Effects of in-cylinder bulk flow and methane supply strategies on charge stratification, combustion and emissions of a dual-fuel DI diesel engine

In order to study the effects of air bulk motion and methane injection strategies on the development and pollutant levels of dual-fuel combustion, an intense experimental campaign was performed on a diesel common rail research engine with variable inlet configurations. Activating only the swirl or the tumble inlet valve of the engine, or both of them, it was possible to obtain, inside the cylinder, three different bulk flow structures. The air-methane mixture was obtained injecting the gaseous fuel into the inlet manifold varying its pressure and the injector position, either very close to the inlet valves, in order to obtain a stratified-like mixture, or more upstream, to obtain a homogeneous-like mixture. By combining the two different positions of the injector and the three air bulk flow structures, seven different inlet setup have been tested, at different values of engine speed and load. Moreover, the pilot injection parameters have been varied, such as diesel fuel quantity, pressure and injection advance. The complex interactions between the injected methane and the air flow structure, result in an evident effect on the emission levels, despite the fact that major differences in pressure and heat release rate time histories were not always observed. In particular, the obtained results show that, at low loads, the motion produced by the swirl inlet valve is able to induce a more rapid and complete combustion of the air-methane mixture after the ignition nuclei of the pilot fuel have burned, reducing at the same time the unburned hydrocarbons and nitrogen oxides content at the exhaust, especially injecting the methane with high pressure and in stratified-like configuration.