The term diesel engine refers to any internal combustion engine that uses the heat of compression to initiate the ignition of the cylinder to burn a fuel that has been injected into the combustion chamber. This differs from positive-ignition engines, such as gasoline engines, which rely on a spark plug to ignite a mixture of air and fuel. The engines have the highest thermal efficiency of any other internal or external combustion engine due to the very high compression ratios required to ignite the fuel. All diesel engines are based on the diesel thermodynamic cycle that was developed by Rudolph Diesel in 1897.

How an engine works is a complicated process to describe. Initially, only air is allowed to enter the combustion chamber, then this air is compressed to a ratio of about 15: 1 to 22: 1, causing the air temperature in the cylinder to rise to about 550 ° C. or 1022 ° F. At the top of the compression stroke, fuel is injected into the compressed air in the combustion chamber. The heat of compression vaporizes the fuel causing combustion. The rapid expansion of combustion gases drives the piston downward, supplying power to the crankshaft, which then drives a piston from different cylinders upward to complete the process over and over again and continuously run the engine.

BMW began using the engines in its vehicles in 1983 with the introduction of the 2.4-liter M21 in-line six. this first version was never available in the UK and was marketed primarily in North America on the Lincoln Continental.

1994 saw the introduction of a new engine offered by BMW, this engine was the M41 1.7 liter inline 4 cylinder engine. This engine was available in the BMW E36 318tds 1994-2000.

1998 saw the addition of three different BMW engines, the 2.0-liter M47 which was a straight 4, then there was the M57 which ranged from 2.5 liters to 3.0 liters and was an inline six, and finally there was the M67 which was 3.9 liter v8 configuration. The M67 won two international awards for Engine of the Year.

In 2006 the N47 was introduced, which was a 2.0-liter straight-line four-cylinder. This latest version is known as a common rail engine and had vast improvements in emissions and fuel efficiency.

In 2008 BMW introduced the N57 engine, this was a 2.5-liter to 3.0-liter inline six-cylinder engine. These engines were turbocharged common rail engines.

Of the engines listed, there are still a few in production today, such as the N57, M57, and N47. BMW has adapted its line of diesel engines to be as environmentally friendly or even more environmentally friendly than some other manufacturers that offer gasoline.

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