Combustion chamber
Otto cilco thermal engine

Used spark plugs
Petrol engine

Motor Otto

Motor Otto

The Otto engine is a type of reciprocating heat engine that runs on the Otto cycle. It is an alternative engine capable of converting the chemical energy of the fuel into mechanical energy through a thermodynamic reaction.

Due to the characteristics of this type of heat engine it can receive various names.

  • Gasoline engine. Due to the fuel used.
  • Otto engine. Because it works through the Otto cycle.
  • Spark ignition engine. Due to the way of starting the ignition of the fuel. Ignition is initiated through a spark generated by a spark plug.

Using the Otto Engine

Most motorsport engines belong to the Otto engine category ( gasoline engine or spark ignition engine). Although in recent years the diesel engine has also become very popular. A large proportion of engines for industrial traction, all engines for motorcycles and aircraft, and a large proportion of engines for nautical and agricultural applications use the Otto cycle.

The Otto engine can run in 4 or 2-stroke cycles. However, the 2-stroke Otto cycle is little used due to the mix losses that occur through the exhaust and the consequent high fuel consumption.

Therefore, the vast majority of Otto engines operate according to the 4-strokeoperating cycle. The 2-stroke spark ignition engine is only adopted in particular cases, such as outboard engines and small motorcycle engines.

Fuel Used in an Otto Engine

The fuel for an Otto engine is gasoline. Gasoline is made up of light hydrocarbons with a high calorific value, which evaporate easily. Gasoline is derived from petroleum, a fossil fuel that is mined underground.

The Otto engine can also use gaseous fuels or liquefied gas as well, but its use is less practical and, therefore, much less widespread.

Gasoline is obtained from  crude oil  at a  refinery  . It is generally obtained from the  naphtha  of  distillation  directly, which is the lightest of the liquid fraction  oil  (excluding gases). The  naphtha is  also obtained from the conversion of heavy fractions of oil (vacuum gas oil) in processing units called FCC (fluid catalytic cracking) or hidrocracatge.

A series of specifications required for the motor to function well and others of an environmental nature must be met, both regulated by law in most countries. The most characteristic specification is the  octane number  , which indicates its tendency to detonate.

There are different types of commercial gasoline, classified according to their octane number.

Otto Motor Power

Otto engines can be carburetted or injection powered. In injection-powered Otto engines, gasoline is mixed into the air by injecting it into the suction line at the valve inlet, or directly into the combustion chamber. Among the two methods of laminting the fuel, the most widely used is the one powered by carburation.

Injection power from the Otto engine, in the case of multi-cylinder engines, has the following advantages:

  • Distributes the fuel evenly across the various cylinders.
  • Not sensitive to acceleration
  • It is not subject to ice formations.

In contrast, this type of gasoline engine supply has the following disadvantages that it is more complicated and expensive, especially with regard to regulation.

Otto Cycle

The Otto engine is governed by the so-called Otto cycle or Beau de Rochas cycle. The Otto cycle is the ideal thermodynamic cycle that is applied in internal combustion ignition engines (gasoline engines). It is characterized by the fact that, in a first theoretical approximation, all the heat is supplied at a constant volume.

Efficiency of the Otto Cycle

The thermal efficiency or performance of such an engine depends on the compression ratio, ratio between the maximum and minimum volumes of the combustion chamber. This ratio is usually 8 to 1 to 10 to 1 on most modern Otto engines. Higher ratios, such as 12 to 1, can be used, thereby increasing engine efficiency, but this design requires the use of high octane fuels to prevent detonation.

A low compression ratio does not require high octane fuel to avoid this phenomenon, similarly, a high compression requires a high octane fuel, to avoid the effects of detonation, that is, self-ignition occurs of fuel before producing the spark in the spark plug. The average performance of a good 4-stroke Otto engine is 25 to 30%, lower than the performance achieved with the diesel engine. Diesel engines reach performances of 30 to 45%, precisely due to its higher compression ratio.

Otto Engine History

Nikolaus August Otto - Inventor of the Otto Cycle Engine

The spark ignition engine was invented, practically in parallel, by two different inventors: Alphonse Beau de Rochas and Nikolaus August Otto.

The first inventor of the Otto engine, around 1862, was the French Alphonse Beau de Rochas. The second, around 1875, was the German doctor Nikolaus August Otto. As neither of them knew about the other's patent until engines were manufactured in both countries, there was a lawsuit. Ultimately, De Rochas earned a certain sum of money, but Otto was left with fame and name.

The thermodynamic principle of the four-stroke engine is still called the Otto cycle.

Otto built his engine in 1866 together with his compatriot Eugen Langen. It was a gas engine that soon after gave rise to the four-stroke internal combustion engine. Otto developed this machine in four and two-stroke versions. Later, this machine would take its name: Otto cyclic motor.


Published: December 18, 2009
Last review: May 3, 2018