Parametric Investigations on Premixed Charged Compression Ignition in a Small-Bore Light Duty Diesel Engine

Achieving stable combustion without misfire and knocking is challenging in premixed charge compression ignition (PCCI) especially in small bore, air cooled diesel engines owing to lower power output and inefficient cooling system. In the present study, a single cylinder, air cooled diesel engine used for agricultural water pumping applications is modified to run in PCCI mode by replacing an existing mechanical fuel injection system with a flexible common rail direct injection system. An advanced start of fuel injection (SOI) and exhaust gas recirculation (EGR) are required to achieve PCCI in the test engine. Parametric investigations on SOI, EGR and fuel injection pressure are carried out to identify optimum parameters for achieving maximum brake thermal efficiency. An SOI sweep of 12 to 50 deg. CA bTDC is done and for each SOI, EGR is varied from 0 to 50% to identify maximum efficiency points. It was found that EGR helps in extending the load range from 20 to 40% of rated load. However, it resulted in increase in unburned hydrocarbon (HC) emissions. To study the effect of injection pressure, SOI, speed and EGR rate were fixed as constant and injection pressure was varied from 300 to 900 bar. Unlike conventional combustion, increasing fuel injection pressure is not beneficial in PCCI in the present small-bore engine wherein significant increase in HC, carbon monoxide (CO) and smoke emissions is observed which could be due to spray wall impingement. It was observed that increasing fuel injection pressure increased the efficiency only slightly whereas the penalty in terms of HC and CO emissions were significant. Based on the results obtained, it is concluded that for achieving PCCI in the present small-bore engine, an early fuel injection with high EGR and lower injection pressures are required.