NUMERICAL PREDICTIONS AND EXPERIMENTAL INVESTIGATIONS ON EXTENDED EXPANSION ENGINE PERFORMANCE AND EXHAUST EMISSIONS 2000-01-1415
This paper deals mainly with the computer simulation and experimental investigations on a single cylinder, four stroke, spark ignited, extended expansion engine. The simulation procedure involves thermodynamic and global modeling techniques. Submodels for predicting gas exchange processes, heat transfer and friction are used. Two-zone model is adopted for combustion process. The combustion model predicts mass burning rate, ignition delay and combustion duration. It uses sub-models for calculating flame-front area, flame-speed and chemical equilibrium composition of ten product species. Experimentally measured valve-lift data along with suitable coefficient of discharge is used in the analysis of gas exchange process. Unburned hydrocarbons, carbon monoxide and nitric oxide emissions have also been predicted.
Experiments have been conducted on a single cylinder, air cooled, four stroke, spark ignition engine. A production engine was modified to run as extended expansion engine. Late intake valve closure (LIVC) associated with variable compression ratio (VCR) concept is adopted to achieve extended expansion. Clearance volume has been altered to achieve variable compression ratio. In this technique, expansion ratio (ER) alone is varied while effective compression ratio (CR) is kept constant, thereby ER/CR ratio is altered. For extended expansion engine, ER/CR ratio ranges from 1.25 to 2 at CR of 7 and 8. Experiments were conducted at a constant rated speed of 1500 rpm. Performance and exhaust emissions have been measured at different loads and ER/CR ratios. The predicted performance and emission characteristics are compared with experimentally measured values. There is a good agreement between the two. It is observed that, for extended expansion engine with CR 8 and at one third load, there is a predicts mass burning rate, ignition delay and upto about 43% and improvement of about 21% in the brake thermal efficiency. At the same condition with CR 7, brake thermal efficiency improvement is about 33%. Also, unburned hydrocarbon emissions are reduced by about 60%. Carbon monoxide emissions are almost reduced to zero level. For the engine configuration considered, it is found that ER/CR ratio of 1.5 gives better performance compared to standard engine at both compression ratios.
From the present investigations and comparisons of results, it is concluded that, the simulation software developed in this work predicts the performance and emission characteristics of extended expansion engine reasonably well. Therefore, it is argued that the developed code can be used with confidence for further parametric studies and to optimize the ER/CR ratio for the given engine configuration.