Experimental comparison of different injection timings in an HCCI engine fueled with n-heptane

Abstract

One of the most effective methods to control the combustion phasing of a homogeneous charged compression ignition (HCCI) engine is injection timing. Injection timing affects the homogeneity of the mixture, start of combustion and heat release rate. In this study, the effects of injection timing on in-cylinder pressure, heat release rate, pressure rise rate and combustion duration were investigated in early direct injection HCCI engine as experimentally. The experiments were performed at 6 different injection timings ranging between 20 ◦bTDC and 270 ◦bTDC using n-heptane as the fuel. Test results showed that maximum in-cylinder pressure increased and start of combustion was advanced when the injection timing was advanced. Maximum in-cylinder pressures of 4.7 bar at 2 ◦bTDC and 3.4 bar at -9 ◦bTDC were obtained at injection timings of 270 ◦bTDC and 20 ◦bTDC, respectively. Moreover, single stage HCCI combustion was observed and combustion duration increased when the fuel was injected towards to TDC. An advance in the injection timing resulted in an increase in maximum pressure rise rate (MPRR). MPRR of 1.1 bar/oCA at 6 oCA bTDC was obtained at an injection timing of 180 oCA bTDC. Therefore, combustion phasing in HCCI engines was controlled by changing the injection timing and the knock limit was estimated.