An Assessment of Cyclic Variations in the Air-Fuel Ratio for RCCI Engine

The potential for simultaneous reduction of soot and NO_x emissions and higher fuel conversion efficiency has already been demonstrated for reactivity-controlled compression ignition (RCCI) engines. The RCCI engine has a relatively higher peak pressure rise rate (PPRR) and cyclic variations compared to the conventional diesel engine. The upper and lower operating load boundaries of the RCCI engine are restricted by higher PPRR and cyclic variations, respectively. The cyclic variations in the air-fuel ratio are one of the main factors which govern the variations in combustion parameters. The cyclic variations in combustion need to be controlled for stable engine operation. The present study estimates the cyclic air-fuel ratio from the measured in-cylinder pressure data for the RCCI engine. The RCCI experiments are performed on a modified single-cylinder compression ignition (CI) engine equipped with a development ECU. In this study, the engine was operated in RCCI combustion mode with port injection of gasoline and direct injection of diesel fuels. The port and direct fuel injection events are controlled through the development ECU. The in-cylinder pressure signals are measured using a piezoelectric pressure transducer installed on the engine head. A crank angle encoder of 0.1 CAD resolution is used to determine the crank angle position for recording in-cylinder pressure data. In this study, 1000 consecutive engine cycles are recorded, and the air-fuel ratio is estimated for each cycle. The pressure moment method is used to estimate the cyclic air-fuel ratio at a particular engine operating condition. Furthermore, statistical methods (coefficient of variation and standard deviation in the parameter) and Wavelet Transform (WT) are used to analyze the cyclic variations in the air-fuel ratio. Correlation between cyclic variations of air-fuel ratio and cyclic variation of combustion parameters is also estimated, and a good relationship is found in some of the engine operating conditions. Results indicate that in conventional dual-fuel operation, only low-periodicity high-frequency variations in the air-fuel ratio are present. Whereas in the case of RCCI combustion, high-frequency and low-frequency variations in the air-fuel ratio are present.