Gas power cycles produce power.

Analysis is complex and confusing.

 Use assumptions to make the analysis simple.

These assumptions help in better

understanding. More

assumptions cause more

deviations from actuality. Otto cycle is a gas power cycle.

Petrol engines use Otto cycle. It

has two isentropic and two constant

volume processes. It is a  4 stroke

cycle. German engineer Otto made

first petrol engine in 1861.


A gas power cycle is one where the working substance is gas. In the analysis of gas power cycles, we take the working substance as air  for simplicity. These cycles are Air Standard Cycles.

List of Air Standard Cycles

  1. Otto cycle

 Petrol engines use it.

  1. Diesel cycle

 Diesel engine use it.

  1. Dual Cycle

 Diesel engine use it.

  1. Brayton Cycle

 Gas turbine use Brayton Cycle.

All these cycles are thermodynamic cycles. Energy  changes from one form to another form. In a thermodynamic cycle, normally four processes are in series. The cycle repeats to get motion.


In actuality, the working substance is different in different cycles. For simplicity, assume the working substance as air. Consequently these cycles become Air Standard Cycles. In all these cycles, there are variations of pressure, temperature, volume, entropy, enthalpy and internal energy. Therefore, real cycles become highly complex and confusing.

Steps for simplicity

In order to have simplicity in cycle analysis, any one aspect of a process is constant. Thus, one or more of the following processes are there in a cycle.

(i) Constant temperature

(ii) Constant pressure

(iii) Constant volume

(iv) Constant enthalpy

(v) Constant entropy

(vi) Constant internal energy processes

  Heat supplied and heat rejected (sink) are  external to the cycle.

Assumptions used in air standard cycles

  1. The working substance is air and it is a perfect gas.

  2. The chemical composition of air does not change in the entire cycle.

  3. The air obeys all the gas laws in Toto.

  4. The specific heat of air is constant in the entire range of temperatures and pressures in the cycle.

  5. The compression and expansion processes are truly adiabatic.  No heat gain or loss  takes place during the process.

  6. Assume the cycle is a closed cycle. Use a constant quantity of air.

  7. All the processes are internally reversible.  There is no friction or any other type of losses in the cycle.


Two Stroke Cycle

In this, the cycle is completed in two strokes i.e. one revolution of the crankshaft. There is one power stroke in each cycle. Examples are Light motorcycles, Scooters, Go carts and Lawn Movers

Four Strokes Otto Cycle

In this, the cycle is completed in four strokes i.e. two revolution of the crankshaft. There is one power stroke in each cycle but in two revolutions. Examples are Gasoline and petrol engines. 

Suction Stroke

Piston moves outwards and Intake valve opens but the exhaust valve remains closed.   A homogeneous mixture of air and fuel is sucked into the cylinder at low pressure.  Piston moves to top dead center (T D C).  It completes the suction stroke.

Compression Stroke

Now piston starts moving towards bottom dead center (B D C),  intake valve closes.  Both the valves are closed. Further piston movement compresses the air fuel mixture  to high pressure and high temperature .

Power Stroke

This high pressure and high temperature mixture is ignited by a spark plug. Piston remaining at BDC. There is a sudden  rise in pressure and temperature. These gases push the piston back.

Exhaust Stroke

Due to high pressure,  exhaust valve is opened and some exhaust gases go out . The piston forces out the remaining gases. Then the exhaust valve is closed. On further movement of piston, the intake valve opens and the cycle repeated.


  1.  Air standard cycle

  2. Closed cycle

  3. Use a fixed amount of air time and again.

  4. Firstly no Combustion process

  5. No intake process

  6. Secondly no Exhaust process

  7. No friction loss

  8. Thirdly no heat loss

  9. Assume constant specific heats

  10. Reversible processes

  In nature, no process is reversible. INTAKE AND EXHAUST DO NOT EXISTS BECAUSE OF CLOSED CYCLE ASSUMPTION. It makes the analysis simpler.  In any case, we are away from reality.

 FOUR REVERSIBLE PROCESSES of the Air Standard Otto cycle are:

Fig. Otto Cycle on p-V diagram

Isentropic compression (ideal) AB
Heat addition at constant volume BC
Expansion as an Isentropic (ideal) CD
Constant volume heat rejection DA



Sr. No.

Four Stroke Engine

Two Stroke Engine

Completes four strokes in two revolutions
Completes two strokes in one revolution
One power stroke in 2 revolutions
One power stroke in 1 revolution
One power stroke in 2 revolutions
Two power strokes in 2 revolutions
Exhaust and suction strokes are separate.
Exhaust and suction strokes are not separate. These strokes take place side by side.
Mean effective pressure is high.
Mean effective pressure is low.
Less smooth in operation because of one power stroke in two revolutions
Smooth in operation because of one power stroke in each revolution
Very less chances of overheating because power stroke every two revolutions give sufficient time for cooling
Chances of overheating because power stroke every revolution does not give sufficient time for cooling
Needs more maintenance
Needs less maintenance
Defective inferior scavenging process
Effective scavenging process
Full exhaust is there and hence volumetric efficiency is high.
Full exhaust is not there and hence volumetric efficiency is low.
Insufficient mixture intake
Sufficient mixture intake
More working parts
Less number of working parts
More lubrication required
Less lubrication required because of less working parts.
Engine is heavy as there are more number of parts.
Engine is light as there are less number of parts.
Requires a heavy flywheel. Engine runs unbalanced as turning moment on the crankshaft is not even. There is one power stroke for every two revolutions of the crankshaft.
Requires a light flywheel. Engine runs balanced as turning moment is more even. There is one power stroke for each revolution of the crankshaft.
 Design of the engine is complicated due to valve mechanism.
 Design of the engine is simple due to absence of valve mechanism.
Engine is water cooled with the help of a radiator.
Engine is air cooled with the help of fins.
Consumes less fuel due to complete combustion.
Consumes more fuel due to incomplete combustion.
Used in heavy machines like cars, buses and trucks.
Used in light vehicles like scooters, motorcycles and lawn movers.








Item/Description SI Engine CI Engine
1. Thermodynamic Cycle used Otto Cycle Diesel Cycle
2. Heat addition At Constant Volume At constant pressure
3. Fuel used Petrol Diesel
4. Ignition temperature Self Ignition temperature is high and needs a spark plug Self ignition temperature is lower and is achieved by compression
5. Carburetor/fuel pump Carburetor and spark plug are required. Voltage required is supplied by the battery Injector and fuel pump are required
6. Fuel air mixture This mixture enters the engine during the suction stroke Air and fuel are premixed.. Air is compressed alone and then the fuel is injected directly into the combustion chamber
7. Fuel regulation Fuel air mixture is controlled as per load on the engine Controls the fuel as per load but cannot control the flow of air as per load
8. Fuel ignition Spark plug ignites the fuel Fuel ignition is self ignition because of high temperature of compressed air
9. Compression ratio 7 to 10 15 to 20
10. Weight of vehicle
Light weight vehicle because of low pressure ratio requiring less thick parts
 Heavy weight vehicles because of high pressures requiring thick parts
11. Speed achieved High speed because of Light weight vehicles Low speed because of Heavy weight vehicles
12. Thermal efficiency Low because of low compression ratio and less power produced High because of high compression ratio and more power produced
13. Knocking Knocking chances are more. Mixture may explode. It may cause severe damage to the engine because of intense pressure waves. Knocking chances are in case of large delay period.

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