Injection timing effects on partially premixed diesel-methane dual fuel low temperature combustion

Dual fuel low temperature combustion (LTC) strategies are attractive for future internal combustion engines due to their promise of very low engine-out emissions of oxides of nitrogen (NOx) and particulate matter. In the present work, experimental results for diesel-ignited methane dual fuel LTC on a compression ignition single cylinder research engine (SCRE) are presented. Methane was fumigated into the intake manifold and diesel injection was used to initiate combustion. The engine was operated at a constant speed of 1500 rev/min, and diesel injection pressure was fixed at 500 bar. The start of injection (SOI) of diesel fuel was varied from 260° to 360° (i.e., TDC) to quantify its impact on engine performance and engine-out, indicated-specific emissions of NOx (ISNOx), carbon monoxide (ISCO), and unburned hydrocarbons (ISHC), and smoke emissions. The SOI sweeps were performed at different net indicated mean effective pressures (IMEPs) of 4.1 and 12.1 bar. Intake manifold pressure and methane percent energy substitution (PES) were fixed at 1.5 bar and 80%, respectively, for 4.1 bar IMEP and at 1.8 bar and 95%, respectively, for 12.1 bar IMEP. For all loads, when SOI was advanced, the longer ignition delays caused the separation between the fuel injection and the combustion events to increase. This was accompanied by a change in the shape of the AHRR curve from a distinct two-stage profile to a smooth, single-stage (almost Gaussian) profile. Advancing SOI to 300° and beyond yielded minimal engine-out ISNOx emissions (∼0.15 g/kW h at 4.1 bar IMEP and ∼1.3-1.5 g/kW h at 12.1 bar IMEP). Smoke emissions were negligible (<0.05 FSN) for all loads and all SOIs. Very high ISHC and ISCO emissions were observed for near-TDC SOI at all loads. The lowest ISHC and ISCO levels occurred for SOIs near 310° and for more advanced SOIs, both emissions increased. High pressure rise rates and the tendency to knock prevented engine operation at intermediate SOIs between 310° and 340° for 12.1 bar IMEP. On the other hand, for both 4.1 bar and 12. 1 bar IMEPs, high coefficient of variation of IMEP (>5%) caused unstable engine operation for SOIs advanced beyond 280°.