Fuel adhesion and oil splash on oil-wet cylinder walls with post diesel fuel injections

Diesel particulate filters are used to capture the particulate matter in engine exhaust, and the post fuel is injected for a regular regeneration of the diesel particulate filter to avoid plugging of the diesel particulate filter with the particulate matter. The concept of the post fuel injection is as follows: the post fuel is injected in the expansion stroke, the vaporized fuel gas is oxidized by the diesel oxidation catalyst, here the high temperature gas oxidizes the particulate matter on the diesel particulate filter, and the diesel particulate filter is regenerated. However, the post fuel impinges and adheres on the cylinder wall due to the low temperature and pressure conditions in the expansion stroke in actual engine operation and this causes diesel engine lubricant oil dilution, the deterioration of fuel consumption characteristics in diesel engines, as well as ash plugging of the diesel particulate filter. In this article, the post fuel spray behavior and adhesion on oil-wet cylinder liners were investigated with a high pressure–temperature optical constant volume chamber instead of engine tests. The in-cylinder temperature and pressure at the 30, 60, and 90 °CA after top dead center, commonly employed in post fuel injection timings, were measured in engine operation. To create the post fuel injection conditions of diesel engines in the constant volume chamber, pre-mixed gas containing ethylene, oxygen, and nitrogen is introduced into the chamber and ignited by the spark plug. Then, fuel masses of 0.5, 1.0, and 1.5 mg per injection hole at the after top dead center settings were injected to a wall adhesion plate with a 5-μm thick oil film that simulates the surface of the cylinder liner and the post fuel impinges on and splashes oil away from the oil film on the cylinder wall. The quantities of splashed oil and adhering fuel on the wall adhesion plate were measured by a precision balance and a thermostatic chamber. With the early post injection, most of the injected fuel vaporizes without penetrating to the cylinder liner and gaseous diesel fuel is condensed on the cylinder wall; however, with the late post fuel injections, the strong penetration of liquid fuel reaches the cylinder wall, and much engine oil becomes splashed away. The droplet size of the fuel spray was measured by telescope ultra-high resolution image analysis with a digital single-lens reflex camera, and the fuel impingement phenomena on the cylinder wall are explained by the Weber number, calculated from velocity and diameter values, droplet density, and surface tension.