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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.

Engine experiments and CFD modeling studies have been carried out and shown that high density-low temperature combustion (H Density-LTC) has the potential of realizing high thermal efficiency and very low engine-out emissions at high and full engine loads. Parametric studies were conducted to explore the mechanism of formation of pollutants in high charge density in this paper. It was found that high charge density was normally favorable to spray atomization, evaporation and fuel/air mixing throughout the entire combustion process, but there was a turning value of charge density above which the improvement of thermal efficiency was reduced. The conversion of CO to CO₂ was accelerated and CO emission was decreased with increasing charge density, which was also proved to be beneficial to re-oxidation of soot formed. The oxygen concentration brings a conflict effect to NOx emissions and exhaust soot. The high density combustion relieved the conflict effect of oxygen concentration.

FIRCRI-a flexible injection rate common rail injector was developed. This paper presents the working principle and the configuration of the injector. As key technologies in development of the injector, a new fast response solenoid valve was developed and 4 dimensionless design parameters of hydraulic system were presented by through computer simulation and experimental study. The solenoid valve was deliberately designed so as to eliminate the hydraulic force acting on the valve. Other configuration parameters were also optimized so that the response time of the solenoid valve is 0.3 ms. It is interesting to find that the response time of the injector is not only determined by the solenoid valve, but all parameters of the hydraulic system of the injector. The injector can realize pilot injection, which is less than 2.5mm3, at a controllable phase and multi-injections.

The effects of jet impingement and turbulence structure on jet mixing rate arc investigated by using a simultaneous measurement system of concentration and velocity. It is found that, after the end of injection, dispersion of momentum of a CO2 jet Is much faster than that of concentration. The residual gas is then left in a rather quiescent atmosphere, that makes a sluggish mixing process of the residual gas. Jet impingement can create intensive air motion in the near impingement region, that promotes the mixing of the residual gas. However, the effect of jet impingement decreases as the distance from the impingement point increases.

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 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.