Internal combustion engine Diesel cycle

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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-electriclocomotives, and submarines.

Contents[hide]

  • 1Diesel vs Otto cycle
  • 1.1Diesel cycle
  • 1.2Otto cycle
  • 2General Information
  • 2.1Other internal combustion engines without spark plugs

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Diesel vs Otto cycle

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Diesel cycle

A Diesel cycle engine draws air by itself, or with the aid of a supercharger, into the engine and compresses it. When the piston reaches approximately Top Dead Center, the fuel is injected directly into the cylinder with a high-pressure fuel injector. The air is extremely hot at this point, and so the fuel ignites immediately; however, diesel fuel has a higher molecular weight than gasoline, and so it vaporizes and burns more slowly. The piston is already moving down by the time the combustion starts, and combustion is usually not 100% 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.

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Otto cycle

In the Otto cycle (4 stroke Gasoline engine) the fuel and air are pre-mixed before being drawn into the cylinder. The mixture is then compressed and ignited at the right moment with a spark plug. The fuel/air charge is ignited just before the piston reaches Top Dead Center. Gasoline burns more quickly than diesel fuel, and since the combustion cycle starts earlier (15 degrees before Top Dead Center is common), the entire combustion process is usually complete by the time the piston reachs Bottom Dead Center. It is critical for this type of engine to have a knock resistant fuel.

Otto cycle engines need a minimum amount of fuel to resist pre-combustion in order to run properly. When fuel is injected into this type of engine it has a cooling effect on the inside of the cylinder. The more fuel injected, the more it cools the cylinder, so that the fuel/air charge does not heat up too much. If too little fuel is injected along with the air, and then compressed, the fuel/air charge will overheat and pre-combust, making the engine knock. This is why gasoline engines have a minimum amount of fuel that must be injected. Efficiency suffers as a result, but only when the engine is not at wide open throttle.

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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. However, cars continue to use gasoline, primarily due to consumer desire for "peppy" cars with a wider range of RPM. In Europe, the use of diesel engines in passenger vehicles is far more common. There is considerable interest today in hybrid cars using diesel engines for better efficiency, with an electric motor added to provide the "pep". Current models seem to suggest that cars with all the performance of modern designs can deliver over 100 mpg, on the less expensive fuel.

Although Diesel engines are more efficient when throttled down, they are not suitable for most aircraft. The higher compression ratios of the Diesel cycle demand a much stronger block, head, and almost all moving parts. These stronger parts add a lot of weight - or a lot of expense, if lighter alloys are used. Otto cycle engines are cheaper to build for these reasons, and others. For the same displacement of the engine, an Otto cycle will produce more actual power than a Diesel cycle can, because the fuel burns at a much faster rate, allowing more power strokes per minute (higher RPM) than diesels can offer. This means that less fuel has to be carried. Additionally, commercial aircraft are usually run at preset limits (slightly less than Wide Open Throttle), so that Otto cycle engines used in aircraft do not suffer anywhere near the efficiency penalties that (stop-and-go) land vehicles do. Jet engines are preferred for commercial aviation because they are more efficient - and more powerful - than either type of engine discussed here.

Heavy equipment, such as that used in mining and construction, almost always uses diesel engines.

Diesel engines are also used in conventional submarines. In these submarines, the diesel engine is run when the submarine is on the surface, which charges the batteries that power the submarine when it is submerged. Most all marine vessels over a certain size are powered by diesel engines, as they allow greater range at lower cost, and are more effective at developing torque at lower speed than gas-powered engines.

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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