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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.

A HCCI engine has been run at different operating boundaries conditions with fuels of different RON and MON and different chemistries. The fuels include gasoline, PRF and the mixture of PRF and ethanol. Six operating boundaries conditions are considered, including different intake temperature (Tin), intake pressure (Pin) and engine speed. The experimental results show that, fuel chemistries have different effect on the combustion process at different operating conditions. It is found that CA50 (crank angle at 50% completion of heat release) shows no correlation with either RON or MON at some operating boundaries conditions, but correlates well with the Octane Index (OI) at all conditions. The higher the OI, the more the resistance to auto-ignition and the later is the heat release in the HCCI engine. The operating range is also correlation with the OI. The higher the OI, the higher IMEP can reach.

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.

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 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.

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.

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.

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.

In the last few years, residual gas trapping has been widely used to achieve CAI combustion operation in the four-stroke gasoline engine by means of the negative valve overlap period. In this paper, a flexible mechanical variable valve actuation system based on the production technologies is described. The 4VVAS system is capable of independent control of intake valve lift and its timing, exhaust valve lift and its timing and it has been incorporated in a specially designed cylinder head for a single cylinder research engine. In addition, an engine simulation program has been developed to investigate the potential of the 4VVAS system for CAI engine operation and the switch between CAI and SI operations on the same engine. The engine simulation program is written with Matlab Simulink and incorporates an engine block, a newly developed CAI ignition and heat release model, a valve profile generator, and an engine control module for spark ignition and fuelling control.

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.

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.

In this paper, HCCI combustion characteristics of three typical high octane number fuels, gasoline, ethanol and methanol, are compared in a Ricardo single cylinder port injection engine with compression ratio of 10.5. In order to trap enough high temperature residual gas to heat intake mixture charge for stable HCCI combustion, camshafts of the experimental engine are replaced by a set of special camshafts with low valve lift and short cam duration. The three fuels are injected into the intake port respectively in different mixture volume percentages, which are E0 (100% gasoline), E50 (50% gasoline, 50% ethanol), E100 (100% ethanol), M50 (50% gasoline, 50% methanol) and M100 (100% methanol). This work concentrates on the combustion and emission characteristics and the available HCCI operation range of these fuels. What's more, the detailed comparison of in-cylinder temperature, ignition timing and other parameters has been carried out.

To achieve homogeneous charge compression ignition (HCCI) combustion in the range of low speeds and loads, special camshafts with low intake/exhaust cam lift and short intake/exhaust cam duration were designed. The camshafts were mounted in a Ricardo Hydra four-stroke single cylinder port fuel injection gasoline engine. HCCI combustion was achieved by controlling the amount of trapped residuals from previous cycle through negative valve overlap. The results show that indicated mean effective pressure (IMEP) depends on valve timings, engine speeds and lambda. Early exhaust valve closing (EVC) timings result in high residual fractions in the cylinder and low air mass sucked into the cylinder. As a result, combustion duration increases, IMEP and peak pressure decrease. However, pumping losses decrease. High engine speed has the similar effect on HCCI combustion characteristics as early EVC timings do. But inlet valve opening timings have slight effect on IMEP compared to EVC timings.

The effects of cooled EGR on combustion and emission characteristics in HCCI operation region was investigated on a single-cylinder diesel engine, which is fitted with port injection of DME and methanol. The results indicate that EGR rate can enlarge controlled HCCI operating region, but it has little effect on the maximum load of HCCI engine fuelled with DME/methanol dual-fuels. With the increase of EGR rate, the main combustion ignition timing (MCIT) delays, the main combustion duration (MCD) prolongs, and the peak cylinder pressure and the peak rate of heat release decreases. Compared with EGR, DME percentage has an opposite effect on HCCI combustion characteristics. The increase of indicated thermal efficiency is a combined effect of EGR rate and DME percentage. Both HC and CO emissions ascend with EGR rate increasing, and decrease with DME percentage increasing. In normal combustion, NOX emissions are near zero.

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.

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.

One-dimensional engine simulation programmes are often used in the engine design and optimization studies. One of the key requirements of such a simulation programme is its ability to predict the heat release process during combustion. Such simulation software has built in it the heat release models for spark ignited premixed flame and compression ignited diesel combustion. The recent emergence of Controlled Auto Ignition (CAI) combustion, also known as Homogeneous Charge Compression Ignition (HCCI), has generated the need for a third type of heat release models for this new combustion process. In this paper, a heat release correlation for CAI combustion has been derived from extensive in-cylinder pressure data obtained from a Ricardo E6 single cylinder research engine and a multi-cylinder Port Fuel Injection (PFI) gasoline engine running with CAI combustion. The experimental data covered a wide range of air/fuel ratios, speed and percentage of residual gas.

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.

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 paper has presented a new reduced chemical kinetic model for the Homogeneous Charge Compression Ignition (HCCI) combustion of n-heptane in an engine, which contains 41 species and 63 reactions. The new model includes three sub-models: the first is the low-temperature reaction sub-model, which is established by determining particular aldehydes and small hydrocarbons in the model developed by Li et al. The second is the sub-model for large molecules decomposing directly into small molecules that is developed for linking the low-temperature reaction with high-temperature reaction. The third is used for high-temperature reaction, which is derived by several modifications to the model developed by Griffiths et al., eliminating several reactions, adding two oxidization reactions related to CO and CH3O.