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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.

The effects of several operating parameters on dual fuel combustion at light load were investigated by means of direct endoscopic observation of the process. Therefore, an intense experimental campaign was performed on a single cylinder diesel common rail research engine, converted to operate in dual fuel mode and equipped with optical accesses and variable intake configuration. Three 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 and 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.

The effect of the shape of the bowl on the combustion process and emissions of a Natural Gas - Diesel dual fuel engine is analyzed. The simulation of the dual fuel combustion is performed with a modified version of the KIVA3V code where diesel is treated as the main fuel and a further reacting specie is introduced as methane (CH4). The auto-ignition of the pilot is simulated with a modified version of the Shell model and the first stage of the combustion, related to the pilot burning process, is simulated with the Characteristic Time Combustion model. When the temperature of the mixture reaches a certain threshold, a kernel of combustion is initialized. Until the kernel reaches a nominal radius the combustion of CH4 is prevented. The combustion of CH4 is simulated with a turbulent characteristic time too. Numerical models were chosen as a compromise between accuracy and computational time.