In the last three articles we studied about Otto cycle, diesel cycle and dual cycle and looked at their thermal efficiency. In this article we will take a collective look at these three cycles in order to compare and contrast them, so that we can come to know the relative advantages and disadvantages of these cycles.
In the last article we compared the three cycles as their peak temperature, peak pressure and heat rejection were kept constant for all cycles. In this article we will focus on peak pressure and heat addition being kept constant. The P-V and T-S diagrams of these three cycles for such a situation are drawn simultaneously as described below.
In the above diagrams the following are the cycles
- Otto cycle: 1 – 2 – 3 – 4 – 1
- Dual cycle: 1 – 2’ – 3 – 3’ – 4’ – 1
- Diesel cycle: 1 – 2” – 3” – 4” – 1
Remember that we are assuming the same peak pressure denoted by Pmax on the P-V diagram. Heat addition is also same for each case thought represented by a different area for each cycle as seen on the T-S chart as follows
- Otto cycle: 2 – 3 – 6 - 5
- Dual cycle: 2’ – 3 – 3’ – 6’ – 5
- Diesel cycle: 2” – 3” – 6” - 5
The heat rejected in each cycle is given by the areas as follows from the T-S diagram
- Otto cycle: Area under 1 – 4 – 6 – 5 say q1
- Dual cycle: Area under 1 – 4’ – 6’ – 5 say q2
- Diesel cycle: Area under 1 – 4” – 6” – 5 say q3
It can also be seen from the same diagram that q1>q2>q3
We know that thermal efficiency is given by 1 – heat rejected/heat supplied
Since heat rejected is same and we know the order of magnitude of heat supplied, we can combine this information to conclude that the thermal efficiency of these engines under given circumstances is of the following order
Diesel>Dual>Otto
Hence in this case it is again the diesel cycle which shows greater thermal efficiency.
In the last four articles we have seen four different scenarios of comparison between the three standard air cycles. In some cases the thermal efficiency of an Otto cycle is more, while in others it is the diesel cycle. Hence you can imagine that the dual or mixed cycle is the optimum choice as it tries to take advantage of the strengths of both types of cycles.