Combustion Development and Exhaust Emissions of a Dual-Fuel DI Diesel Engine with Variable in-Cylinder Bulk Flow and Methane Supply Strategies

An extensive experimental campaign was performed on a diesel common rail research engine, converted to operate in dual-fuel mode, in order to investigate the effects of in-cylinder bulk flow and methane supply strategies on combustion and emissions. Three different bulk flow structures of the charge were induced inside the cylinder by activating/deactivating the two different inlet valves of the engine (i.e. swirl and tumble). Methane was injected into the inlet manifold at different pressure levels, varying the injector position. In order to obtain a stratified-like air-methane mixture, the injector was mounted very close to the inlet valve, while, to obtain a homogeneous-like one, methane was injected more upstream. By combining the two different positions of the injector and the three bulk flow structures, seven different engine inlet setups were tested, at different values of engine speed and load. Moreover, the effects of various pilot injection parameters, such as injection advance, diesel fuel quantity and rail pressure, were investigated. From in-cylinder pressure and heat release curves some output parameters, representative for combustion development, were calculated while emissions concentrations in the exhaust gases were measured. Main and interaction effects on dual-fuel combustion and exhaust emissions of all the operating parameters described above were studied by means of DOE technique, in order to perform a synthetic and complete analysis of the combustion behavior. The obtained results showed that the charge bulk motion produced by the swirl inlet valve is able to induce a more rapid and complete combustion of the air-methane mixture inside the cylinder. At low engine loads, this results in a contemporary reduction of unburned hydrocarbons and nitrogen oxides content at the exhaust, especially injecting the methane at high pressure and in stratified-like configuration. Furthermore, the analysis of the exhaust pollutant concentrations revealed that complex interactions effects between some unexpected operating parameters exist, such as between pilot injection pressure and engine inlet configuration. Generally, it was demonstrated that the stratified-like configuration of methane injection is an effective method to reduce unburned hydrocarbon emissions at the engine exhaust.