Effects of Multi-Injection Mode on Diesel Homogeneous Charge Compression Ignition Combustion

Early injection, well before top dead center (TDC), has perhaps been the most commonly investigated approach to obtain homogeneous charge compression ignition (HCCI) combustion in a direct-injection (DI) diesel engine. However, wall wetting due to overpenetration of the fuel spray can lead to unacceptable amounts of unburned fuel and removal of lubrication oil. Another difficulty of diesel HCCI combustion is the control of combustion phasing. In order to overcome these difficulties, a multipulse fuel injection technology has been developed for the purpose of organizing diesel HCCI combustion, by which the injection width, injection number, and the dwell time between two neighboring pulse injections can be flexibly regulated. In present paper, the effects of a series of multipulse injection modes realized based on the prejudgment of combustion requirement, on engine emissions, thermal efficiency, and cycle fuel energy distribution of diesel HCCI combustion are studied. The designed injection modes include so-called even mode, hump mode, and progressive increase mode, and each mode with five and six pulses, respectively. Engine test was conducted with these modes. The experimental results show that diesel HCCI combustion is extremely sensitive to multipulse injection modes and that thermal efficiency can be improved with carefully modulated ones. There are many modes that can reach near zero $NOx$ and smoke emissions, but it is significant to be aware that multipulse injection mode must be carefully designed for higher thermal efficiency.