Diesel cycle

The Diesel cycle is the combustion process of a type of internal combustion engine, in which the burning of the fuel is triggered by the heat generated in first compressing air in the piston cavity, into which is then injected the fuel - as opposed to it being ignited by a spark plug, as combustion is in the Otto cycle (four-stroke/petrol) engine. Diesel engines (heat engines utilizing the Diesel cycle) are used in automobiles, power generation, diesel-electric locomotives, and submarines.



Diesel cycle

The diesel cycle was introduced by Dr. Rudolph Diesel in 1897. In a Diesel cycle engine, heat is supplied (by the burning fuel) while maintaining the cylinder at a constant pressure.

The Diesel cycle generally refers to compression ignition power plants, called the Diesel engine. The engine can be two or four stroke and may draw in air by using the piston or with the aid of an engine-driven supercharger or exhaust gas-driven turbocharger. As the air is compressed it gets hot. When the piston reaches approximately "top dead center", the fuel is injected directly into the cylinder with a high-pressure fuel injector. The fuel ignites immediately; however, as diesel fuel has a higher molecular weight than gasoline, it vaporizes and burns more slowly. The piston is already moving down by the time the combustion starts, and combustion is usually not finished when the piston reaches "bottom dead center." Because of this incomplete combustion, diesel engines actually lose some of the potential energy of the fuel.

Diesel cycle engines are nevertheless more efficient than Otto cycle engines overall, but only when power needs to be scaled. Most land vehicles almost never use the maximum rated power of the engine. Unless the vehicle is at wide open throttle, when the pedal is 'floored', the engine is only producing a fraction of its rated power. Since diesel engines use the heating effect of compressing the air to ignite the fuel, a diesel engine can inject as little or as much fuel as the situation demands. It is important to note that Otto cycle engines can be more efficient than Diesel cycle engines, but only when the engine is running at or near its maximum power. This is dependent on the Otto cycle engine's compression ratio.



General information

The diesel engine has the lowest specific fuel consumption of any large internal combustion engine, 0.26 lb/hp.h (0.16 kg/kWh) for very large marine engines. In fact, two-stroke diesels with high pressure forced induction, particularly turbocharging, make up a large percentage of the very largest diesel engines.

In North America, diesel engines are primarily used in large trucks, where the low-stress, high-efficiency cycle leads to much longer engine life and lower operational costs. These advantages also make the diesel engine ideal for use in the heavy-haul railroad environment.



Other internal combustion engines without spark plugs

Many model airplanes use very simple "glow" and "diesel" engines. Glow engines use glow plugs. "Diesel" model airplane engines have variable compression ratios. Both types depend on special fuels (easily obtainable in such limited quantities) for their ignition timing.

Some 19th century or earlier experimental engines used external flames, exposed by valves, for ignition, but this becomes less attractive with increasing compression. (It was not until Nicolas Léonard Sadi Carnot that the thermodynamic value of compression was known.) An historical implication of this is that the diesel engine would eventually have been invented without the aid of electricity.