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The Diesel Engine

    Diesel engines are having a surge of popularity right now because they are stronger and more fuel efficient than a gasoline engine.
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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.

  1. 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.
  2. 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. 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 to $4 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.

The amount of power which an engine can produce is limited by how much fuel it can burn, and the amount of fuel it can burn is limited by the amount of oxygen in the cylinder. The amount of oxygen in the cylinder is limited by the amount of air in the cylinder. So, if more power is wanted then more air is needed, how do we achieve this? Well a large engine has more air, so it can produce more power, or we can compress the air before it gets to the cylinders.

This is what a super-charger or turbo-charger does; they are a simple air compressor. A super-charger is a mechanically driven compressor run by a belt off the crankshaft. A turbo-charger is made to spin by blowing exhaust gasses over a turbine on one end of a shaft and compresses air with a turbine at the other end of the shaft. The diesel injection pump has a pressure sensor so that when the turbo-charger is winding up, extra fuel can be injected. There is one problem with compressing air, it gets hot and hot gas is less dense and therefore has less oxygen in it, so you lose a little bit of the advantage you would expect, unless you have an intercooler.

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Additional Information provided courtesy of and Warranty Direct
© 2000-2007 Vincent T. Ciulla

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