with the increase of the octane number, the ignition timing delayed, the combustion rate decreased, and the cylinder pressure decreased. The HCCI combustion can be controlled and then extending the HCCI operating range by burning different octane number fuel at different engine mode, which engine burns low octane number fuel at low load mode and large octane number fuel at large load mode. There exists an optimum octane number that achieves the highest indicated thermal efficiency at different engine load.

With the application of valve timing strategy to inlet and exhaust valves, Homogeneous Charge Compression Ignition (HCCI) combustion was achieved by varying the amount of trapped residuals through negative valve overlap on a Ricardo Hydra four-stroke port fuel injection engine fueled with ethanol. The effect of ethanol on HCCI combustion and emission characteristics at different air-fuel ratios, speeds and valve timings was investigated. The results indicate that HCCI ethanol combustion can be achieved through changing inlet and exhaust valve timings. HCCI ethanol combustion range can be expanded to high speeds and lean burn mixture. Meanwhile, the factors influencing ignition timing and combustion duration are valve timing, lambda and speeds. Moreover, NOx emissions are extremely low under HCCI combustion. The emissions-speed and emissions-lambda relationships are obtained and analyzed.

Homogeneous Charge Compression Ignition (HCCI) has the potential of reducing fuel consumption as well as NOx emissions. However, it is still confronted with problems in real-time control system and control strategy for the application of HCCI, which are studied in detail in this paper. A CAN-bus-based distributed HCCI control system was designed to implement a layered close loop control for HCCI gasoline engine equipped with 4VVAS. Meanwhile, a layered management strategy was developed to achieve high real-time control as well as to simplify the couplings between the inputs and the outputs. The entire control system was stratified into three layers, which are responsible for load (IMEP) management; combustion phase (CA50) control and mechanical system control respectively, each with its own specified close loop control strategy. The system is outstanding for its explicit configuration, easy actualization and robust performance.

In Spark Ignition (SI)-Controlled Auto Ignition (CAI) hybrid combustion, the in-cylinder temperature and total mass of dilution charge are usually increased compared to the traditional SI engine in order to achieve and control the auto-ignition combustion, which would in turn lead to the variations of the diluted flame propagation combustion. In this study, the optical measurements were performed to understand the flame characteristics at highly diluted conditions. The results showed that the decrease of the flame propagation speed of rich mixture was less than that of lean mixture at highly diluted conditions. However, the inhomogeneous distribution of residual gas led to asymmetric development of flame propagation. The high temperature, strong dilution and rich mixture created local auto-ignition sites which were located in front of the main flame and gradually merged with the main flame.

A strategy to actualize the dual-mode (SI mode and HCCI mode) operation of gasoline engine was investigated. The 4VVAS (4 variable valve actuating system), capable of independently controlling the intake and exhaust valve lifts and timings, was incorporated into a specially designed cylinder head for a single cylinder research engine and a 4VVAS-HCCI gasoline engine test bench was established. The experimental research was carried out to study the dynamic control strategies for transitions between HCCI and SI modes on the HCCI operating boundaries. Results show that equipped with the 4VVAS cylinder head, the engine can be operated in HCCI or SI mode to meet the demands of different operating conditions. 4VVAS enables the rapid and effective control over the in-cylinder residual gas, and therefore dynamic transitions between HCCI and SI can be stably achieved. It is easier to achieve transition from HCCI to SI than reversely due to the influence of thermo-inertia.

In order to understand premixed charge compression ignition (PCCI) combustion, a new combustion model of kinetic-and-turbulent characteristic-time has been developed. A ununiformity function H(ϕ)was presented by analysis of the effect of fuel/air distributions on the role of turbulent timescale in the combustion model, then an analytical turbulent timescale coefficient f was deduced, which was proved to be able to correlate the fuel ununiformity with the turbulent timescale in the combustion model. The new model was employed for simulation of a PCCI combustion organized by various multi-pulse injection strategies in a heavy duty diesel engine. The simulation results agreed with the experimental data well. The ignition process of a PCCI combustion organized by multi-pulse injection was a separated volume autoignition process, which was strongly influenced by the condition of fuel stratification.

In this research, numerical simulation using Star-CD is performed to investigate the mixing process of a single-cylinder experimental gasoline engine equipped with 4VVAS (4 Variable Valve System). Different engine operating conditions are studied with respect to valve parameters, including EVC (Exhaust Valve Closing), IVO (Intake Valve Opening), and IVL (Intake Valve Lift). The definitions of RGF (Residual Gas Fraction)/temperature statistical distribution and inhomogeneity are proposed and quantified, on which the influences of the aforementioned valve parameters are analyzed. Results reveal that, the distribution of in-cylinder residuals varies with valve parameter combinations. Intake valve timing has a greater effect on the in-cylinder distribution and inhomogeneity of residuals than intake valve lift. Earlier IVO leads to lower RGF inhomogeneity around TDC.

This paper concerns the secondary influence factors of combustion noise under transient conditions of DI-Diesel engines. By designing combustion noise test in transient and steady conditions, the secondary influence factors of combustion noise are measured. At the same load and rotational speed, the secondary influence mechanism of combustion noise is studied by analyzing indirect influence factors of combustion noise, such as the temperature of combustion chamber wall, the pressure of fuel injection and the needle lift between transient and steady conditions. The difference of the secondary influence factors in two conditions affects the aerodynamic load and the high-frequency oscillation, which will further influence the combustion noise of steady and transient conditions. The secondary influence mechanism of combustion under transient conditions is also studied at different fuel supply advance angles.

The simulation of main bearing load plays an important role in engine multi-body dynamics simulation, seemingly influencing the simulation of strength, vibration and acoustics. It is necessary to conduct engine bench test to validate the result of simulation. More attention has been paid to the flexibility of engine blocks and crankshafts, but not on the installing conditions of engine test bench, such as the stiffness of mounts, the presence of the connecting flange and the elasticity of shaft coupling, which are easy to ignore. The work presented here focuses on the influence of bench installation conditions on the multi-body dynamics simulation of an engine. A flexible multi-body dynamics model of a 4100QB diesel engine is built by employing the modal synthesis technique in the software ADAMS. By comparing the simulation results of different models, the effects of the connecting flange, the stiffness of mounts and the elasticity of shaft coupling are discussed.

A numerical study was carried out to investigate the chemical reaction mechanism encountered in the homogenous charge compression ignition (HCCI) process of dimethyl ether (DME) and methanol dual fuel mixture by using a zero-dimensional thermodynamic model coupled with a detailed chemical kinetic model. The results show that methanol affects the DME oxidation path, low temperature reaction (LTR) of DME is inhibited and the heat release shape of dual-fuel only shows a one-stage heat release, owning to the heat released by high temperature reaction (HTR) of DME and methanol, including blue-flame and hot-flame reactions. In dual fuel reaction, the second molecular oxygen addition of DME is restrained, and the thermal decomposition reaction of the methoxymethyl radical (CH3OCH2) named β -scission plays a more important role in DME oxidation. Also, HTR of DME and methanol, including blue-flame and hot-flame reactions, almost occur at the same time.

The effect of a new exhaust catalyst system consisting of the traditional three way catalyst converter (TWC) and adsorber-reduction catalyst converter along the emission systems has been investigated in this research under different schemes of catalyst converter arrangement and different ranges of engine speed and engine load. The effect on BSFC (Brake Specific Fuel Consumption) of a lean burn gasoline engine has also been studied at the same time. The upstream placement of TWC ahead of the NOx Adsorber Catalyst has been selected, which is corresponding to the highest NOx converting efficiency. The effect of engine speed on the exhaust emission and BSFC is related to absolute time and the ratio of lean burn time to rich burn time. Engine load is the major factor in affecting exhaust emission characteristics and BSFC of lean burn gasoline engine.

Both in conventional diesel combustion and the low temperature combustion represented by PCCI and EGR-diluted combustion, high mixing rate at the whole combustion history is the key to achieve comparative clean and high-efficiency combustion. In this study, a newly developed combustion chamber, vortex-induced combustion chamber which can enhance middle and late cycle combustion is developed based on BUMP combustion chamber investigated in previous study. And then, the combustion and emission characteristics in the circumstances of diluted combustion are studied. For low oxygen concentration cases, heat release rate goes down and combustion efficiency decreases due to decreased mixing efficiency. The results of chamber design indicate complex structure of flow can be realized by special designed chamber geometry. The velocity difference in the interface of the vortexes will benefit to mixing of fuel and air, therefore combustion and emissions.

Engine experiments have shown that simultaneous reductions of NOx and soot emissions can be achieved by the so called BUMP (Bump-up mixing process) combustion chamber. In order to understand the underlying mechanism of emission reduction, a STAR-CD based multi-dimensional combustion modeling was carried out for a heavy-duty diesel engine with the BUMP combustion chamber. The results from an impingement gas jet experiment were also presented and compared with computer modeling. The results showed that complex air motion with high turbulence was obtained by adoption of the bump ring. The fuel/air mixing rate was promoted greatly. Therefore, for the BUMP combustion chamber, much fuel fell in the optimum equivalence ratio range than that of the baseline chamber.

In order to understand the effects of pulse injection mode on power output and emissions in an HCCI diesel engine, the pulse injection mode modulation was investigated. A computer simulation code of common rail injector FIRCRI was developed based on previous work by the authors, including the simulation of dynamic response and injected fuel amount. Then the injector parameters were partly revised to meet the requirement of pulse injections. By variation of control signals, a series of injection modes were realized based on the prejudgment of combustion requirement. The designed injection modes included so called even mode, staggered mode, hump mode and progressive increase mode with four, five and six pulses. Engine test was conducted with the designed injection modes. The experimental results showed that the HCCI diesel combustion was extremely sensitive to injection mode.

The influence of injection pressure fluctuations on cavitation inside a nozzle hole under diesel engine conditions was simulated using a two-fluid model. Injection pressure fluctuations with different amplitudes and frequencies were introduced at the inlet boundary. A comparison of the calculated results with experimental results available in the literatures was performed to verify the model. The simulation results indicate that both partial cavitation and supercavitation are just sensitive to the inlet pressure fluctuations with higher amplitude. As the amplitude decreases, the influence of the pressure fluctuations on cavitation process diminishes and it becomes negligible at the amplitude of around 5%. The frequency of inlet pressure fluctuation has an obvious influence on the inception of cavitation and quasi-periodic behaviors of supercavitation.

A concept of high density-low temperature combustion (HD-LTC) is put forward in this paper, showing potential of its high thermal efficiency and very low engine-out emissions by engine experimental and CFD modeling study. A single cylinder test engine has been built-up equipped with mechanisms of variable boost pressure and intake valve closing timing (IVCT). By delaying IVCT and raising boost pressure to certain values according to engine loads, the in-cylinder charge density is regulated much higher than in conventional engines. It is found that the high charge density can play the role of rising of heat capacity as exhaust gas recirculation (EGR) does. Thereby low temperature combustion is realized with less EGR (about 18~19% oxygen concentration) to achieve very low NOx and soot emissions, which is extremely important at high and full loads.

Diesel-fueled HCCI combustion was achieved by multi-pulse injection before top dead center (TDC). However, the multi-pulse injections strategies have not been sufficiently studied previously due to the large number of parameters to be considered. In the present work, a series of multi-pulse injection modes with four or five pulses in each mode are designed, and their effects on diesel HCCI Combustion are experimentally studied. The results showed that the HCCI diesel combustion was extremely sensitive to injection mode. There were many modes to achieve very low NOx and smoke emissions, but the injection parameters of these modes must be optimized for higher thermal efficiency. A micro-genetic algorithm coupled with a modified 3D engine simulation code is utilized to optimize the injection parameters including the injection pressure, start-of-first-injection timing (SOI), fuel mass in each pulse injection and dwell time between consecutive pulse injections.

The effects of Exhaust Gas Recirculation (EGR) and octane number of PRF fuel on combustion and emission characteristics in HCCI operation were investigated. The results show that EGR could delay the ignition timing, slow down the combustion reaction rate, reduce the pressure and average temperature in cylinder and extend the operation region into large load mode. With the increase of the fuel/air equivalence ratio or the fuel octane number (ON), the effect of EGR on combustion efficiency improves. With the increase of EGR rate, the combustion efficiency decreases. The optimum indicated thermal efficiency of different octane number fuels appears in the region of high EGR rate and large fuel/air equivalence ratio, which is next to the boundary of knocking. In the region of high EGR rate, HC emissions rise up sharply as the EGR rate increases. With the increase of octane number, this tendency becomes more obvious.

Studies on combustion process of Dimethyl Ether (DME) were carried out on a constant volume combustion bomb (CVCB) and a visualization engine, and the photograph of combustion of DME was taken by high speed digital CCD. The results show that the ignition delay of DME is shorter than that of diesel fuel. When the fuel delivery amounts of DME and diesel in volume are the same, the combustion duration of DME is shorter than that of diesel fuel, and the flame temperature of DME is lower than that of diesel. At the end of combustion, the second injection occurred. The results of high-speed photograph in visualization engine show that, as soon as DME leaves the nozzle, it evaporates rapidly, and under the effect of air swirl, the spray“core” is blown off. Compared to diesel, the penetration of DME is shorter, and the wall combustion of DME is apparently smaller.