Abstract
Large power Diesel engine for Heavy Duty transport applications can be efficiently modified to be fuelled with compressed natural gas (CNG) in order to achieve better pollutants emissions performance together with lower CO2 levels for power unit. Nevertheless natural gas engines still present operating problems about specific fuel consumption and the release of unburned hydrocarbons (UHC). Both engine piston-ring design and valve timing originally designed for diesel engine could be incorrectly tailored for gas fuelling conditions, so leading to high UHC emissions from the cylinder. In the present work a simulation model developed to represent engine performance and pollutant emissions for any arbitrary engine geometry has been intensively used to optimise valve timing of a turbocharged Heavy Duty spark ignition 6-cylinder engine. The optimisation process has been accomplished with respect to maximum volumetric efficiency, minimum specific fuel consumption and HC emissions constraints. The model has been tested by comparing simulation results with extensive experimental data measured at CNR Istituto Motori with the engine operating at a stoichiometric conditions. After validating model accuracy to predict the engine behaviour, through the numerical optimisation process new cams profiles better satisfying all the constraints have been designed.