Homogeneous charge compression ignition (HCCI) combustion of diesel fuel with external mixture formation

Abstract

Abstract HCCI (Homogeneous Charge Compression Ignition) has been touted for many years as the alternate technology of choice for future engines, preserving the inherent efficiency of CIDI (Compression Ignition Direct Injection) engines while significantly reducing emissions. The current direction for all published diesel HCCI research is mixture preparation using the direct injection – system, referred to as internal mixture formation. The benefit of internal mixture formation is that it utilizes an already available direct injection system. Direct injected diesel HCCI can be divided into two areas, early injection (early in the compression stroke) and late injection (usually after Top Dead Center (aTDC)). Early direct injection HCCI requires carefully designed fuel injector to minimize the fuel wall-wetting that can cause combustion inefficiency and oil dilution. Late direct injection HCCI requires a long ignition delay and rapid mixing rate to achieve the homogeneous mixture. The ignition delay is extended by retarding the injection timing and rapid mixing rate was achieved by combining high swirl with toroidal combustion-bowl geometry. There is a compromise between Direct Injection (DI) and HCCI combustion regimes. Even under ideal conditions, it can prove difficult to form a truly homogeneous charge, which leads to elevated emissions when compared to true homogenous charge combustion and also strongly contribute to the high sensitivity of the combustion phasing to external parameters. The alternative to the internal mixture formation is, predictably, external mixture formation. By introducing the fuel external to the combustion chamber one can use the turbulence intake process to create a homogeneous charge regardless of engine conditions. This eliminates the need for combustion system changes which were necessary for the internal mixture formation method. With this method, the combustion system remains fully optimized for direct injection and also capable of running in HCCI combustion mode with nearly ideal mixture preparation. The key to the external mixture formation with diesel fuel is proper mixture preparation. In the present investigation a fuel vapouriser was used to achieve excellent HCCI combustion in a single cylinder air-cooled direct injection diesel engine. No modifications were made to the combustion system. In this study a vaporized diesel fuel was mixed with air to form a homogeneous mixture and inducted into the cylinder during the intake stroke. To control the early ignition of diesel vapour–air mixture, cooled (30 °C) Exhaust Gas Recirculation (EGR) technique was adopted. Experiments were conducted with diesel vapour induction without EGR and diesel vapour induction with 10%, 20% and 30% EGR and results are compared with conventional diesel fuel operation (DI @ 23° before Top Dead Center (bTDC) and 200 bar injection pressure).