Effect of piston bowl geometry on combustion and emissions of a direct injected diesel engine
The effect of piston bowl geometry on the performance and exhaust emissions from a modern, high speed direct-injection (HSDI) diesel engine was investigated. Four piston bowl geometry’s (shape) were designed, manufactured and tested in a pre-production HSDI diesel engine installed on an eddy-current dynamometer. A series of experimental tests were performed to determined the optimum injector configuration for each piston bowl shape, the best bowl shape for minimum drive-cycle simulated emissions, and the effect of in-cylinder swirl ratio at various engine operating conditions. Results from computational fluid dynamics (CFD) combustion simulation of extreme injector configurations, correlated well with the experimental trends observed. Full-load testing to determine the optimum injector configuration for each piston bowl shape, indicated that exhaust emissions were very sensitive to the point of fuel impingement on the piston bowl walls. In particular, the trend in the emission of particulates and NOx was explained in relation to the point of fuel impingement, and supported by CFD combustion simulation. The emission of smoke and particulates was found to be dependent on wall wetting and late combustion. Key features for the successful design of future HSDI piston bowl shapes were identified, based on the results form piston bowl comparison tests at a selection of the European drive cycle simulation conditions. The effect in-cylinder swirl ratio on engine performance and emissions was determined. An increase in the rate of mixing and heat release from higher swirl generally raised the emissions of NOx, but reduced smoke formation at low engine speeds. Benefits of an increase in swirl on emissions were negated at high engine speeds due to throttling of the intake charge.