2. You should set the initial conditions (green boxes) to reasonable values, explaining your choices. You should also explain the effects that increasing or decreasing these values have on the model. The initial conditions for density of air, ambient pressure, and ambient temperature for the engine have been input in the relevant green boxes in the excel file. The air pressure at sea level is 1.225 kg/m3, rounded off to 1.2 kg/m3 (Ahrens et al. 2012). This is at a temperature of 15°C (The Engineering Toolbox 2013). This air density is chosen because it is the density of air present at sea level. The ambient temperature is set at 300 K because this is generally taken as the average room temperature. The ambient pressure is chosen as 1.000 bar because the standard atmospheric pressure is generally indicated as 1 bar, which is equated to 100,000 Pa (Ahrens et al. 2012). Air density affects the pressure in the cylinder, ambient pressure affects the power generation and ambient temperature affects the efficiency of the engine. The effect of increasing the given value of air density would be that the pressure in the cylinder will increase for the compression ratio mentioned in the table. There will be more air to mix with fuel if the air density is high and therefore, there will be more power. Increase in the pressure at the same temperature will result in increase in power generation. Increasing the given value of ambient pressure will cause an increase in the maximum pressure in the cylinder. This will in turn increase the power and torque of the engine and vice versa. At low pressure, the air per cubic meter is lesser due to lesser pressure. This will lead to a reduction in performance of the engine. This effect is dependent on the speeds, as shown by Shannak and Alhasan (2002). At a low speed of up to 2500 rpm, volumetric efficiency and fuel consumption of the engine are found to decrease at with decrease in ambient pressure. On the other hand, at a higher speed of 3000 rpm, engine performance is found to decrease with increase in atmosphere pressure. Furthermore, while it can be assumed that at higher pressure the power and torque of the engine increases, there are more structural loads on the car and the fuel consumption is also increased (Ayers 1997). If the ambient temperature is reduced, the efficiency of the engine will increase. Increase in the ambient temperature results in a reduction in the engine efficiency as well as output (Energy and Environmental Analysis, Inc. 2008). All engines are known to have a better performance at lower temperatures (Ayers 1997). This will happen because reduction in ambient temperature will cause a higher energy transfer to the crankshaft. Thus, the energy loss will be reduced and the engine will have higher efficiency. 3. You now need to select suitable values for the polytropic index for compression and expansion (pink boxes) explaining your choices with reference to adiabatic, isothermal and polytropic processes and the differences between them. The suitable values for the polytropic index for compression and expansion have been input into the relevant pink boxes in the excel file. The values are given as 1.3 and 1.2 respectively for polytropic index of compression and polytropic index of expansion. The polytropic index of compression for petrol engines is generally reported as 1.3 (Huleihil and Mazor 2012). A polytropic index of 1 is considered isothermal while that of 1.4 is considered adiabatic (The Engineering Toolbox 2013). In our engine, the
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