Effects on performances, emissions and particle size distributions of a dual fuel (methane-diesel) light-duty engine varying the compression ratio

Abstract

Economic and technological issues related to meet the future harmful emissions and greenhouse gases (GHG) standards are pushing the scientific community to consider studying alternative routes for low emissions and high efficient propulsion systems. A possible approach is represented by the dual-fuel (DF) concept applied to high efficient compression ignition engines.In this context, the results of a wide experimental campaign performed on a single-cylinder engine platform equipped with modern combustion and injection systems operated in dual-fuel diesel/methane mode are presented. The effects of the compression ratio, injection parameters and air-throttling on the global performances and emissions, also in terms of particle size spectrum, are assessed.The tests were performed in several operating points representative of real working conditions of an automotive light-duty (LD) diesel engine in order to carry out the fuel consumption (FC), the GHG and the pollutant emissions estimation on the New European Driving Cycle (NEDC) test procedure. A proper DF engine parameter calibration was set-up observing constraints in terms of in-cylinder peak firing pressure, pressure rise rate, cycle-to-cycle variation and engine-out emissions.A significant impact of the CR variation and injection parameters on the combustion characteristics and emissions is revealed in DF mode (MHC). In particular, benefits on methane unburnt (MHC) and combustion noise can be attained reducing the CR. A global CO2 reduction of about 12%, over the NEDC, can be definitely obtained with an average CH4 substitution rate of about 50% and independently of the CR at the expense of the DF- CO2 equivalent that is higher (22%) compared to D mode. The DF particles concentration in the accumulation mode is generally reduced as well as the estimated particle number on the NEDC. Moreover, within the tested engine class, a CR of 15.5 appears to be the best compromise among the global efficiency and pollutant emissions outputs to operate in DF mode.