Homogeneous Charge with Direct Multi-Pulse Injection - A Promising High Efficiency and Clean Combustion Strategy for Diesel Engines 2021-01-1156

Extensive experimental investigations done over a decade in different engine types demonstrated the capability of achieving high efficiency along with low levels of oxides of nitrogen (NO_x) and soot emissions with low temperature combustion (LTC) modes. However, the commercial application of LTC strategies requires several challenges to be addressed, including precise ignition timing control, reducing higher unburned hydrocarbon (UHC) and carbon monoxide (CO) emissions. The lower exhaust gas temperatures with LTC operation pose severe challenges for after-treatment control systems. Among the available LTC strategies, Reactivity Controlled Compression Ignition (RCCI) has emerged as the most promising strategy due to better ignition timing control with higher thermal efficiency. Nevertheless, the complexity of engine system hardware due to the dual fuel injection system and associated controls, high HC and CO emissions are the major limiting factors in RCCI. Homogeneous Charge with Direct Injection (HCDI) strategy is recently proposed to address the above limitations of RCCI. Unlike RCCI, HCDI is a single fuel LTC strategy with port and direct injection of diesel fuel, and thus, there is no reactivity stratification. However, the equivalence ratio and thermal stratification with direct-injected (DI) diesel fuel result in better combustion control lower HC and CO emissions in HCDI. The HCDI strategy is investigated with multiple injections of direct-injected (DI) fuel to examine the benefits in the present work. A production light-duty diesel engine used for agricultural water pumping applications is modified to run in HCDI mode through suitable changes in the intake manifold and fuel injection system. Experiments are conducted at the rated engine speed under varying load conditions in conventional diesel combustion (CDC), HCDI with single and double pulse DI modes. The results obtained show that at 4.6 bar imep, the indicated thermal efficiency is increased by 3.9% compared to CDC. However, NO_x emissions are increased from 2.7 g/kW-hr to 13.41 g/kW-hr, CO increased from 2.8 g/kW-hr to 10.49 g/kW-hr, but UHC decreases drastically from 4.18 g/kW-hr to 0.10 g/kW-hr in single DI pulse HCDI. In the case of double-pulse HCDI at 3.5 bar imep, indicated thermal efficiency increases by 5.5%, CO decreases by 13%, UHC increase by 48%, smoke increase by three times, and NO_x increase by 49.7% in comparison of single pulse HCDI.