Water Injection for Combustion Phasing Control and Load Expansion on an Ethanol HCCI Engine

Homogeneous charge compression ignition (HCCI) combustion is low-temperature combustion (LTC) mode that offers an alternative to conventional combustion modes. The advantages of HCCI combustion include high conversion efficiency and low NOx emissions. On the other hand, a direct control mechanism for combustion phasing control is not attainable as in conventional SI (spark ignition) or CI (compression ignition) engines. This limits the HCCI operational range and provides one of the biggest challenges in HCCI mode commercial implementation. High heat release rates and knock initiation limit the high load operation, whereas combustion instabilities limit the low load operation. In this context, this paper explores the use of water injection technique to control the combustion phasing and expand the load of an ethanol HCCI engine. The experiments were conducted on a three-cylinder diesel engine, where all the exhaust gases from a diesel cylinder were used to achieve the HCCI combustion in another cylinder. The results showed that the water injection technique was effective in controlling the charge reactivity and consequently the combustion phasing while enabling higher engine load. The water injection reduced combustion speed and heat release rates, which led to higher combustion duration, better combustion phasing control, and a reduction of ringing intensity. It was possible to expand the initial engine load from 2.0 bar without water injection to 6.0 bar IMEP with water injection. In addition, the ethanol HCCI combustion achieved high indicated efficiencies, ranging from 34 to 39%, while maintaining good combustion stability and low emissions characteristics.