Multiple injection strategies for reducing HC and CO emissions in diesel-methane dual-fuel low temperature combustion

Dual fuel low temperature combustion (LTC), while promising extremely low engine-out emissions of oxides of nitrogen (NOx) and particulate matter (PM), is beset with high unburned hydrocarbon (HC) and carbon monoxide (CO) emissions, especially at low engine loads. In the present work, diesel dual injection is experimentally shown to achieve simultaneous reduction of HC and CO emissions without compromising NOx and PM benefits. The motivation to use a second late diesel injection (typically after top dead center (ATDC)) is to oxidize HC arising from incomplete methane oxidation in dual-fuel combustion initiated by the first early diesel injection (around 310 CAD). Since the second diesel injection occurs during the expansion stroke, the NOx-formation propensity is reduced due to low local temperatures. The experimental matrix consisted of twenty-two distinct operating points on a single cylinder research engine (SCRE) adapted for diesel-ignited methane dual fueling. All the experiments were performed at a constant intake pressure of 1.5 bar and fixed first diesel injection timing at 310 CAD whereas, the second diesel injection timing was varied between 320 and 375 CAD and the injection pressure was varied between 500 and 1500 bar. In addition to reductions in the following emissions: indicated specific hydrocarbons or ISHC (54%, to 13.7 g/kW-hr), ISCO (46%, to 2.7 g/kW-hr), and indicated specific oxides of nitrogen or ISNOx (7%, to 0.31 g/kW-hr) emissions, an 11% increase in indicated fuel conversion efficiency (IFCE), and a 12% increase in combustion efficiency (eta c) were achieved relative to the baseline single injection (at 310 CAD) dual-fuel LTC. Although smoke emissions increased slightly from 0.03 to 0.06 Filter Smoke Number (FSN), they are still considerably lower than for conventional diesel operation.