Combustion, performance and emissions characteristics of compression-ignition engines fuelled by sustainable fuels

Abstract

Internal combustion engines are approaching their theoretical maximum efficiency, which could indicate limited future technological improvements in performance and exhaust emissions with standard fuels. In addition, fossil fuel dependence can only be reduced by implementing appropriate renewable fuel sources. The experimental investigation in this work only concerns the compression-ignition (CI) engine combustion process both in normal operation and “dual-fuel” operation. The dual-fuel mode allows low-cetane number fuel to be used in CI engines, with a “pilot” fuel spray injection of high-cetane number fuel to provide ignition. Initially, rapeseed methyl ester (RME) and two water-in-RME emulsions were compared with normal diesel fuel during normal operation. Neat RME generally performed similarly to diesel fuel, while giving higher specific fuel consumption (SFC) levels. Both water- in-RME emulsions performed fairly similarly to neat RME. This suggests that the cooling effect of water vapourisation was a negligible factor throughout the operating range. Natural gas dual-fuel operation reduced NOx at certain conditions and overall CO2 emissions while thermal efficiencies were maintained compared with normal operation. However, significantly higher unburnt hydrocarbons (HC) and CO emissions were recorded at low and intermediate engine loads. For the emulsified pilot fuels, better fuel-air mixing (possibly as a result of “microexplosions”) increased NOx after an equivalence ratio of about 0.6. Hydrogen dual-fuel operation generally increased NOx emissions while CO2 emissions were reduced compared with normal operation. Thermal efficiencies remained comparable for all pilot fuels. NOx emissions in the emulsified fuel cases were generally comparable to the neat RME pilot. Lower volumetric efficiency was also recorded, while power output was limited to maintain engine stability and avoid abnormal combustion caused by excessively high pressure-rise rates (called “hydrogen knock”). Overall, significant optimisation is needed to improve combustion efficiency at low and intermediate engine loads during dual-fuel CI engine operation. As these engines are designed specifically for liquid fuels, substantial engine customisation or even complete redesign (particularly in the fuel supply system) is needed to improve the combustion quality on a scale larger than that seen in this work.