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Differences between the Otto engine and the diesel engine

Differences between the Otto engine and the diesel engine

The Otto engine and the diesel engine are two types of thermal engines. These are two types of endothermic engines that, through thermodynamic reactions, convert the internal energy of the fuel into mechanical work. However, there are certain differences between them.

The most important difference is found in its theoretical cycle. The Otto engine operates according to the Otto cycle by spark ignition while the diesel engine is controlled according to the diesel cycle by compression ignition.

The other important difference is in the ignition of the fuel. In the Otto engine the ignition is made by the spark caused by the spark plug, for this reason it is also known as the spark ignition engine. On the other hand, in the diesel engine, ignition is done by compressing the fuel; for this reason, the diesel engine is also known as a compression ignition engine.

Mechanically there are no big differences between both types of alternative engines. However, it should be noted that the diesel engine, when combustion is carried out at a much higher pressure, requires a more robust structure than in the gasoline engine.

The fundamental differences between the two engine types are derived from the differences in their cycles:

  • Differences in the fuel input to the engine
  • Difference in fuel ignition
  • Differences in the compression ratio

Differences in fuel input

The input of fuel in the diesel engine and the Otto engine is one of the most important differences between both types of thermal engines:

Fuel inlet in the gasoline engine.

In most petrol gasoline engines (otto cycle), air and fuel are introduced into the combustion chamber in the form of a gas mixture.

The mixture between air and gasoline is made in the carburetor, and the regulation of the amount of mixture introduced is obtained by means of a butterfly valve.

At present, this mixture is generated electronically and its proportions vary depending on different factors such as the temperature of the engine and fuel. Formerly, the motors of the automobiles used two types of mechanisms to regulate the entry of the fuel when starting: the starter, to increase the amount of gasoline or the air restrictor that limited the access of air increasing the proportion of gasoline,

Fuel input in the diesel engine

In the diesel engine, the air is introduced into the combustion chamber through ducts that go to the suction valve. In this type of thermal engines the fuel (diesel) is introduced directly by means of an injector.

In this type of engine it is important to take into account that the pressure inside the cylinder is higher than in the Otto engine. One of the functions of the injector is to print enough pressure to the fuel so that it can enter.

The air-fuel mixture is made in the combustion chamber. In the diesel engine there is no regulation of the amount of air, there is only a regulation of the amount of fuel introduced.

Fuel ignition difference

Once the fuel inside the cylinder has been introduced, the way in which it is ignited is different between both types of engines. In one case the ignition is done by supplying temperature and in the other the ignition of the fuel is done by increasing the pressure inside the cylinder. According to the laws of thermodynamics and the law of gasos, both state variables (pressure and temperature) are related. If the volume remains constant, increasing one increases the other.

Ignition of fuel in the gasoline engine

In internal combustion engines, the explosion cycle is caused by the inflammation of the air and fuel mixture found in the cylinder body.

On the Otto engine, the fuel is ignited by a spark. This type of motor requires an ignition system to generate a spark in the combustion chamber between the electrodes of a spark plug. This spark implies an increase in the temperature of the mixture. This increase in temperature is sufficient to allow combustion to start.

Ignition of fuel in the diesel engine

In diesel engine ignition is done by compression. This type of engine uses the high temperature and pressure obtained by compressing the air in the cylinder to start combustion when the fuel is injected.

In the diesel engine this temperature is achieved by air compression. During the intake cycle the air enters the explosion chamber that will be compressed in the next cycle, that compression generates an increase in air temperature until the autoignition temperature value is passed.

It is at that moment when the injectors put the fuel under pressure, in a pulverized way for a better mixture with the air, and the combustion takes place.

Differences in the compression ratio

The compression ratio in an endothermic engine is the number that allows to measure the proportion in which the air-fuel mixture (Otto engine) or air (diesel engine) has been compressed inside the combustion chamber of a cylinder.

The value of the compression ratio in Otto cycle engines varies from 6 to 10, except in exceptional cases, while in diesel engines the compression ratio ranges from 14 to 22.

In the Otto engine (spark ignition), the upper limit of the compression ratio is essentially determined by the anti-knock quality of the fuel in the market; for diesel engines, it is determined, above all, by the weight of the engine structure, which increases with increasing compression ratio. This increase occurs in a special way with large displacements.

Difference of weight of the Otto engine with the diesel engine

In this case, the way of lighting affects the weight. That is, in the diesel engine, the ignition of the fuel is done by compression. As we have commented on the mechanical differences, this means that it must be made with a design that can withstand these high pressures. In practice, this means a larger motor, and therefore, heavier.

The diesel engine is, generally, heavier than an Otto engine of equal displacement. If it is the engine of a vehicle, the weight of the engine will also have to be displaced, so the engine will need more power to have similar performance. The amount of work of the vehicle will have to be higher since it displaces more quantity of massa per meter displaced.

Engine Brake

Another difference that does not affect performance but if braking.

In the case of gasoline engines, the engine brake is produced by the air intake system. When we stop accelerating the butterfly key, it closes the air flow, at that moment air stops entering the combustion chamber and vacuum is generated. The force that the piston has to make to generate that vacuum is the energy that is obtained from braking.

In the case of diesel vehicles, and since they do not have an acceleration body, it is not possible to close the air flow to generate that vacuum. The natural process would be as follows: the valves are opened and air enters, the valves are closed and the air is compressed, when the piston reaches the top dead center it begins its descent aided by the pressure of the compressed air. In this way there would be hardly any loss of energy.

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Last review: December 5, 2017