The HCCI engine could offer low NOx, PM emissions and high efficiency. However the operation region of the HCCI combustion is limited because of the knocking at high load and the misfire at low load. Moreover the HCCI principle lacks direct combustion control and needs a system to control the combustion phasing with high accuracy. Today there exists various ways to control the HCCI combustion, such as Variable Valve Train, Variable Compression Ratio, Inlet Air Heating and Dual Fuels. However such variable mechanisms and Inlet Air Heating tend to be heavy and complex. Dual Fuels method needs two types of fuels and has a challenge in infrastructure.
In this study, in order to develop a small and light HCCI engine, a simple HCCI combustion control system is proposed. DME (Di-methyl Ether) is used as the fuel to keep the structure small and light. In this system, the mixing ratio of three gases: stoichiometric pre-mixture, hot EGR gas and cold EGR gas is changed by only throttles. Characteristics of the proposed HCCI combustion control system are evaluated experimentally. Both the load and the combustion phasing can be controlled by only two throttles.
Focusing on each cycle data, cycle variability is observed. Therefore the nature of cycle variability is examined. It is caused by change of the mixing ratio of three gases and EGR property. For automatic control, output signals from engine are used as feedback signals. Therefore cycle variability must be low. The delay of CA50 (Crank Angle of 50% Heat Release), leads the high indicated thermal efficiency, but it also leads the increase of cycle variability. Though high indicated thermal efficiency is scarified, to avoid large cycle variability in automated control, CA50 around ATDC 7 degree is suitable in this system. Furthermore repeatability is observed in cycle variability of IMEP. There is a possibility to predict the future combustion.