Research On Interaction Of Physical And Chemical Factors In MULINBUMP Compound Combustion

Due to the shortage of energy and noble metal resources and the deterioration of environment, low temperature combustion including Homogeneous Charge Compression Ignition (HCCI) combustion and EGR-diluted combustion has attracted more and more attention in recent years. However, it is difficult to prepare homogeneous mixture for diesel fuel because of its low volatility, high viscosity and low resistance to self-ignition. Therefore, high pre-ignition mixing rate is the key factor to achieve HCCI combustion. For EGR-diluted low temperature combustion, the late cycle mixing rate determines the oxidation of unburned hydrocarbon (uHC) and CO. That is to say, the enhancement of the mixing rate in the middle and late cycle is the basis to obtain high efficiency combustion of engine. Consequently, achieving sufficiently rapid fuel-air mixing rates throughout the whole combustion history is the key technology to achieve new concept combustion. Based on the previous research, MULINBUMP compound combustion concept is further studied in this dissertation by CFD numerical simulation, engine experiment and visualization of the complicated physical and chemical processes involved in the new concept combustion by PLIF method. The main conclusions are as follows:The results from the PLIF test and CFD simulations in the constant-volume system indicate that low in-cylinder pressure and temperature in the condition of early injection lead to low in-cylinder density, therefore, the increase of penetration results in wall-wetting which restricts the pre-mixing of fuel and air. This problem can be avoided by employing the multi-pulse fuel injection strategy. Concentration and temperature stratifications by different fuel injection strategy effectively may be employed for control of the ignition and combustion process of premixed combustion. The optimization of multi-pulse fuel injection mode should target on the elimination of wall-wetting and the controlling of fuel concentration stratification.A combustion chamber named as spray-wall-guided combustion chamber is innovated based on the BUMP (Bump-Up Mixing Process) combustion chamber. The simulation results show that different configuration of the combustion chambers can lead to completely different flow patterns. The secondary space jet formed by spray-guiding and abrupt structure induces a pair of eddies turning reversely. Both eddies have similar scales to the structure of combustion chamber. In the spray-wall-guided combustion chamber, combustion process converts from the"semi-surface"combustion by the wall jet to"whole-surface"combustion by the secondary space jet. It results that the combustion rate in the middle and late cycle is intensified and the oxidation of soot is obviously enhanced, and therefore engine-out soot emission decreases up to 55% compared with the originalωcombustion chamber, while NOx emissions are slightly increased.When low temperature combustion is achieved with EGR which prolongs chemical time-scale (chemical factor), NOx and soot emissions decrease obviously. The typical heat-release rate diagram for heavy EGR rate case is the rapid premixed combustion phase, and following by a mixing-controlled process. Due to the decreased O2 concentration, a greater amount of ambient fluid must be mixed with the fuel to achieve the same fuel-air equivalence ratio, leading a tending incomplete combustion. Therefore, the increasing of the ratio of the premixed combustion or rapid late-cycle mixing rate is a promising approach to high efficiency power generation. The investigation combining with the control of fuel injection strategy (physical factor) indicates that MULINBUMP compound combustion can obtain trade-off among NOx, soot and thermal efficiency. Furthermore, the engine load is extended obviously compared with that of PCCI combustion.