Separate effect of H2, CH4 and CO on diesel engine performance and emissions under partial diesel fuel replacement

The use of gaseous fuels in internal combustion engines has gained increasing attention in last years, mainly when coming from renewable sources. This paper evaluates the separate effect of hydrogen, methane and carbon monoxide addition on the performance, combustion efficiency and pollutant emissions of a diesel engine. The gaseous fuels tested have been selected among those with high interest either as a single fuel (H2 and CH4, the latter is the main component of natural gas) or as constituents of a mixture (the three compounds being the main components of gas from biomass gasification or reforming gas). The experimental tests were carried out in a supercharged 2 l single-cylinder diesel engine under two engine loads (90 and 105 N m, which corresponds to IMEP values of 6.28 and 7.22 bar respectively) and EGR rates (0% and 15%). The effect of the intake temperature (around 45 and 60 °C) was analyzed for 15% EGR. The indicated thermal efficiency was reduced when increasing the diesel fuel replacement because of the unburnt gaseous fuel, as confirmed by an increase in HC and CO emissions when CH4 and CO were used. However, a significant decrease in the PM mass and number concentration was achieved, while NOx emissions were not much affected by the diesel fuel replacement. Higher engine loads significantly improved the engine efficiency and the pollutant emissions. The recirculation of the unburnt gaseous fuel derived from the use of EGR showed a relevant role for reducing emissions, mainly when using hot EGR, which suggest removing EGR cooling when diesel fuel is replaced. Thermodynamic diagnostic results suggest that the combustion of the gaseous fuel is governed by the gas entrainment into the diesel jet.