A) Time losses

B) Heat transfer losses

C) Exhaust blow down losses

D) Pumping losses

A) Time Losses:

In ideal cycles the heat addition process was assumed at constant volume whereas in actual cycles the burning of fuel does not take place instantaneously and the entire process of combustion takes a definite time interval.

During this period of combustion the gases experience a change in volume.

The increased volume due to motion of piston results in lower maximum pressure and less work on the piston. Fig 1 shows the difference between ideal, fuel-air and actual cycles.

Point 3 represents the maximum pressure in case of ideal cycle while point 3’ represents the pressure in case of fuel-air cycles if burning would have been carried out instantaneously.

However, due to time lag in combustion the maximum pressure achieved corresponds to point - a. The shaded area represents the loss of work due to time losses or burning time losses which includes the incomplete combustion losses also.

Fig. Otto cycle-time losses

B) Heat Losses:

The ideal compression and expansion processes were assumed to be adiabatic, however in actual processes there is heat transfer from the working substance to the cylinder walls.

There is considerable amount of heat loss particularly during the combustion and expansion Processes due to which there Is loss of work and efficiency. These losses we called heat losses.

C) Exhaust Blow-down Loss:

Exhaust valve is assumed to open at bottom dead Centre (BDC) in case of ideal cycles while in actual engines 50 before B.D.C.

It helps in reducing the pressure in the cylinder during the expansion stroke so that the work required in pushing out the exhaust gases by the piston is reduced.

Due to this lot of heat energy is carried away by exhaust gases resulting into the loss of work Fig. 2 shows the ideal and actual cycles.  The shaded area represents the loss of work called exhaust blow down losses.

Fig. Exhaust blow down losses

D) Pumping Losses:

In case of ideal cycles the suction and exhaust processes were assumed to be at atmospheric pressure.

However some pressure differential is required to carry out the suction and exhaust processes between the fluid pressure and cylinder pressures.

During suction the cylinder pressure is lower than the fluid pressure in order to induct the fluid into the cylinder and the exhaust gases are expelled at a pressure higher than the atmospheric pressure. Fig.

Therefore some work is done on the gases during suction and exhaust stroke. This work is called pumping work as shown in fig by shaded area.