Optimization of inlet valve closure timing and clearance volume of a SI engine for better performance at part loads - A numerical and experimental approach

In this paper, a computer simulation and experimental investigations on a single cylinder, four-stroke, spark ignited engine are carried out to optimize inlet valve closure timing and clearance volume for better part-load performance. The simulation procedure involves thermodynamic and global modeling techniques. Many sub-models have been used for predicting heat transfer, friction and gas exchange processes. Unburned hydrocarbons, carbon monoxide and nitric oxide emissions are also predicted. Experiments have been conducted on a single cylinder, air-cooled, four-stroke, spark-ignited engine. In this work, by varying inlet valve closure timing (IVCT) and clearance volume, geometric expansion ratio (GER) of engine alone is varied, while effective compression ratio (ECR) is kept constant, thereby GER/ECR ratio is altered. For modified engine, GER/ECR ratio is varied from 1.25 to 2. Experiments have been conducted for two effective compression ratios, viz., 7 and 8 at a speed of 1200 rpm. Performance and exhaust emission characteristics have been measured at different loads and GER/ECR ratios. The predicted performance and emission characteristics are compared with measured values and the agreement between the two is found to be good. It is observed that, for modified engine, considerable improvements have been found in the reduction of pumping losses (about 25.8 to 56%), increase in the brake thermal efficiency (about 13.6 to 25%), and reduction in unburned hydrocarbon emissions (about 18.5 to 58%). Finally, for modified engine, it is found that GER/ECR ratio of 1.5 gives the best performance compared to standard engine for both compression ratios.