Among the existing concepts that help to improve the efficiency of spark ignition engines at part load, Controlled Auto-Ignition™ (CAI™) is an effective way to lower both fuel consumption and pollutant emissions without major modifications of the engine design. The CAI™ concept is based on the auto-ignition of a fuel mixture highly diluted with burnt gases in order to achieve high indicated efficiency and low pollutant emissions through low temperature combustion.
In a 4-stroke engine, large amounts of burnt gases can be trapped in the cylinder by re-breathing them through the exhaust ports during the intake stroke using a 2-step exhaust valve-lift profile. The interaction between the intake and exhaust flows during the intake stroke was identified as a key parameter to control the subsequent combustion in a CAI™ PFI engine. Consequently, the intake valve-lift profile as well as the exhaust re-opening profile can potentially be used as control parameters for this combustion mode.
In the present study, a fully Variable Valve Actuator was used to investigate the influence of each valve-lift profile (opening/closure timing, opening/closure speed, peak valve-lift and valve-lift duration) on the engine behaviour. Furthermore, the operating points were analysed with a 0D code to investigate the mean in-cylinder conditions (species, pressure, temperature) and to help to understand the consequences of the valve actuation choices. The influence of the valve-lift profile characteristics on the combustion process and the engine efficiency are presented. Finally, the profile characteristics with the best potential to control the engine in real operating conditions are identified.