Welcome to the Knowledge-Swami.com blog. This blog is about internal combustion I.C. engine Engine-Auto & Diesel operating cycles. Whether it’s a petrol engine or diesel engine, both have different operating cycles.
Before we proceed with I.C. Engine-Auto & Diesel Cycles and p-v diagram details, please refer to the following diagram to understand engine terminology eg. T.D.C, B.D.C.
Let’s understand different types of IC Engine cycles,
- Diesel Cycle
- Otto Cycle
Typically the I.C. engine-Auto & Diesel cycle diagram is drawn in the Pressure-Volume (PV) diagram.
To understand more precisely about Otto Cycle and Diesel cycle please refer to the following diagram.
Diesel Cycle:
Diesel engine works on a constant pressure cycle. Let’s understand the theoretical and practical (actual) diesel cycle.
Characteristics of an ideal diesel cycle:
The above diagram indicates the theoretical/idle diesel cycle.
The X-axis indicates volume (Cylinder Volume) & Y-axis indicates cylinder pressure.
- 1-2: Adiabatic compression
- 2-3: Combustion at the constant pressure
- 3-4: Adiabatic expansion
- 4-1: Exhaust at the constant volume
In reality, the diesel engine does not work on an ideal cycle process.
The ideal cycle indicates adiabatic compression and adiabatic expansion. The adiabatic process means no mass OR heat transfer in the thermodynamic system. In reality, heat transfer happens during compression and expansion.
The theoretical diesel cycle does not indicate a suction stroke process. In a suction stroke, air sucked inside the chamber as the piston creates a vacuum effect as the position moves from T.D.C to B.D.C
Actual Diesel Cycle:
Let’s review the following elements of the diesel cycle.
1-2 Air Intake / Suction:
In this cycle, air sucked inside the engine cylinder through a suction/inlet valve. The piston moves from T.D.C to B.D.C.
2-3 Compression stroke:
In this stroke, the piston moves from B.D.C to T.D.C, and the air gets compressed. Due to air compression, the heat generated in this process.
3-4 Combustion / Power stroke:
In this step, diesel fuel is injected inside the cylinder under high pressure. As per the theoretical diesel cycle, this process is executed under constant pressure, however, in the actual diesel cycle, it’s not really a ‘constant-pressure’ combustion process. In the combustion/power stroke, the pressure inside the cylinder increases, and heat generated. In this process, both the suction and exhaust valve will remain closed.
4-5 Expansion:
Gases generated inside the combustion chamber pushes piston from T.D.C to B.D.C. Pressure drop rapidly, however, pressure still be above atmospheric pressure.
5-1 Exhaust stroke:
At the end of the power stroke, the exhaust valve opens and exhaust stroke is initiated. In exhaust stroke, the piston moves from B.D.C to T.D.C. Exhaust gases passed out of the engine cylinder through the exhaust valve.
Characteristics of an ideal Otto cycle:
The above diagram indicates the theoretical/idle Otto cycle.
The X-axis indicates volume (Cylinder Volume) & Y-axis indicates cylinder pressure.
- 1-2: Adiabatic compression (piston moves from B.D.C to T.D.C)
- 2-3: Combustion at the constant volume
- 3-4: Adiabatic expansion (piston moves from T.D.C to B.D.C)
- 4-1: Exhaust at the constant volume
As explained in the Diesel cycle diagram, the petrol (gasoline) engine also does not work on an ideal cycle process.
The ideal cycle indicates adiabatic compression and adiabatic expansion, which is impossible to achieve in practice.
Actual Otto Cycle:
Let’s review the following elements of the Otto cycle.
1-2 Air Intake / Suction:
In this cycle, air & fuel mixture sucked inside the engine cylinder through a suction/inlet valve. The piston moves from T.D.C to B.D.C.
2-3: Compression stroke:
In this stroke, the piston moves from B.D.C to T.D.C, and the air & fuel mixture gets compressed.
3-4 Combustion / Power stroke:
In this step, the spark plug is ignited to burn air and fuel mixture. In the combustion/power stroke, the pressure inside the cylinder increases, and heat generated. In this process, both the suction and exhaust valve will remain closed.
4-5 Expansion:
Gases generated inside the combustion chamber pushes piston from T.D.C to B.D.C. Pressure drop rapidly, however, pressure still be above atmospheric pressure.
5-1 Exhaust Stroke
At the end of the power stroke, the exhaust valve opens and exhaust stroke is initiated. In exhaust stroke, the piston moves from B.D.C to T.D.C. Exhaust gases passed out of the engine cylinder through the exhaust valve.
Comparison of ideal and actual P-V diagram.
| Ideal p-v diagram | Actual p-v diagram |
| Adiabatic compression and expansion i.e. no heat & mass transfer. | In the real engine, heat is generated & transferred both in compression and in the expansion stroke. Engine cylinders are cooled by air-cooling or by liquid cooling. Heat transfer also happens from hot engine components to engine oil. Further engine oil is cooled in the engine oil pan and cooled oil is recirculated inside the engine. |
| Combustion at constant Pressure in the Diesel cycle | The combustion of a diesel engine cannot be done at constant pressure, the pressure inside cylinder increase after fuel injection. |
| Combustion at constant volume (isochoric process) cycle in Otto cycle. | The combustion of petrol (gasoline) engines cannot be done at constant volume. The air-fuel mixture cannot be ignited instantaneously as there will be an ignition delay. |
We hope this blog helped to understand the ideal and actual P-V diagram of an internal combustion engine.
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