Some Techniques to Improve Performance of Si Engine at Part Loads - A Numerical and Experimental Approach

Work in this paper involves the computer simulation and experimental investigations on a single-cylinder, four-stroke, spark-ignition engine in which inlet valve closure timing and clearance volume are optimized 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. A two-zone model is adopted for combustion process. The combustion model used here predicts mass burning rates, ignition delay and combustion duration, etc. Sub-models for calculating flame-front area, flame -speed and chemical equilibrium composition of ten product species are used in combustion analysis. Measured valve -displacement along with suitable coefficient of discharge is used in the analysis of gas exchange processes. Unburned hydrocarbons and carbon monoxide emissions have been predicted. Experiments have been conducted on a modified single-cylinder, air-cooled, four-stroke, spark-ignition engine. A standard diesel engine was modified to carryout the experiments. Late intake valve closure (LIVC) associated with variable compression ratio (VCR) concepts have been used in the modified engine. Clearance volume was altered suitably to achieve variable compression ratio. In this work, geometric expansion ratio (GER) of modified alone is varied, while effective compression ratio (ECR) is kept constant, thereby GER/ECR ratio is altered. For modified engine, GER/ECR ratio was varied from 1.25 to 2. Experiments were conducted for two effective compression ratios, viz. 7 and 8 at a speed of 1200 rpm. Performance and exhaust emissions have been measured at different loads and GER/ECR ratios. The pre dicted performance and emission characteristics are compared with measured values and the agreement between the two is found to be good. From the present investigations, it is concluded that, this concept is a viable one and the simulation software developed in this work predicts the performance and emission characteristics of modified 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.