Theoretical and Experimental Investigations of Extended Expansion Concept for SI Engines 2002-01-1740
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. Sub-models have been used for predicting heat transfer, friction and gas exchange processes. A two-zone model is adopted for combustion process. Combustion model predicts mass burning rate, ignition delay and combustion duration. It uses sub-models for calculating flame-front area, flamespeed 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 diesel engine was modified to run as extended expansion engine. Late intake valve closure (LIVC) associated with variable compression ratio (VCR) concept has been used to achieve extended expansion. Clearance volume were altered to achieve variable compression ratio. In this technique, geometric expansion ratio (GER) alone is varied, while effective compression ratio (ECR) is kept constant, thereby GER/ECR ratio is altered. For extended expansion engine, GER/ECR ratio was varied from 1.25 to 2. Experiments were conducted for two compression ratios, viz. 7 and 8 at the rated speed of 1500 rpm. Performance and exhaust emissions 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 extended expansion engine with ECR 8 and at onethird load, the pumping losses are reduced by 43% and improvement in the brake thermal efficiency is found to be 21%. For the same condition with ECR 7, brake thermal efficiency improvement is about 33%. Also, unburned hydrocarbon emissions have reduced by about 60%. Carbon monoxide emissions are almost reduced to zero level. For the engine configuration considered, it is found that GER/ECR ratio of 1.5 gives the best performance compared to standard engine for both compression ratios.
From the present investigation and comparison of results, it is concluded that, extended expansion concept is a viable one and the simulation software developed in this work predicts the performance and emission characteristics of extended expansion engine quite well. Therefore, it is argued that the developed code can be used with confidence for further parametric studies and to optimize the GER/GCR ratio for the given engine configuration for practical applications.