Parametric Analysis of Compression Ratio Variation Effects on Thermodynamic, Gaseous Pollutant and Particle Emissions of a Dual-Fuel CH4-Diesel Light Duty Engine

The paper reports the results of an experimental campaign aimed to assess the impact of the compression ratio (CR) variation on the performance and pollutant emissions, including the particle size spectrum, of a single cylinder research engine (SCE), representatives of the engine architectures for automotive application, operated in dual-fuel methane-diesel mode. Three pistons with different bowl volumes corresponding to CR values of 16.5, 15.5 and 14.5 were adopted for the whole test campaign. The injection strategy was based on two injection pulses per cycle, as conventionally employed for diesel engines. The test methodology per each CR included the optimization of both 1st injection pulse quantity and intake air mass flow rate in order to lower as much as possible the unburned methane emissions (MHC). The testing points were selected in order to analyse the CR impact on SCE performance at partial and high loads, including the performance estimation over the New European Driving emission homologation Cycle (NEDC) of a real four-cylinder automotive multi-cylinder engine. The results evidence benefits on combustion noise, MHC and particle size spectrum when lowering the CR in DF mode. However, with respect to the tested engine class, a CR of 15.5 appears to be the best compromise among energy efficiency and global pollutant emissions.