06-04-2012, 11:05 AM
PERFORMANCE AND EMISSION CHARACTERISTICS OF A VARIABLE COMPRESSION SI ENGINE USING ETHANOL-GASOLINE BLENDS AS FUEL
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INTRODUCTION
The demand of energy increases rapidly with the growth in civilization. The majority of this demand is fulfilled from the combustion of fossil fuels. As a result, the reserve of the fossil fuel is rapidly exhausting. And the combustion of huge amount of fossil fuel creates enormous pollution to the atmosphere by emitting various pollutants. Researchers are trying to find out an alternative to petrol fuel. Ethanol is found to be one of the promising alternative fuel for SI engine. The most attractive properties of ethanol as a SI engine fuel are that it can be produced from renewable energy sources such as agricultural feedstock and it has high octane number and flame speed.
PROPERTIES OF ETHANOL
Performance and emission characteristics of the engine depend on chemical characteristics of a fuel. As ethanol contains 35% oxygen, the combustion of the fuel is improved due to leaning effect [10]. Ethanol has higher octane number than gasoline thus it can lead in operation at higher compression ratios. Also due to low calorific value and high latent heat of vaporization of ethanol, engine volumetric efficiency may increase.
EXPERIMENTAL SETUP
The experiment is carried out in a single cylinder (MK-25) variable compression ratio spark ignition engine. Typical views of test engine have shown in Fig. 1. The specifications of test engine are shown in Table 2. The tests were performed keeping the speed constant at 2800 rpm at all loads. The test fuels used are gasoline; gasoline and ethanol blends (maximum 20% ethanol with gasoline by volume). The experiment was performed at compression ratios of 6:1. 8:1 and 10:1.
Effect of Ethanol on Brake Specific Fuel Consumption
Brake specific fuel consumption (BSFC) is defined as the fuel consumption rate to produce unit brake power, i.e. it is the ratio of the fuel consumption rate and the brake power. As the heating value of ethanol is lower than the gasoline, fuel consumption (kg/s) for blended fuel is expected to be more than that of pure gasoline. Figures 3a-3c shows the variation of BSFC with brake power of the engine running on gasoline and various blends of ethanol and gasoline at different compression ratios. It is observed that the BSFC of the engine with pure gasoline as well as different blends of gasoline and ethanol decreases with the increasing loads from 0.8473 to 1.964 kW at all compression ratios.
The Effect of Ethanol on Volumetric Efficiency
The volumetric efficiency is defined as the ratio of actual volume of air-fuel mixture flow into the cylinder at atmospheric pressure and temperature to the volume displaced by piston. Figures 4a-4c shows the variation of volumetric efficiency with the brake power for different percentages of ethanol-gasoline blends (E10, E15 and E20) and pure gasoline (E0) at compression ratios of 6:1, 8:1 and 10:1 respectively. The results obtained by the experiment indicate that the volumetric efficiency increases with the increase of ethanol percentage in the blend compared to pure gasoline for a particular compression ratio. Figure 4b indicates the maximum increase of volumetric efficiency for E20 blend is 21.72% at a compression ratio of 8:1 in comparison to E0 at loads ranges from 0.8473 to 1.964 kW.
Effect of Ethanol on CO2 Emission
It is known that if the complete combustion takes place inside the combustion chamber then CO2 emission increases rapidly.
CONCLUSIONS
The following conclusion can be drawn from this experimental investigation on ethanol-gasoline blends.
The brake thermal efficiency increased but the BSFC decreased with the increase in compression ratio and ethanol % in the blend, the volumetric efficiency was maximum for E20 blend and compression ratio 10:1. CO2 emission increases and emission of CO decreases with the Ethanol % in the blend and also decreases due to increasing compression ratios and brake power.