1. What are Actual Cycles ?
In thermodynamics and internal combustion (IC) engine analysis, Actual Cycles represent the real working cycles of engines as they operate in practice. Unlike air-standard or fuel–air cycles, actual cycles account for all real-world losses and imperfections such as friction, heat transfer, incomplete combustion, gas exchange losses, and time effects.
Actual cycle analysis is essential for understanding:
- Real engine performance
- Deviation from ideal behavior
- Efficiency losses
- Practical limitations of engines
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2. Why Actual Cycles Are Needed
Ideal cycles (air-standard and fuel–air cycles) assume simplified conditions that do not exist in real engines. Actual cycles are required because real engines experience:
- Heat loss to cylinder walls
- Friction between moving parts
- Incomplete combustion
- Finite time for combustion
- Valve timing losses
- Pumping work during intake and exhaust
3. Definition of Actual Cycle
An Actual Cycle is the real thermodynamic cycle followed by the working fluid in an internal combustion engine, considering combustion, heat transfer, friction, gas exchange, and mechanical losses.
It is usually represented by an actual indicator diagram (P–V diagram) obtained experimentally using an engine indicator.
4. Characteristics of Actual Cycles
Actual cycles differ significantly from ideal cycles due to the following features:
- Compression and expansion are not perfectly isentropic
- Combustion occurs over a finite crank angle
- Heat is lost during all processes
- Intake and exhaust processes consume power
- Peak pressure and temperature are lower than ideal
5. Actual Cycles Processes
1. Suction (Intake) Process
- Inlet valve opens before top dead center (TDC)
- Air or fuel–air mixture enters the cylinder
- Pressure inside cylinder is slightly below atmospheric
- Some work is required to draw in charge (pumping loss)
2. Compression Process
- Both valves closed
- Compression is polytropic, not isentropic
- Heat loss occurs to cylinder walls
- Actual compression pressure is lower than ideal
3. Combustion Process
- Combustion is not instantaneous
- Occurs over a finite crank angle
- Heat loss to cylinder walls occurs
- Peak pressure occurs after TDC
- Combustion may be incomplete
4. Expansion (Power) Process
- High-pressure gases expand and produce work
- Expansion is polytropic
- Heat is lost during expansion
- Work output is lower than ideal cycle
5. Exhaust Process
- Exhaust valve opens before bottom dead center (BDC)
- High-pressure gases escape (blowdown)
- Residual gases remain in the cylinder
- Some work is consumed in pushing exhaust gases out
6. Actual Cycles Indicator Diagram
The actual indicator diagram differs from ideal cycles due to:
- Rounded corners instead of sharp points
- Lower maximum pressure
- Pumping loop (intake–exhaust loop)
- Smaller enclosed area (less work output)
7. Comparison of Actual Cycle with Ideal Cycles
| Aspect | Ideal Cycle | Actual Cycle |
|---|---|---|
| Compression | Isentropic | Polytropic |
| Combustion | Instantaneous | Finite time |
| Heat loss | Neglected | Present |
| Friction | Neglected | Present |
| Exhaust & intake | Idealized | Pumping losses |
| Efficiency | Higher | Lower |
| Practical relevance | Theoretical | Realistic |
8. Losses in Actual Cycles
a) Heat Losses
- Heat transferred to cylinder walls
- Reduces thermal efficiency
b) Friction Losses
- Piston, bearings, valves, camshaft
- Reduces brake power
c) Pumping Losses
- Work required during intake and exhaust
- Reduces net work output
d) Combustion Losses
- Incomplete combustion
- Flame quenching near walls
9. Efficiencies Related to Actual Cycles
- Indicated Thermal Efficiency
Ratio of indicated work to fuel energy supplied - Mechanical Efficiency
Ratio of brake power to indicated power - Brake Thermal Efficiency
Ratio of brake power to fuel energy supplied
10. Actual Otto and Diesel Cycles
Actual Otto Cycle
- Used in spark ignition engines
- Combustion starts before TDC
- Heat addition occurs partly at constant volume and partly at constant pressure
Actual Diesel Cycle
- Used in compression ignition engines
- Combustion controlled by fuel injection
- Pressure does not remain constant throughout combustion
11. Advantages of Actual Cycle Analysis
- Realistic engine performance evaluation
- Helps in engine design and optimization
- Useful for diagnostic and testing purposes
- Accurate prediction of power and efficiency
12. Limitations
- Highly complex analysis
- Requires experimental data
- Difficult to express analytically
- Depends on engine speed, load, and condition
13. Applications
- Engine testing and performance analysis
- Design improvement of IC engines
- Estimation of real efficiency and power
- Research and development