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The Diesel Engine
05/22/12
In 1892 Rudolf Diesel developed and obtained the German patent for the diesel
engine. His goal was to create an engine that was highly efficient. Much more
efficient than the gasoline engine that was invented in 1876 and was not very
efficient at all. Especially at that point in time.
There are two main differences between a diesel engine and a gasoline engine.
* A gasoline engine intakes a mixture of gas and air, compresses it and ignites
the mixture with a spark. A diesel engine takes in just air, compresses it and
then injects fuel into the compressed air. The heat of the compressed air lights
the fuel spontaneously.
* A gasoline engine compresses at a ratio of 8:1 to 12:1, while a diesel engine
compresses at a ratio of 14:1 to as high as 25:1.
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The higher compression ratio of the diesel engine leads to better efficiency.
Gasoline engines use either a carburetor or a fuel injection system to deliver
the fuel to the cylinder. With a carburetor the fuel is mixed as it enters the
intake manifold, long before it gets to the cylinders. In a fuel injection system
the fuel is injected just before the intake stroke at the intake valve
Diesel engines use direct fuel injection (DI), that is to say the diesel fuel
is injected directly into the cylinder. The diesel engine has no spark plugs.
The air it takes in is compressed and the fuel is injected directly into the
cylinder where the heat caused by the air compression ignites the fuel. In the
old days this meant that it exploded and expanded very quickly, making a noisy
engine. This is why most diesel cars were IDI (indirect injection); the rough
behavior was fixed by injecting the fuel into a small pre-combustion chamber
that is connected to the cylinder by a narrow passage.
This slows down the explosion, as the gasses have to escape from through the
narrow passage into the cylinder. This gives a softer bang and a smoother engine,
but the gasses have to work harder, which lowers the efficiency a little. However
the newer breed of DI engines use other techniques to tame the behavior of the
engine, such as two stage injection, electronic control, and acoustic shrouds
and shock absorbing engine mounts to mask the rattle.
The injector on a diesel engine is its most complex component and has been
the subject of a great deal of development and innovation. On any specific engine
it may be located in a variety of places. The injector has to be able to withstand
the temperature and pressure inside the cylinder and still deliver the fuel
in a fine mist. Getting the mist circulated in the cylinder so that it is evenly
distributed is also a problem, so some diesel engines employ special induction
valves, pre-combustion chambers or other devices to swirl the air in the combustion
chamber or otherwise improve the ignition and combustion process.
One major difference between a gas engine and a diesel engine is in the injection
process. Most car engines use port injection or a carburetor rather than direct
injection. In a car engine all of the fuel is loaded into the cylinder during
the intake stroke and then compressed. The compression of the fuel/air mixture
limits the compression ratio of the engine. If it compresses the air too much,
the fuel/air mixture spontaneously ignites and causes knocking. A diesel compresses
only air, so the compression ratio can be much higher. The higher the compression
ratio, the more power is generated.
In the United States and Canada, diesel engines are most commonly found in
trucks and buses. In Europe, where fuel prices fluctuate around $3.00 to $4.00
per gallon, the fuel efficiency of the diesel has made it a popular choice for
cars, and most European-market cars are available with a diesel or a turbo-diesel
engine.
Compared to gasoline-powered vehicles, diesels are more fuel efficient, and
they can travel significantly farther on a tank of fuel than their gasoline
counterparts. Diesel engines produce more torque, and they tend to be more durable.
They don't need an electric ignition system, which reduces their complexity.
However, they also create more noise, they can be difficult to start in extremely
cold weather and they sometimes require more frequent routine maintenance than
gasoline engines.
Most passenger car diesel engines have a glow plug of some type. When a diesel
engine is cold, the air compression may not raise the air to a high enough temperature
to ignite the fuel. The glow plug is an electric heater that glows red-hot and
helps to ignite the fuel when the engine is cold so that the engine can start
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