The Impact of Engine Design Constraints on Diesel Combustion System Size Scaling

A set of scaling laws were previously developed to guide the transfer of combustion system designs between diesel engines of different sizes [ 1 , 2 , 3 , 4 ]. The intent of these scaling laws was to maintain geometric similarity of key parameters influencing diesel combustion such as in-cylinder spray penetration and flame lift-off length. The current study explores the impact of design constraints or limitations on the application of the scaling laws and the effect this has on the ability to replicate combustion and emissions. Multi dimensional computational fluid dynamics (CFD) calculations were used to evaluate the relative impact of engine design parameters on engine performance under full load operating conditions. The base engine was first scaled using the scaling laws. Design constraints were then applied to assess how such constraints deviate from the established scaling laws and how these alter the effectiveness of the scaling effort. The considered design parameters included engine speed, fuel injection pressure, nozzle hole size, injection duration, compression ratio, intake valve closing, squish height and effective compression ratio. For each test case a start of injection (SOI) sweep was carried out. The impact of the engine parameters on engine-out emissions was evaluated using averaged deviation of the emission data over the range of the SOI tests. The results revealed that duration of injection (DOI) has the biggest impact, followed by the injection velocity both of which are parameters that are related to local equivalence ratio. Injection velocity was found to affect the mixing as well. It was also found that engine speed had a negligible effect on the combustion characteristics within the tested range. This is attributed to the rapid fuel vaporization and the short ignition delay characteristic of the tested conditions.