Homogeneous Charge Compression Ignition Engine with Two-Stage Diesel Fuel Injection

Abstract

A diesel-fueled homogeneous charge compression ignition (HCCI) combustion technique using a two-stage injection strategy has been investigated in a single cylinder optical engine equipped with a commonrail fuel system. Although HCCI combustion has the advantages of reducing NOx and PM emissions, difficulties in vaporization of a diesel fuel and control of the combustion phase hinder the development of the HCCI engine. A two-stage injection strategy was applied to relieve these problems. Premixing of diesel fuel with air was realized mainly by the early injection strategy. The first injection, named as main injection, was an early direct injection of diesel fuel into the cylinder to achieve premixing with air. The second injection was a diesel injection of a small quantity (1.5 ㎣) as an ignition promoter and combustion phase controller near TDC. Effects of injection pressure and intake air temperature were studied with two-stage injection strategy. Concerning the injector geometry, a hole-type injector (5 holes) with small injection angle (100°) instead of conventional angle (150°) was applied to minimize the over-penetration that may prevent forming a premixed charge due to wall wetting. The multi-hole injector (14 holes) was also tested to maximize the atomization of spray. The experimental results showed that the two-stage injection could be used as a combustion phase controller only in the case of a low intake air temperature, but maintained the effect of ignition promotion although the ignition timing was advanced by the high intake air temperature. The main injection timing should be advanced earlier than BTDC 100°CA for the homogeneous and non-luminous combustion. The small injection angle was effective to minimize the overpenetration, and the multi-hole injector was effective to maximize the atomization so that the power output was increased, and exhaust gas was decreased. Exhaust results of the HCCI combustion optimized by this parametric study showed reduced NOx by more than 90% but increased HC, CO emissions compared with the conventional direct-injection (DI) diesel engine operation in the same engine. In addition, the application of the alternative gaseous fuel such as dimethyl ether (DME) was discussed implying the practical potential of HCCI operation.