Dual Combustion Cycle
Dual combustion cycle is the combustion of otto and diesel cycles. It is sometimes called semi-diesel cycles because semi-diesel engines work on this cycle. In this cycle, heat is absorbed partly at constant pressure.
The dual combustion cycle has two adiabatic, two constant volume and constant pressure processes. These processes are described in the p-v and t-s diagram as shown in the figure.
Let the engine cylinder contain m kg of air at point 1. At this point let p1, T1 and V1 be the pressure, temperature and volume of the air.
Read also: Thermodynamic Cycle: Its Classification, Working, Terms Used in Thermodynamics and More.
Stages of an Ideal Dual Combustion Cycle
Following are the five stages of an ideal dual combustion cycle.
- Constant Pressure Heating (Process 5-1)
- Constant Pressure heating (Process 1-2)
- Adiabatic Expansion (Process 2-3)
- Constant Volume Cooling (Process 3-4)
- Adiabatic Compression (Process 3-4)
1. Process 5-1 (Constant Pressure Heating)
The air is heated at constant pressure from initial temperature T1 to a temperature T2 represented by the graph 1-2 in the figure.
2. Process 1-2 (Constant Pressure heating)
The air is expanded adiabatically from temperature T2 to a temperature T3 as shown by the graph 2-3 in Fig. In this process, no heat is absorbed or rejected by the air.
3. Process 2-3 (Adiabatic Expansion)
The air is now cooled at constant volume from temperature T3 to temperature T4 as shown by the graph 3-4 in the figure.
4. Process 3-4 (Constant Volume Cooling)
The air is expanded adiabatically from temperature T4 to a temperature T5 as shown by the graph 4-5 in Fig. In this process, no heat is absorbed or rejected by the air.
5. Process 3-4 (Adiabatic Compression)
The air finally heated at constant volume from temperature T5 to temperature as shown by the graph 5-1 in Fig.
We see that the air has been brought back to its original condition of pressure, volume and temperature, Thus completing the cycle.
We know that
Work done = Heat absorbed – Heat rejected
Air standard efficiency,
Compression ratio,
Cut-off ratio,
Pressure ratio,
For constant pressure process 1-2
Adiabatic expansion
For constant volume heating 5-1
Adiabatic compression 4-5
Substituting values of T2, T3, T4 & T5 in equation (1)
If p = 1 in the above equation, we get
Note:
For the otto cycle, p=1 substituting this value in the equation.
For the diesel cycle, a=1 substituting this value in the equation.
The efficiency of duel combustion cycle is greater than the Diesel cycle and loss than the Otto cycle, for the same compression ratio.
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