Internal combustion engine Diesel cycle - 542-04-70

Internal combustion engine 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.

Contents

 1 Diesel vs Otto cycle

 1.1 Diesel cycle

 1.2 Otto cycle

 2 General Information

 2.1 Other internal combustion engines without spark plugs

Diesel vs Otto cycle

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.

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. 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