Combustion modelling for direct injection diesel engines

Abstract

Successful modelling of combustion systems depends on an adequate as well as a computationally efficient description of the reaction mechanisms. For a typical medium speed compression ignition direct injection (CIDI) engine under normal operating conditions, from 70 to 95 per cent of the injected fuel is in the vapour phase at the start of combustion and 10-35 per cent of the vaporized fuel has mixed to within flammability limits. This indicates that both a premixed combustion mechanism and a diffusion combustion mechanism are important to model CIDI engines. In the early stages of combustion, a premixed combustion mechanism should be utilized while, late in the combustion process, a diffusion-based mechanism will be dominant. The former is chemically based, while the latter is physically based. In this article, the two combustion mechanisms mentioned above and various combinations of the two mechanisms for the intermediate stages of combustion are investigated. The study was done on a Caterpillar 3401 diesel engine. It was found that by utilizing a hybrid scheme whereby the premixed combustion mechanism is employed up to a user-specified crank angle, beyond which point a diffusion combustion method is utilized, reasonable agreement for both in-cylinder pressure and heat release rate with experimental data could be obtained.