COMPARISON OF OTTO AND DIESEL CYCLE EFFICIENCY ON THE BASIS OF SAME MAXIMUM TEMPERATURE AND PRESSURE

Comparison of Otto & Diesel cycles efficiency on the basis of maximum temperature and maximum pressure:

Both Otto cycle and Diesel cycle are idealised thermodynamic cycles which can convert heat energy into useful work done and hence, be the basis of spark ignition and compression ignition internal combustion engines.

Although, both the cycles produce useful work, they have different efficiencies and they can be compared under different constraints and parameters. The most important of them is the comparison on the basis of same maximum cycle temperature and pressure, which is obviously a design constraint. In both the cycles, maximum temperature occurs at the end of compression ie., the state after compression will be same for both the cycles.

In the figure processes 1-2'-3-4-1 is the p-v diagram for Otto cycle, where as processes 1-2-3-4-1 represents diesel cycle and in both the cycles maximum temperature and pressure occurs at state 3, hence both have same maximum temperature and pressure. From the diagrams it has been seen that heat rejection is same for both the cycles and equal to Q_4-1. Where as heat input in diesel cycle is Q_2-3 and in Otto cycle it is Q_2'-3.

Efficiency_Otto = 1 - (Q_4-1 /Q_2'-3)

Efficiency_diesel = 1 - (Q_4-1 /Q_2-3)

From the T-s diagram, we can say area A-2-3-B represents heat transfer during heating in diesel cycle or Q_2-3 and area A-2'-3-B represents heat transfer during heating in Otto cycle or Q_2'-3. As the area A-2-3-B is larger than area A-2'-3-B, we can conclude,

Q_2-3 > Q_2'-3

Therefore,
(Q_4-1 /Q_2'-3) > (Q_4-1 /Q_2-3)
or, 1 - (Q_4-1 /Q_2'-3) < 1 - (Q_4-1 /Q_2-3)
or Efficiency_otto < Efficiency_diesel