Injection Strategies for POMDME- and Diesel-Natural Gas Dual Fuel Engines

Abstract Conventional dual-fuel combustion (DFC) has been studied for decades due to its ability to produce high fuel conversion efficiency and its potential to defeat the well-known diesel NOx-PM trade-off. In DFC, a low-reactivity fuel (LRF) such as natural gas is premixed with air and ignited with the help of a high-reactivity fuel (HRF) such as diesel. In DFC, along with its low PM emissions, NOx emissions are reduced significantly by advancing the start of injection (SOI) to earlier than 320 CAD (40 degrees before TDC). However, earlier SOIs result in combustion instability, due to overleaning of HRF. It is more evident for a case with a high percentage energy substitution (PES) of LRF, due to the limited injected quantity of HRF. Research has shown that combustion instabilities can be improved by split injection (more than one injection), especially when they are close-coupled. In this study, the typical diesel PM-NOx tradeoff was nullified with an oxygenated, high cetane fuel, polyoxymethylene dimethyl ether (POMDME) as HRF. POMDME-natural gas DFC produced zero measurable soot emissions at all investigated conditions. However, since the lower heating value of POMDME is nearly half that of diesel, it needs longer injection durations (greater mass injected) for the same energy input. Therefore, it is important to understand the effect of end of injection (EOI) of POMDME compared to diesel for the same SOI on performance and emissions In addition, the impact of split injection strategies on performance and emissions are explored, especially at the heat release rate shape transformation region (i.e.) between SOI 320 and 330 CAD.