The small engine for two-wheel vehicles has generally high possibility to be optimized at low speeds and high loads. In these conditions fuel consumption and pollutants emission should be reduced maintaining the performance levels. This optimization can be realized only improving the basic knowledge of the thermo-fluid dynamic phenomena occurring during the combustion process.
It is known that, during the fuel injection phase in PFI SI engines, thin films of liquid fuel can form on the valves surface and on the cylinder walls. Successively the fuel films interact with the intake manifold and the combustion chamber gas flow. During the normal combustion process, it is possible to achieve gas temperature and mixture strength conditions that lead to fuel film ignition. This phenomenon can create diffusion-controlled flames that can persist well after the normal combustion event. These flames induce the emission of soot and unburned hydrocarbons.
In this paper, experimental activities were carried out in the combustion chamber of a single-cylinder 4-stroke optical engine. The engine was equipped with a four-valve head of a commercial Piaggio engine for 2-wheel vehicle. Two injectors were tested: standard 3-hole injector and 10-hole injector for multi-cylinder engine of a passenger car.
High spatial resolution digital imaging was used to characterize the flame propagation. The presence of diffusion-controlled flames near the valves and on the cylinder walls was investigated. The flames induced the formation of unburned hydrocarbons and soot particles. The in-cylinder optical investigations were correlated with particulate emissions. The effect of three-way catalyst on the exhaust particles was examined.