How Does a Diesel Engine Work?

The Science Behind a Diesel Engine

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Who Invented the Diesel Engine?

Diagram of a Diesel Engine

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Rudolf Diesel (1858-1913) understood engines, but his early understanding was at the most basic of levels—heat. After battling Typhoid and a spotty education, Diesel ended up working in development at a company called Linde, and his specialty was refrigeration. What does this have to do with a diesel engine? Lots. Unlike most internal combustion engines, Diesel's development didn't rely on spark plugs and a fancy mechanical ignition system to make the fuel explode. Instead, his invention relied on principals of thermodynamics, or the way heat behaves and the way it affects its surroundings. He did have a few stumbling blocks along the way. Diesel was determined to invent a better engine than the internal combustion gasoline engine that Benz was using in his newly invented motor cars after 1887.

Unfortunately, sometimes his ideas blew up in his face, literally. An accident involving Diesel trying to reinvent the steam engine using ammonia almost killed him. He recovered after a hospital stay, and reportedly suffered some vision and other health problems.

Fast forward to 1898, and Rudolf Diesel is finalizing development on an internal combustion engine that relies only on its own compression to ignite the fuel. At almost 500psi in the combustion chamber, the Diesel engine has as much as 5 times the compression you'd find in a gasoline engine, and Diesel obtained the patent for this technology.

Unfortunately, Diesel didn't live long enough to continue to develop the engine to the potential it eventually realized -- the rest of the world had to do that part. In 1913 he disappeared while sailing to London. His body was discovered days later floating at sea. Most experts and biographers have said the death was likely a suicide.

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Diesel vs. Gas, What's the Difference?

There are too many differences between gas engines and diesel engines to go into here, but let's go over some of the major parts. The most fundamental difference between the two engines—besides the type of fuel they burn (more on that in a minute) is the compression inside the combustion chamber. There can be variation in the compression ratio of gas engines, but for the sake of argument let's say it's around 150 psi. Diesel engines have more than three times that amount of compression in the chamber. Even Rudolf Diesel's original patent had a compression of 500 psi! That's a big difference in how much the air and fuel mixture is being compressed inside the cylinder!

This difference in compression leads us to all of the other differences between gas and diesel internal combustion engines. Take spark, for instance, or "ignition" as it's called in the field because it's what ignites the air-fuel mixture in the combustion chamber of an engine. A gasoline engine has a spark plug that is installed in the cylinder head. The tip of this plug makes an electric spark right inside the chamber, at exactly the right time so that the air-fuel mixture explodes and forces the piston back down to the bottom of the chamber. Here comes the big difference—diesel engines don't have spark plugs. Rudolf Diesel knew from his studies in thermodynamics that if he could compress the air-fuel mixture enough, like 500 psi enough, he could get it to explode without an external sparking mechanism. Modern diesel engines do have what's called a "glow plug," which helps the engine run more efficiently even when cold, and helps the engine to start, but once it's going the engine has enough internal heat and compression to keep running. Rudolf Diesel also knew from his studies that a diesel engine would be many times more efficient than other engines, especially the popular steam engine which loses a huge percentage of its energy to lost heat via escaping steam.

There have been countless advances in diesel engines since they started being used in cars and trucks. Diesel reliability is amazing, with engines getting 500,000 miles without a rebuild on a regular basis. Turbocharging has given diesel engines more power so that cars and trucks will have better acceleration. Direct injection has made them run much cleaner than the smoky messes we saw in the 1970s. Diesel fuel prices have been on the rise for years now, so it's unlikely we will see many more diesel developments, but the diesel engine's place in history has been and continues to be very important.