Energy efficiency trade-off with phasing of HCCI combustion

Abstract

Homogeneous charge compression ignition (HCCI) combustion in diesel engines offers the potential of simultaneous low NOx and soot emissions. However, this is normally accompanied by high hydrocarbon (HC) levels in the exhaust and an early combustion phasing before the top-dead-center (TDC) that may drain out substantial amounts of fuel energy from the engine cycle. Exhaust gas recirculation is usually applied to delay the onset of combustion, thereby shifting the phasing of the heat release close to the TDC. Although the retarded phasing improves the engine energy efficiency, a significant increase in HC and carbon monoxide emissions will deteriorate the combustion efficiency. Therefore, an inherent trade-off exists between the combustion phasing and the combustion efficiency that needs to be minimized for improved energy efficiency. In this work, both theoretical and experimental studies have been carried out to evaluate the combustion efficiency-phasing (CEP) trade-off. Engine tests have been conducted to analyze the losses in combustion (burning) and phasing efficiencies, and along with theoretical analyses, the CEP trade-off has been evaluated in terms of a ‘coefficient of combustion inefficiency’ (CCI). The CCI quantitatively correlates the losses in combustion and phasing efficiencies and provides a reference for improving the combustion phasing of the HCCI operation vis-à-vis the combustibles in the exhaust. The focus of this research is to carry out a quantitative analysis of the energy efficiency of HCCI cycles. Copyright © 2011 John Wiley & Sons, Ltd.