Sources of Hydrocarbon Emissions from Direct Injection Diesel Engines

Some results of a systematic study on sources of unburned hydrocarbons from direct injection diesel engines are presented. The following possible sources are considered and investigated experimentally and/or analytically: local over-mixing, local under-mixing, bulk quenching, cyclic misfire, cyclic variation, and wall effects. The significance of each source under a variety of operating conditions including simulated deceleration, light loads, high loads, and simulated acceleration are discussed.

The results show that the formation of unburned hydrocarbons is mainly controlled by transient fuel-air mixing and bulk quenching processes. The fraction of fuel appearing as unburned hydrocarbons in the exhaust is greatest at light loads and retarded conditions.