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

Abstract

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 multihole injector was studied in a single-cylinder optical directinjection spark-ignition engine under part-load conditions (0.5 bar intake plenum pressure) at 1500 RPM. A highspeed 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 multicomponent fuel (commercial gasoline) and a singlecomponent 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.

An analysis of spray development with iso-octane, n-pentane, gasoline, ethanol and n-butanol from a multi-hole injector under hot fuel conditions

“…one at the intake side and the other at the exhaust side of the engine. More details about the injector geometry, nozzle-hole angles and spray formation in a quiescent environment and in a running DISI engine can be found in previous studies by the current authors [3,4,[8][9][10].…”

“…towards the spark plug or any other in-cylinder area of interest, and by flexibility of when and how fuel is injected into the cylinder i.e. independence over piston position [2][3][4].…”

Cavitation, primary break-up and flash boiling of gasoline, iso-octane and n-pentane with a real-size optical direct-injection nozzle

“…The investigation used the same injection system and facilities described in previous publications by the authors [22][23][24][25][26][27]. Therefore primarily deviations from standard equipment, i.e.…”

“…This was achieved using a real-size optical nozzle to simulate the flow in one of 8 the holes of a real multi-hole injector previously used by the authors for macroscopic spray imaging and related engine studies [22][23][24][25][26][27]. These studies showed that the geometry and break-up of sprays from such injectors is significantly affected by operating conditions, which result in a high degree of superheating of the fuel components, i.e.…”

“…The six nozzle holes have different turning angles that direct fuel to different areas of the combustion chamber but they are symmetric with respect to a centerline, therefore, only 3 'sprays' can be seen from the main imaging view corresponding to the 3 plume pairs (1,6; 2,5; 3,4). Further details can be found in [22][23][24][25][26][27].…”