Performance And Emission Characteristics Of Spark Ignition Engines Based On Engine Operating Parameters

ABSTRACT

Poor vehicle maintenance culture and high proportions of old vehicles are major contributing factors to high vehicle emission levels. The purpose of this study was to determine performance and emission characteristics of vehicles that were taken for inspection at the Vehicle Inspection Centre in Nairobi City based on engine operating parameters and to develop performance and emission prediction models. The specific objectives were; to determine vehicle exhaust emission levels, engine’s performance and emission characteristics and to develop performance and emission models. The sample size comprised 384 petrol vehicles randomly selected. The key observations included vehicle usage, compression pressure, ignition angle, engine speed, spark plug gap, and vehicle category. The key variables examined were emission of CO, HC and CO2, excess air factor (λ) and factors that influence emissions. Logistic regression model was fitted to determine the probability of tested vehicles failing emission tests based on the test variables. Field data were simulated using engine test bed and the effects of engine input variables on engine performance and emission were determined. Sub-model equations were generated from engine performance and emission curves and superimposed to develop engine performance and emission models. Validation, optimization and sensitivity analysis of the models were done. The mean vehicle usage ranged between 14328 km/yr and 19640 km/yr and the lowest compression pressure of 6.8 bar was recorded in the non-catalytic vehicles manufactured before 1986. Both categories of non-catalytic vehicles operated at a rich mixture. There was significant difference between the measured and standard values of exhaust emission gases. The models developed predicted well for engine performance and emission as expressed by low percentage error in most of the points. Optimization of Specific Fuel Consumption (SFC) model gave input variables of 2839 rpm, 16o BTDC, 1.05 λ and spark plug gap of 0.8 mm. Excess air factor was found to be the most sensitive variable when adjusted by ±10%, it mostly affected engine performance and emissions. In conclusion, exhaust emission levels from vehicles measured in Nairobi City were 6.8% vol., 4.41% vol., 1.16% vol. and 0.46% vol. CO for non-catalytic vehicles manufactured before 1986, non-catalytic vehicles manufactured between 1986 and 2002, catalytic vehicles manufactured between 1986 and 2002, and catalytic vehicles manufactured after 2002 respectively. The mean values for HC were 1814 ppm, 1884 ppm, 333 ppm, and 253.4 ppm for non-catalytic vehicles manufactured before 1986, non-catalytic vehicles manufactured between 1986 and 2002, catalytic vehicles manufactured between 1986 and 2002, and catalytic vehicles manufactured after 2002 respectively. These values were significantly different from the limits given in KS 1515-2000. Excess air factor for noncatalytic vehicles manufactured before 1986 and those manufactured between 1986 and 2002 oper were 1.14 and 1.08 respectively, while for catalytic vehicles manufactured between 1986 and 2002, and catalytic vehicles manufactured after 2002 were within the required limit of 1±0.03. There were significant changes in engine performance and emissions when the input variables were changed from optimal values. The models developed predicted well engine performance and emission characteristics. It is recommended that emissions control mechanisms be put in place to reduce emission levels. There is also need to include more test parameters in the models so as to improve on the prediction levels.