Study on the twin-pilot-injection strategies for the reduction in the exhaust emissions in a low-compression-ratio engine

An investigation of twin-pilot-injection strategies was performed in a low-compression-ratio engine in order to improve the combustion performance and to reduce the exhaust emissions. All tests were conducted on a modified four-cylinder test engine running on only one cylinder with a reduced compression ratio (15.3:1), and analyses of the combustion pressure, the heat release rate and the net work output in the cylinder were performed. At the same time, the reduction in the exhaust emissions such as carbon monoxide, hydrocarbons and soot under different twin-pilot-injection strategies was compared with other combustion cases (single-injection strategy and one-pilot-injection strategy). Higher combustion pressures, lower heat release rates, shorter ignition delay times, higher indicated mean effective pressures (about 2.1%) and lower coefficients of variation in the indicated mean effective pressure (approximately 29.4%) in all multiple-injection (one-pilot-injection or twin-pilot-injection) cases, which indicated improved operation of the low-compression-ratio engine, were observed. Retarded injection timing (t_p,SOE = 20–10° crank angle before top dead centre) in the twin-pilot-injection case proved to the best timing for reduction in the carbon monoxide emissions because the relatively short ignition delay with the high in-cylinder temperature enhanced the carbon monoxide oxidation rate. Twin-pilot-injection strategies formed higher hydrocarbon emissions because of the rich fuel mixture during the ignition delay time than one-pilot-injection combustion did. As more multiple injections were applied, lower nitrogen oxide emissions (up to 45.7%) were observed because the early termination of the first injection event and the restart of injection created a time interval between injections in which freshly charged air could be entrained into the flame.