An Experimental Database Dedicated to the Study and Modelling of Cyclic Variability in Spark-Ignition Engines with LES

In spark-ignition engines, cyclic variability limits the optimisation of operating conditions (choice of spark advance and/or injection timing) since it induces load variations and the occurrence of misfire and/or knock. This, in turn, restricts the operation range of new concepts such as downsizing or stratified combustion. To understand the basic physical phenomena behind cyclic variations, careful experimental studies are necessary to simultaneously characterise the combustion and the unsteady flow in the complete engine set-up. With a well-characterised experimental engine set-up, Large Eddy Simulation (LES) modelling can be easily combined with experiment in order to tackle intricate physical phenomena couplings.

This paper describes an experimental database acquired on an optical research engine. The single-cylinder spark-ignition engine is equipped with four valves, a pentroof combustion chamber and a flat piston. The database is dedicated to the validation of LES models. It includes crank-resolved pressure measurements at different points in the air path and exhaust line and over several operating conditions. Volumes are implemented in the intake and exhaust lines to ensure LES to be imposed with accurate experimental boundary conditions. Local flow motion as well as combustion are characterised with optical diagnostics. The experimental campaign includes both stable (low cyclic variability) and unstable (high cyclic variability) conditions. Stable operating points are selected to check LES ability to reproduce simple engine behaviours (spark advance sweep, engine speed and load variations, influence of mixture strength or dilution). Unstable operating points are considered to evaluate engine LES reliability to capture cycle-to-cycle variations and to maximise the benefits from the combination of experiment and modelling for cyclic variability investigation.