Spray Development, Flow Interactions and Wall Impingement in a Direct-Injection Spark-Ignition Engine

Levels of liquid fuel impingement on in-cylinder surfaces in direct injection spark ignition engines have typically been higher than those in port-fuel injection engines due to in-cylinder injection and higher injection pressures. The result is typically an increase in the levels of un-burned hydrocarbons and smoke emissions which reduce the potential fuel economy benefits associated with direct injection engines. Although different injection strategies can be used to reduce these effects to some extent, full optimisation of the injection system and combustion process is only possible through improved understanding of spray development that can be obtained from optical engine investigations under realistic operating conditions. To this extent, the spray formation from a centrally mounted multi-hole injector was studied in a single-cylinder optical direct-injection spark-ignition engine under part-load conditions (0.5 bar intake plenum pressure) at 1500 RPM. A high-speed camera and laser illumination were used to obtain Mie-scattering images of the spray development on different in-cylinder planes for a series of consecutive engine cycles. The engine temperature was varied to reflect cold-start (20 °C) and fully warm (90 °C) engine conditions. A multi-component fuel (commercial gasoline) and a single-component fuel (iso-octane) were both tested and compared to investigate the effects of fuel properties on spray formation and wall impingement. An experimental arrangement was also developed to detect in-cylinder liquid fuel impingement using heat flux sensors installed on the cylinder liner. Two different injection strategies were tested; a typical single-injection strategy in the intake stroke to promote homogeneous mixture formation, as well as a triple-injection strategy around the same timing to assess the viability of using multiple-injection strategies to reduce wall impingement and improve mixture preparation. A sweep of different locations around the cylinder bore revealed the locations of highest fuel impingement levels which did not correspond directly to the nominal spray plume trajectories as a result of spray-flow interactions. These results were analysed in conjunction with the observed effects from the parallel imaging investigation.