Suitability Study of n-Butanol for Enabling PCCI and HCCI and RCCI Combustion on a High Compression-ratio Diesel Engine

This work investigates the suitability of n-butanol for enabling PCCI, HCCI, and RCCI combustion modes to achieve clean and efficient combustion on a high compression ratio (18.2:1) diesel engine. Systematic engine tests are conducted at low and medium engine loads (6∼8 bar IMEP) and at a medium engine speed of 1500 rpm. Test results indicate that n-butanol is more suitable than diesel to enable PCCI and HCCI combustion with the same engine hardware. However, the combustion phasing control for n-butanol is demanding due to the high combustion sensitivity to variations in engine operating conditions where engine safety concerns (e.g. excessive pressure rise rates) potentially arise. While EGR is the primary measure to control the combustion phasing of n-butanol HCCI, the timing control of n-butanol direct injection in PCCI provides an additional leverage to properly phase the n-butanol combustion. With the existing engine hardware, n-butanol PCCI outperforms HCCI by offering preferable combustion controllability while both combustion modes meet prescribed targets of low emissions and high efficiency. In RCCI, the auto-ignition of port injected n-butanol tends to occur early in the compression stroke, and thus high EGR and late diesel injection are generally required to attain proper ignition timing and combustion phasing. Although NOx emissions of butanol-diesel dual-fuel combustion meet the target, soot emissions are much higher compared to those in PCCI and HCCI modes.