Engine performance and exhaust emissions in stoichiometric combustion engines fuelled with dimethyl ether

Abstract

This paper describes the effects of stoichiometric combustion on the engine performance and exhaust emissions in a single-cylinder compression ignition engine fuelled with dimethyl ether. In order to realize stoichiometric combustion in a direct-injection compression ignition engine, high exhaust gas recirculation was used to control air-to-fuel ratios based on a lambda meter. The combustion pressure, heat release rate, and indicated mean effective pressure, which are the main indicators of the engine performance, were derived from in-cylinder pressure data in a single-cylinder direct-injection compression ignition engine. The test engine had a displacement volume of 373.3 cm3, a compression ratio of 17.8, and a common-rail injection system. Exhaust emissions characteristics were obtained from emission gas analysers at the tailpipe. The experimental results show that increasing the equivalence ratio dramatically reduced the indicated specific nitrogen oxide emissions (i.e. by over 90 per cent) owing to the high exhaust gas recirculation in the conventional diesel combustion regime. Furthermore, soot emissions were negligible owing to the chemical properties of the dimethyl ether fuel in the operation range of the injection timings. For injection timings near top dead centre, the indicated specific carbon monoxide emission levels rise excessively as the equivalence ratio increases and the maximum value is 80.9 g/kW h. However, for advanced injection timing (i.e. near 25° crank angle before top dead centre), the indicated specific carbon monoxide emission level shows minimal variation as the equivalence ratio changes. In contrast, the levels of indicated specific hydrocarbon emissions vary according to the injection timing. Increasing the equivalence ratio causes the indicated mean effective pressure to deteriorate slightly (i.e. up to 10 per cent loss) owing to the intake conditions for high exhaust gas recirculation, which results in a low oxygen concentration and a high carbon dioxide concentration.