Otto engines and diesel engines are two types of heat engines. These are two types of endothermic engines that convert the fuel's internal energy into mechanical work through thermodynamic reactions. However, there are specific differences between them.
The two most notable differences between the Otto engine and the diesel engine are the thermal cycle and the type of fuel used. However, there are other differences that we will discuss in this article:
Differences in thermal cycles
The most crucial difference between these two internal combustion engines is their theoretical cycle. The Otto engine works according to the Otto cycle using spark ignition. On the other side, diesel engines compress the fuel to increase the temperature and ignite it.
The main difference between the Otto cycle and diesel is that in the Otto cycle, the fuel intake and gas expulsion are done in a constant volume process. On the other hand, the diesel cycle is carried out at constant pressure.
Diesel cycle
The ideal diesel cycle consists of four processes:
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Compression: an adiabatic compression of the air standard in the cylinder is initiated to increase the temperature.
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Combustion: the injectors introduce the fuel, which, due to thermodynamic conditions, self-ignites by supplying isobaric heat.
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Expansion: the piston goes down, performing an adiabatic expansion of the working fluid.
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Last stage: Isochoric heat rejection of the working fluid.
Each diesel cycle generates two complete revolutions of the crankshaft.
Otto cycle
Nicolaus August Otto, in 1876, designed the ideal Otto cycle. It consists of four processes:
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Intake: The piston moves down, and the intake valve remains open, allowing the fuel/air mixture to be drawn into the cylinder.
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Compression: the valves remain closed, and the piston rises, compressing the mixture. In the Otto cycle, heat addition is made by a spark plug igniting the mix when the piston reaches the top.
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Combustion: releasing energy that causes the expansion of gases and moves the piston down.
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Exhaust: the exhaust valve opens, and the piston moves upwards, expelling the gases produced.
Differences in fuel input
Most Otto engines introduce air standards and fuel into the combustion chamber as a gaseous mixture. Currently, this mixture is generated electronically, and its proportions vary depending on different factors.
In the diesel engine, the air is introduced into the combustion chamber through ducts that go to the suction valve. The fuel is introduced directly through an injector with a higher pressure than the Otto engine. The air-fuel mixture takes place in the combustion chamber.
Fuel ignition difference
In Otto engines, the explosion cycle is produced by igniting the air-fuel mixture found in the cylinder. The fuel is ignited by a spark caused by a spark plug. This engine is also known as a spark ignition or petrol engine.
During fuel intake in a diesel engine, air enters the chamber to be compressed in the next cycle. As the pressure increases, the temperature increases until it has reached the self-ignition value. At this moment, the injectors insert the pulverized fuel under pressure, and combustion occurs. The diesel engine is also known as a compression ignition engine.
Other differences
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 between 14 and 22.
Concerning weight, the diesel engine is heavier than the Otto engine because it has to be more robust to withstand the high pressures generated during air compression.