Mixture Preparation and Combustion Variability in a Spray-Guided DISI Engine

Abstract

In an attempt to study the numerous contributors towards cyclic variations in combustion in a direct injection spark ignition engine, simultaneous high-speed imaging of fuel injection and flame growth are undertaken on a crankangle resolved basis in a single-cylinder optical research engine. Batches of images from 100 consecutive cycles are acquired for all conditions with synchronised incylinder pressure logging. The engine is motored and fired at stoichiometric conditions at 1500 RPM under part-load and wide-open-throttle conditions (0.5-1.0 bar intake pressure), with injection timing set early in the intake stroke to promote homogeneous mixture formation with a centrally mounted multi-hole injector. Liquid impingement is observed on the cylinder walls and on the piston crown with early intake injection and multiple injection strategies are employed in an attempt to reduce impingement and alter mixture preparation and subsequent combustion. The effects are investigated for iso-octane and pump-grade gasoline at engine coolant temperatures of 50 and 90 °C. Gasoline sprays showed severe deformation and partial collapse of the individual spray plumes at 90 °C, leading to a different mixture formation process relative to iso-octane, which showed no such effects at this engine temperature. Using multiple fuel injections per cycle, but maintaining the same overall air-to-fuel ratio with single-injection strategies, a significant reduction in direct impingement on the walls was observed, together with different flame growth relative to single injection. These results suggest that injection strategy and type of fuel is playing an important role in the mixture preparation process, even for homogeneous early direct injection, and that different types of multiple-injection strategies alone have the potential to modify significantly in-cylinder phenomena, affect combustion and potentially exhaust emissions.

Effect of Fuel Properties on Spray Development from a Multi-Hole DISI Engine Injector

“…more liquid in the footprint) when viewed from the chamber base (equivalent to the piston crown in an engine). It should be borne in mind that the injector is mounted at an angle of 19° in the chamber and hence the view shown in the images is angled to that which would be seen from the piston crown in an engine, as presented in [12]. Figure 23 shows the total spray area as seen from the base view during spray development from 'first fuel' seen (333 μs ASOI) to a fully-developed spray form (1110 μs ASOI).…”

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

“…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.…”