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The difference between a gasoline and a diesel engine

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2011-10-20

¬"What is the difference between a gasoline and a diesel engine?"
Diesel's story actually begins with the invention of the gasoline engine. Nikolaus August Otto had invented and patented the gasoline engine by 1876. This invention used the four-stroke combustion principle, also known as the "Otto Cycle," and it's the basic premise for most car engines today. In its early stage, the gasoline engine wasn't very efficient, and other major methods of transportation such as the steam engine fared poorly as well. Only about 10 percent of the fuel used in these types of engines actually moved a vehicle. The rest of the fuel simply produced useless heat.
¬I¬n 1878, Rudolf Diesel was attending the Polytechnic High School of Germany (the equivalent of an engineering college) when he learned about the low efficiency of gasoline and steam engines. This disturbing information inspired him to create an engine with a higher efficiency, and he devoted much of his time to developing a "Combustion Power Engine." By 1892 Diesel had obtained a patent for what we now call the diesel engine.
¬If diesel engines are so efficient, why don't we use them more often? You might see the words "diesel engine" and think of big, hefty cargo trucks spewing out black, sooty smoke and creating a loud clattering noise. This negative image of diesel trucks and engines has made diesel less attractive to casual drivers in the United States -- although diesel is great for hauling large shipments over long distances, it hasn't been the best choice for everyday commuters. This is starting to change, however, as people are improving the diesel engine to make it cleaner and less noisy.
If you haven't already done so, you'll probably want to read How Car Engines Work first, to get a feel for the basics of internal combustion. But hurry back -- in this article, we unlock the secrets of the diesel engine and learn about some new advancements.

¬In theory, diesel engines and gasoline engine are quite similar. They are both internal combus¬tion engines designed to convert the chemical energy available in fuel into mechanical energy. This mechanical energy moves pistons up and down inside cylinders. The pistons are connected to a crankshaft, and the up-and-down motion of the pistons, known as linear motion, creates the rotary motion needed to turn the wheels of a car forward.
Both diesel engines and gasoline engine covert fuel into energy through a series of small explosions or combustions. The major difference between diesel engines and gasoline engine is the way these explosions happen. In a gasoline engine, fuel is mixed with air, compressed by pistons and ignited by sparks from spark plugs. In a diesel engines, however, the air is compressed first, and then the fuel is injected. Because air heats up when it's compressed, the fuel ignites.
The following animation shows the diesel cycle in action. You can compare it to the animation of the gasoline engine to see the differences.
Image courtesy Baris Mengutay
The diesel engines uses a four-stroke combustion cycle just like a gasoline engine. The four strokes are:
• Intake stroke -- The intake valve opens up, letting in air and moving the piston down. ¬
• Compression stroke -- The piston moves back up and compresses the air.
• Combustion stroke -- As the piston reaches the top, fuel is injected at just the right moment and ignited, forcing the piston back down.
• Exhaust stroke -- The piston moves back to the top, pushing out the exhaust created from the combustion out of the exhaust valve.
Remember that the diesel engine has no spark plug, that it intakes air and compresses it, and that it then injects the fuel directly into the combustion chamber (direct injection). It is the heat of the compressed air that lights the fuel in a diesel engine. In the next section, we'll examine the diesel injection process.

Edited by Placido
Articles from ET Power Machinery Co., Ltd

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