Homogeneous Charge Reactivity-Controlled Compression Ignition Strategy to Reduce Regulated Pollutants from Diesel Engines

Reactivity-controlled compression ignition (RCCI) is a dual fuel low temperature combustion (LTC) strategy which results in a wider operating load range, near-zero oxides of nitrogen (NO_x) and particulate matter (PM) emissions, and higher thermal efficiency. One of the major shortcomings in RCCI is a higher unburned hydrocarbon (HC) and carbon monoxide (CO) emissions. Unlike conventional combustion, aftertreatment control of HC and CO emissions is difficult to achieve in RCCI owing to lower exhaust gas temperatures. In conventional RCCI, an early direct injection (DI) of low volatile diesel fuel into the premixed gasoline-air mixture in the combustion chamber results in charge stratification and fuel spray wall wetting leading to higher HC and CO emissions. To address this limitation, a homogeneous charge reactivity-controlled compression ignition (HCRCCI) strategy is proposed in the present work, wherein the DI of diesel fuel is eliminated. HCRCCI strategy is achieved by inducting diesel and gasoline vapor along with inlet air in the intake manifold during the suction stroke, and the premixed diesel-gasoline-air mixture is autoignited during the compression stroke. Unlike RCCI, the charge stratification is eliminated in HCRCCI owing to the induction of gasoline and diesel fuel vapor and a longer time available for fuel-air mixing which in turn results in higher degree of homogeneity. A production light-duty diesel engine used for agricultural water pumping applications is modified to run in RCCI and HCRCCI through suitable changes in the cylinder head and intake and exhaust systems. The unmodified test engine is initially run under conventional combustion with diesel fuel to generate the baseline reference data for comparison. The DI diesel fuel timings, gasoline-to-diesel energy ratio, and exhaust gas recirculation (EGR) are optimized in RCCI, while gasoline-to-diesel energy ratio and EGR percent are optimized in HCRCCI at each load condition to achieve higher brake thermal efficiency. After parametric optimization, the engine combustion, performance, and emissions are compared among RCCI, HCRCCI, and conventional combustion. The comparison shows that the engine could be operated in both RCCI and HCRCCI over the entire operating regime with near-zero NO_x and smoke emissions along with higher brake thermal efficiency compared to the conventional combustion. As compared to RCCI, HCRCCI result in higher brake thermal efficiency at part load conditions along with ~87% and ~37% lower CO and HC emissions, respectively. Thus, HCRCCI could be a promising LTC strategy for diesel engines to reduce all the regulated pollutants with higher brake thermal efficiency without any compromise on achievable load range.