Effect of Reverse Squish on Fuel Spray Behavior in a Small DI Diesel Engine under High Pressure Injection and High Charging Condition

Modern small DI diesel engines are operated at high loads and high speeds. In these engines the spray spreading on the cavity walls during the main combustion is kept approximately constant at all engine speeds to optimize the air utilization. However, spray spreading on the wall during the early and late part of combustion changes with engine speed due to the changes in air motion. At the end of impingement much of the spray moves outside the cavity due to a strong reverse squish when the injection timing is set near TDC. This causes incomplete combustion of fuel and increase emissions of HC and soot. Therefore, the study of the behavior of spray affected by the reverse squish is very important.

In this study the fuel spray development under high injection pressure and high gas charging pressure was investigated photographically in a small direct injection diesel engine with a common rail injection system. Improvements in the spray distribution inside the cavity were attempted by changing the lip shape, side wall shape, and injection angle. It was found that the roundness at the top and bottom side of the cavity entrance causes leakage of fuel to outside the cavity during the expansion stroke. Larger injection angles with the horizontal can confine much fuel inside the cavity but produces long floor jets at the bottom. A small 2 mm step at the cavity side wall produces larger spray volume to inside the cavity and improves the mixture distribution.