• OTTO CYCLE: How does an internal combustion engine work?

What is the Otto Cycle? Have you heard it before? We are surrounded by engines: in cars and motorcycles, on boats, and of course, in planes. But, how do they work? And in the case of our aeroplanes, how does a combustion engine work?

This is where the Otto Cycle comes into play, the process by which the majority of internal combustion engines work. But, before we get carried away, here is an educational video summarising visually what we’re about to explain. Enjoy!

What is Otto Cycle?

The Otto cycle is the thermodynamic process used in internal combustion engines with electric spark ignition.

The reason for this name is due to all the energy generated by the engine, which has its origin in the internal chamber destined for this.

This cycle was invented by the german engineer Nikolaus Otto and has been in use since 1876, a little over 145 years.

Within the Otto cycle, we can find two types of engines: four and two-stroke engines. This difference is based on the number of turns the crankshaft makes; but do not worry, we provide you with detailed information on this in the following lines.

Diagram of the four-stroke Otto Cycle engine

The diagram of the Otto cycle is based on four phases which correspond to the four processes it carries out to generate the power. They are as follows:


In this phase, the downward movement of the piston within the cylinder allows the mixture of fuel and oxygen to go in. The intake valve is open and the exhaust valve, closed.

In addition, the fuel does not enter the cylinder in a liquid state, but in a gaseous state. This significantly improves its combustion properties.


Once the piston reaches the lowest part of the cylinder, both valves close, and it starts its upward stroke.

The higher the compression ratio generated by an engine, the greater the power generated.


When the top of the cylinder is reached, the maximum pressure is also reached. In this phase, both valves are closed; it is the only phase in which work is produced.

In Otto Cycle engines, the spark is generated by the spark plug; whereas, in diesel engines, the mixture ignites due to the high temperature and pressure generated during compression.

Once the mixture has combusted, the piston starts its downward stroke at high speed.


It is the final process of the Otto cycle. The cylinder travels upwards again and, this time, the exhaust valve is open, expelling the gases generated during combustion.

Once the piston reaches the top of the cylinder, the exhaust valve closes, starting the intake phase again.

Two-stroke Otto Cycle vs. four-strike

As we have already mentioned, there are also 2-stroke Otto cycle engines. Let’s see how they differ from the 4-stroke ones.

The first thing to keep in mind is that every time the piston travels inside the cylinder, the crankshaft moves 180º.

In 4-stroke engines, the piston makes four complete strokes, so the crankshaft will make two complete revolutions.

However, in the 2-stroke engine, the process changes significantly; the process is reduced to two phases instead of four.

In this type of engine, the intake and compression processes are carried out together, as well as the combustion and exhaust processes. This causes the number of piston strokes inside the cylinder to be halved, and the crankshaft to make only one revolution instead of two.

Two-stroke engines are those used in small motorcycles or outboard engines.

Otto Cycle efficiency

You already know that the Otto cycle is the process by which internal combustion engines work; this is because they produce great power with good efficiency.

For an Otto Cycle engine to operate at full performance, there must be a balanced ratio between the air and fuel entering the engine.

This ratio is called the lambda factor and its stoichiometric ratio, which is considered to be the most accurate, is 14.7 parts of air to 1 part of fuel.

This is the reason why, as we ascend in our aeroplane, we must cut down the mixture.

Formula for power in the Otto Cycle

The formula that explains the operation of Otto cycle engines is presented below:

𝞢 Work = W1-2 + W 3-4 = (U2-U1) + (U4-U3) = +4 -5 =-1

It is important to keep in mind that the only process in which work is generated is the explosion process; the other steps subtract energy. This is shown in the formula of the work generated, in which each minus sign indicates a loss of energy.

Another factor to take into account is that the higher the compression of an engine, the greater the power generated. The compression ratio is the reduction in volume inside the cylinder from bottom dead centre to top dead centre.

And yes, we know what you’re thinking. It’s a bit confusing. But let’s compare with something simpler:

Imagine taking a syringe and filling it with air up to the 10 mark. If you close the inlet and apply force to the plunger until you reach the number 1 mark, you will have achieved a compression ratio of 10. Easier that way, right?

Atkinson Cycle vs. Otto Cycle

The Atkinson cycle is becoming very popular in the construction of hybrid vehicles, as it offers high efficiency and low fuel consumption.

The main differences between the Otto and the Atkinson cycle are that, in the Atkinson, the intake valve closes later, reducing compression.

Furthermore, once the cycle has been completed inside the cylinder, the pressure is equal to atmospheric pressure; whereas, in Otto cycle engines, this pressure is much higher.

In general, we can say that Atkinson engines have a lower fuel consumption, as well as a longer service life, due to their lower compression ratio. However, they generate considerably less power than an Otto cycle engine.

Nikolaus Otto, the inventor of the internal combustion engine

Nikolaus Otto was born in Cologne on June 10, 1832. This German inventor has gone down in history as one of the most important engineers for the development of mankind, because in 1872, he materialised for the first time a four-stroke engine.

Despite not having a solid educational background, Otto came to prominence in 1864 when, together with Eugen Langen, he founded the world’s first engine factory, the NA Otto & Cie.

On a personal level, Nikolaus Otto was the father of Gustav Otto, co-founder of BMW together with Karl Rapp in 1917.

The role of Alphonse Beau de Rochas

As it usually happens with great advances, sometimes there are discrepancies about the authorship and, as it could not be otherwise, with Otto Cycle engines the same thing happened. Sometimes it is not so easy to define the author in a single person; sometimes a breakthrough is the result of several milestones of different people.

The Otto Cycle engine was designed on paper by the French inventor Alphonse Beau de Rochas, in 1862, although an engine of these characteristics was not built until 4 years later.

The German Nikolaus August Otto, who in 1866, built the first internal combustion engine without knowing about Beau de Rochas’ invention, was responsible for its construction.

After a series of lawsuits, Otto had to financially compensate Beau de Rochas for plagiarism and although he obtained a very hefty amount, there is no doubt who actually took credit for it.

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