| Homogeneous charge compression ignition (HCCI) combustion, as a method of in-cylinder emission reduction, is considered as a key technique to make diesel engines satisfy more strict emission regulations. However, rough combustion at medium-to-high load restricts further load expansion. Quasi-homogeneous charge compression ignition (qHCCI) combustion is one kind of generalized HCCI combustion modes. In this paper, fuel injection during negative valve overlap period is used to realize qHCCI combustion, and in-cylinder CO2 injection is used to control combustion. By using this new method, effects of injection parameters, load expansion and closed-loop combustion control are studied. Considering that diesel-fueled HCCI engines use both HCCI and DICI combustion modes, mode transition is experimentally studied.First, fuel vapor distribution and gas injection are simulated in this paper. Fuel injection simulation reveals that, fuel injection must be near the exhaust TDC so that fuel can be injected into the combustion bowl. In addition, fuel-air mixture becomes richer from top to bottom of combustion chamber. Advancing fuel injection timing is beneficial to fuel air mixing. Estimation of CO2 injection energy reveals that momentum and kinetic energy of injected gas are larger than those of in-cylinder gas and injected fuel. Gas injection simulation reveals that better fuel air mixing is achieved, and gas injection direction is chosen according to the simulation.Based on the simulation results, experimental study of in-cylinder CO2 injection is conducted. Firstly, parameters of gas injector's nozzle are optimized. The results reveal that, nozzle direction has a large effect on experimental results. Gas should not only be injected directly into the piston bowl, where the mixture is rich according to simulation, to expedite air-fuel mixing, but also keep a certain angle with swirl so as not to weaken it. Secondly, by using the optimized gas injector, effects of gas injection timing, quantity and pressure on qHCCI combustion are studied. The results reveal that both advancing injection timing and increasing injection quantity will lower NO emission, and large pressure brings large disturbance and hence fuel-air mixing is improved and smoke opacity is decreased. By adjusting CO2 injection timing and injection quantity, NO emission can be reduced by 97%.Based on the optimized gas injection parameters, effects of external EGR and CO2 injection on combustion and load expansion of qHCCI combustion are studied. Smoke increases as EGR rate increases. However, CO2 injection does not affect smoke because it has little effect on fresh air quantity. In the experiment, CO2 injection can increase the upper load limit of qHCCI combustion by 15%; however, the maximum pressure rise rate becomes big.Considering that different operating condition (load and speed) needs different amount of CO2, a cycle-based closed-loop combustion phase control system is developed, realizing cycle-based cylinder pressure sampling, analysis and CA50 feedback control. CA50 is chosen to the feedback signal and Rassweiler-Withrow method is used to calculate CA50. The experimental results reveal that, the system can well follow target step input and can restrain system disturbances such as load and speed.By using closed-loop system, loading characteristics and qHCCI/CI combustion mode transition are studied. As to external EGR, the loading rate should not be too large to keep low emission level; as to CO2 injection, there is no limit on loading rate. The mode transition experiments reveal that, using direct transition from one combustion mode to the other, the output will overshoot or undershoot. The author proposes to insert several transition cycles with different fuel injection parameters, and effectively solve the problem.
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