Mixture Preparation Strategies for a Four Valve Centrally Injected Flat Piston DISI Engine

This paper reports the Computational Fluid dynamic modelling to study the mixture preparation strategies for a 4 valve DISI engine under motoring condition. The suitability of air guided mixture preparation concept for a flat piston engine is investigated. Three different valve configurations viz., standard valve, forward tumble shroud valve and reverse tumble shroud valve are used to create different bulk air flow pattern viz., standard tumble, forward tumble and reverse tumble inside the engine cylinder. Two speed viz., 1000 and 2000 rpm with different fuel injection timings (90°, 180° and 270°) have been considered. The fuel injector is located near central axis of the cylinder and the spark plug is located between the intake valves on the fire deck of the engine. Gambit Pre-Processor is used to create the computational domain and the commercial CFD package STAR CD is used for simulation and post processing. The effect of air motion inside the cylinder on the turbulent kinetic energy and equivalence ratio prevailing near the spark gap are studied. Compared to standard valve configuration the forward tumble and reverse tumble valve configurations are able to produce higher tumble index for the speeds simulated. The mean turbulent kinetic energy prevailing inside the engine cylinder is sustained for the forward tumble and reverse tumble cases, well into the end of compression stroke which is attributed to the higher tumble for these cases, which may enhance burning rate. For the simulated cases of no shroud valve the equivalence ratio at the spark gap for higher speed (2000 rpm) is higher than the lower speed (1000 rpm) for the same injection timing. This may be attributed to higher turbulence at higher speed. The same trends is noticed for the forward tumble shroud valve and reverse tumble shroud valve configuration cases except for 180° injection timing where the higher speed case produces lesser strength of mixture at the spark plug gap. From the analysis of the simulated cases for different bulk flow inside the cylinder the forward tumble configuration with wide spacing arrangement of spark plug and injector, consistently produces a better mixture near the spark plug gap for all the injection timing for the two speeds simulated. Hence the forward tumble air motion with wide space concept may be a viable strategy for maintaining proper equivalence ratio at the spark plug at different loads and better combustion stability.