How to compare the Otto cycle and Diesel cycle to know which is better?
To know which is the better and more efficient cycle between the Otto cycle and diesel cycle, cycle analysis are made using important parameters like compression ratio, maximum pressure, maximum temperature, heat input, work output, etc.
Comparison of Otto Cycle and Diesel cycle
Usually, comparisons are made between these cycles under the 4 conditions i.e. (i) For the same compression ratio and same heat input, (ii) For constant maximum pressure and same heat input, (iii) For the same maximum pressure and temperature and (iv) For same maximum pressure and output. let us discuss one by one
(i) Comparision of Otto cycle and Diesel Cycle for the same compression ratio and same heat input
The figure shows the two cycles on the p-V and T-s diagrams. All the cycles start from the same initial state 1 and the air is reversible adiabatically (isentropically) compressed to the same state 2, as the compression ratio is the same.
Under this condition, the Otto cycle has the highest efficiency The diesel cycle has the least efficiency.
Reason
It is practically known that for the same compression ratio and same heat input, adding the heat at constant volume (Otto cycle) results in the highest maximum temperatures and pressures. Adding the same quantity of heat at constant pressure (diesel cycle) results in the lowest maximum temperatures and pressures.
On the T-s diagram the areas under the line 4-1, and b-1 represent the heat rejected for Otto, and Diesel cycle, respectively. The heat supplied for the two cycles is given by areas 2-3-6-5 and 2-a-6′-5. These areas are equal.
Hence we can easily conclude from the diagram that for the same compression ratio and same heat input, minimum heat is rejected in the Otto cycle and the maximum heat is rejected in the diesel cycle. Consequently, the Otto cycle has the largest work area and highest efficiency. The diesel cycle has the least efficiency
(ii) Comparision between the Otto cycle and Diesel Cycle for constant maximum pressure and the same heat input
Under this condition, the Diesel cycle is more efficient than the Otto cycle.
Reason
For the same maximum pressure, points 3 and “b” must be on the same pressure line and for the same heat input the areas 2-3-6-5 and a-b-6′-5 on the T-s diagram must be equal.
We can see from the T-s diagram that the heat rejected by the diesel cycle (area c-1-5-6′) is less than the heat rejected by the Otto cycle (area 4-1-5-6); hence the Diesel cycle is more efficient than the Otto cycle
(iii) Comparision of the Otto cycle and Diesel Cycle for the same maximum pressure and temperature
Under this condition once again Diesel cycle is more efficient than the Otto cycle.
Reason
We can see from the figure that the heat rejected by both Otto and diesel cycles is equal (i.e.area 4-1-5-6 on the T-S diagram). But the heat supplied to the diesel cycle (area a-3-6-5 on the T-S diagram) is more than that of the Otto cycle (area 2-3-6-5 on the T-S diagram).
Hence for this condition, the diesel cycle is more efficient than the Otto cycle.
(iv) Comparision of the Otto cycle and Diesel cycle for the same maximum pressure and output
Under this condition, the diesel cycle is more efficient than the Otto cycle.
Reason
Referring to Figure in the second condition(i.e. for constant maximum pressure and the same heat input), the work done in Otto and diesel cycles (areas 1-2-3-4 and 1-a-b-c respectively) can be equal only if point “b” has a greater entropy. In that case, the heat rejected by the diesel cycle is less than that of the Otto cycle. Hence diesel cycle is more efficient than the Otto cycle.
Conclusion
We have seen that for a given compression ratio Otto cycle is more efficient than the Diesel cycle but for other conditions, the diesel cycle is more efficient.
However, one should carefully consider the practical implications of the difference in efficiency between the Otto and Diesel cycles.
Even though the Otto cycle has the potential for higher thermal efficiency than the Diesel cycle, it is limited by the fact that a Petrol engine cannot handle high compression ratios.
As a result, we cannot fully utilize the advantages of the theoretical Otto cycle in a practical engine at a given compression ratio.
From the above discussion, we can conclude that the Diesel cycle is a better choice for the engines if the size and the cost of the engine are not a big issue.
References
- Thermal Engineering by R.K Rajput
- Internal Combustion Engine by M.L Mathur, R.P Sharma