How does varying speed affects the engine efficiency at constant load?

Answer 1

For constant load, if you want to increase the speed, you have to use more amount of fuel. Efficiency of engine will increase upto certain speed and decreases above that certain speed. Factors affecting efficiencies are 1) volumetric efficiency (while suction): let volume of air-fuel mixture at atmosphere temperature be Va total volume of cylinder be Vcy volumetric efficiency=Va/Vcy At higher speeds, during suction stroke, the displacement of piston will be very fast from TDC to BDC, at the same time the air-fuel mixture from inlet will not fullfill the the engines total volume,because air-fuel mixtures need some time to fullfill the engine,though the piston is moving with high speed within short duration from TDC to BDC (during suction stroke).Occupation of air-fuel mixture depends upon the pressure difference between inlet and inside the cylinder (more pressure difference increases the velocity of air-fuel mixture). 2)While compression: At higher speeds,volumetric efficiency will be low.Volumetric efficiency affects the pressure at the end of compression stroke. (i.e, In the end of suction stroke, air-fuel mixture occupies more volume of the cylinder at low speeds and less volume at high speeds.If we are compessing more volume to state1,pressure per kg of air-fuel mixture will be high at the end of compression and compressing less volume to the same state1,pressure per kg of air-fuel mixture will be low at the end of compression.) 3)While ignition: a) At lower speeds, gas will be ignited within a short time, compared to the movement of piston, speed of burning is compact.At higher speeds,gas will be ignited within a same short time but compared to the movement of piston, speed of burning is negligibly low. b)low volumetric efficiency(at high speed) results in low pressure at the end of compression.For this condition,after burning the pressure per kg of gas will be low but it maintains it piston speed because of presence of more amount of gas(i.e, inlet fuel is more at high speed). 4)While expansion: During expansion,the pressure at the end of stroke will be always high compared to the atmosphere pressure.To protect the engine from the pressure exerted at the end of stroke ,the outlet valve will be opened before piston reaches BDC. If fuel used is more, pressure(Ph) exerted at the end of stroke will be very high compared to pressure(Pl) exerted by the use of less fuel.Here,energy interms of pressure in not extracted. Amount of energy extracted during expansion stroke , (at high speed) =((P2)x(V2))-((P1)x(V1)) (at low speed)=((P4)x(V4))-((P3)x(V3)) ((P2)x(V2))-((P1)x(V1)) is greater than ((P4)x(V4))-((P3)x(V3)) where (P2) is pressure at the end of ignition at higher speed (P1) is pressure at the end of expansion at higher speed (V2) is volume at the end of ignition at higher speed (V1) is volume at the end of expansion at higher speed (P4) is pressure at the end of ignition at lower speed (P3) is pressure at the end of expansion at lower speed (V4) is volume at the end of ignition at lower speed (V3) is volume at the end of expansion at lower speed but {(P2)-(P1)}/(P2) is less than {(P4)-(P3)}/(P4). 5)While exhaust: At high speed (more amount of fuel will be used),energy (interms of pressure and temperature) carried by the gas after expansion stroke will be high compared to the energy carried by the gas at low speed.