Fuel-Air Mixing and Diesel Combustion in a Rapid Compression Machine

The influence of charge motion and fuel injection characteristics on diesel combustion was studied in a rapid compression machine (RCM), a research apparatus that simulates the direct-injection diesel in-cylinder environment. An experimental data base was generated in which inlet air flow conditions (temperature, velocity, swirl level) and fuel injection pressure were independently varied. High-speed movies using both direct and shadowgraph photography were taken at selected operating conditions. Cylinder pressure data were analyzed using a one-zone heat release model to calculate ignition delay times, premixed and diffusion burning rates, and cumulative heat release profiles. The photographic analysis provided data on the liquid and vapor penetration rates, fuel-air mixing, ignition characteristics, and flame spreading rates.

Results show that charge temperature is the most significant parameter controlling the ignition delay period and it significantly affects the heat release profile in the premixed combustion phase. Temperature and pressure have little effect on the burning rates after the premixed phase which supports the theory that this latter phase is a mixing controlled process. Variations in injection rate and swirl level have a significant impact on the burning rates. Injection rate affects the entire heat release profile; swirl primarily affects the mixing controlled phase.