OTTO CYCLE: Introduction, Stages, Advantages & Disadvantages


Otto cycle is used for most car engines and other 4-stroke SI engines.

For analysis, this cycle is approximated by the air standard cycle.

This ideal air standard cycle is called the Otto cycle and is named after one of the early developers of this type of engine.


The Otto cycle intake stroke begins with the TDC piston and is a constant pressure process at an inlet pressure of 1 atmosphere.

This is a good approximation of the actual engine intake process in WOT .

This is actually a little lower than atmospheric pressure due to the pressure drop in the intake airflow.

The temperature of the air during the intake stroke rises by as the air passes through the hot intake manifold.

Otto cycle
T-S Diagram

The temperature at point 1 is usually about 25-35 °C higher than the temperature of the surrounding air.

The second stroke of the cycle is the compression stroke, and in the Otto cycle it is isentropic compression from BDC to TDC.

This is a good approximation of the actual engine compression, except at the beginning and end of the stroke.

In a real engine, the start of the stroke is affected by the fact that the inlet valve is not fully closed until just after the BDC.

The end of compression is affected by the ignition of the spark plug in front of the TDC.

Not only does the pressure of the rise during the compression stroke, but the compression heating also significantly raises the temperature inside the cylinder.

The compression stroke is followed by the constant heat supply process at TDC.

It replaces the actual combustion process of an engine cycle, which takes place under nearly constant volume conditions.

In a real engine, combustion starts shortly before TDC, reaches top speed near TDC, and ends shortly before TDC.

A large amount of energy is supplied to the air inside the cylinder during combustion and heat input.

This energy raises the temperature of the air to a very high level, resulting in a peak cycle temperature at point .

This temperature rise during the closed constant volume process also results in a significant increase in pressure.

Therefore, the peak cycle pressure of at point is also reached.

Very high pressure and enthalpy values ​​in the TDC system produce a working cycle (or expansion cycle) following combustion.

When high pressure is applied to the piston surface, the piston is pushed back towards the BDC, producing engine work and power.

The working cycle of the actual engine cycle is approximated by the isentropic process of the Otto cycle.

This is a good approximation, subject to the same discussion as the compression stroke, as it is frictionless and adiabatic.

In a real engine, the start of work cycle is affected by the last part of the combustion process.

The end of the power stroke is affected by the opening of the exhaust valve in front of the UT.

During the power stroke, as the volume increases from TDC to BDC, both the temperature and pressure values ​​in the cylinder decrease.

Near the end of the power stroke of the actual engine cycle, the exhaust valve is opened and the cylinder is exhausted.

A large amount of exhaust gas is exhausted from the cylinder, reducing the pressure on the exhaust manifold.

The exhaust valve opens at BDC, allowing a finite blowdown time.

The blowdown should be completed by the BDC so that there is no high pressure in the cylinder to resist the piston in the next exhaust stroke.

Therefore, the blowdown in a real engine is almost constant, but not perfect.

A large amount of enthalpy is carried away with the exhaust gas, limiting the thermal efficiency of the engine.

The Otto cycle replaces the exhaust gas blowdown process with the open system of the actual cycle, with a constant volume pressure drop, closed system process.

The enthalpy loss during this process is replaced by the heat dissipation of the engine analysis.

The pressure inside the cylinder at the end of the exhaust port dropped to about 1 atmosphere, and the temperature dropped significantly due to expansion and cooling.

The final stroke of the 4-stroke cycle occurs when the piston moves from the BDC to the TDC.

Process is the exhaust stroke that occurs at a constant pressure of at 1 atm due to the exhaust valve being open.

This is a good approximation of the actual exhaust stroke.

This occurs at a pressure slightly higher than the ambient pressure due to the small pressure drop across the exhaust valve and within the exhaust system.

At the end of the exhaust stroke, the engine completes two revolutions, the piston returns to the TDC, the exhaust valve closes, the intake valve opens and a new cycle begins. `

When analyzing the Otto cycle, it is more convenient to handle a particular property by dividing by the mass in the cylinder.

The reason for this is that these two processes thermodynamically cancel each other out and are not needed for circuit analysis.


This cycle has a good power-to-weight ratio.

Engines using the Otto cycle (gasoline engines) are low cost.

The hydraulic fluid (that is, gasoline) used to operate this circuit is low cost.

High thermal efficiency.


It has a low compression ratio.

It is less efficient than the Diesel cycle.

This cycle is not best for engines (i.e., diesel engines) of heavy vehicles.

This has low efficiency.

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