Investigation of an Engine Concept for CNG-OME Dual Fuel Operation Using External and Internal EGR 2022-32-0067

Requirements to future internal combustion engines (ICEs) regarding sustainability and efficiency are continuing to rise while on the other hand, pollutant emission regulations are continuously tightened. Dual-fuel combustion (DFC) of diesel and natural gas is an approach to reduce soot emissions while still profiting from the high efficiency of the diesel combustion process. Using natural gas as the main fuel also helps to reduce carbon dioxide (CO₂) emissions due to the favorable C/H-ratio of methane (CH₄) as its primary constituent. To reduce both pollutant and greenhouse gas emissions further, diesel can be replaced by an e-fuel. The use of C1-oxygenates – such as polyoxymethylene dimethyl ether (POMDME or “OME”) – as pilot fuel promises to reduce both soot and nitrogen oxide (NO_x) emissions. For the present investigation, a 4.5l tractor diesel engine has been converted to a biogas-OME dual-fuel engine. A fully variable valve train has been integrated into the cylinder head. A second exhaust valve lift during aspiration is used to vary the internal exhaust gas recirculation (EGR) rate, while an EGR valve controls the cooled external EGR introduced into the intake runner. This study investigates the influence of external and internal EGR on equivalence ratio and the maximum substitution rate for stable combustion, as well as the resulting engine efficiency and raw emissions. Internal EGR was adjusted by varying the second exhaust valve lift in several discrete steps from 0 to 1.9 mm, while the external EGR rate was varied between 0 and 30 %. The investigations show that CH₄ and carbon monoxide (CO) emissions rise with increasing substitution rate, whereas NO_x and CO₂ emissions are reduced. Soot emissions are well below the EU stage V emission limit throughout the entire measuring range. As the soot-NO_x trade-off is no longer an issue, NO_x emissions can be reduced by up to 88 % by increasing the EGR rate. Combining internal and external EGR also showed a positive effect on both unburnt CH₄ (up to -62 %) and CO emissions (up to -87 %).