Homogeneous Charge Compression Ignition (HCCI) of Diesel Fuel

This paper describes the ongoing homogeneous charge compression ignition (HCCI) research being carried out at Southwest Research Institute (SwRI). Summaries of the results of testing to date are presented and discussed. HCCI is a process whereby a premixed charge of diesel fuel and air is admitted into the power cylinder and compression ignited. Ignition occurs homogeneously throughout the cylinder. HCCI reduces flame temperatures and oxides of nitrogen (NOx) emissions. The lack of fuel rich zones within the cylinder eliminates soot formation (1-pull Bosch smoke numbers of 0, 5-pull = 0). The limits of HCCI start of combustion timing are defined by knock before top dead center (BTDC) and misfire after top dead center (ATDC). Stable and repeatable HCCI combustion has been demonstrated over a wide range of air-fuel (A/F) ratios, intake temperatures, compression ratios (CR), exhaust gas recirculation (EGR) rates, and for two fuels. A/F ratios of 14 to 80 are possible. CRs of 8 to 13 have been demonstrated. EGR rates from 0 to 50 percent have been operated successfully. Results for ceramic coating of the piston crown, valves, and fire-deck have been compared to those of plain steel parts. Stable combustion for diesel fuel as well as a blended fuel (19 percent hexadecane, 81 percent heptane) has been demonstrated. While HCCI tests have yielded a higher indicated specific fuel consumption (ISFC) value than the direct-injection (DI)-diesel, results indicate a 98 percent reduction in NOX emissions for diesel-fueled HCCI over the DI-diesel configuration, and a reduction of 27 percent in particulates. They also reveal a strong dependence of particulate emissions on intake air temperature.