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Diesel engine had invented by germen engineer Rudolf diesel in 1892. it was based on hot bulb engine and received the patent on feb.23,1893

The diesel engine is a type of internal combustion engine. It is a compression ignition engine, in which the fuel ignites as it is injected into the engine. By contrast, in the gasoline engine the fuel is mixed first and then ignited by a spark plug. Also, diesels generally have high compression ratios, to enable compression ignition, whereas in gasoline-burning engines, compression ignition is undesirable.

When a gas is compressed, its temperature rises .a diesel engine uses this property to ignite the fuel. Air is drawn into the cylinder of a diesel engine and is compressed by the moving piston at a compression ratio as high as 25:1, much higher than needed for a spark-ignition engine.

At the end of the piston stroke, diesel fuel is injected into the combustion chamber at high pressure through an atomising nozzle. The fuel ignites directly from contact with the air, the temperature of which reaches 700–900 °C (1300–1650 °F). The combustion causes the gas in the chamber to heat up rapidly, which increases its pressure, which in turn forces the piston outward.

The connecting rod transmits this motion to the crankshaft, which delivers rotary power at its output end. Scavenging (pushing the exhausted gas-charge out of the cylinder and drawing in a fresh draught of air) of the engine is done either by ports or valves.

To fully realize the capabilities of a diesel engine, use of a turbocharger to compress the intake air is necessary. Use of an aftercooler or intercooler to cool the intake air after compression by the turbocharger further increases efficiency.

In cold weather it can be difficult to start diesel engines because the diesel fuel thickens and can form wax crystals or a gel. This can make it difficult for the fuel injector to get fuel into the cylinder in an effective manner. Recent advances in diesel fuel technology have made these difficulties rare.

A commonly applied advance is to electrically heat the fuel filter and fuel lines. Other engines use small electric heaters called glow plugs inside the cylinder to warm the cylinders prior to starting. A small number use resistive grid heaters in the intake manifold to warm the inlet air until the engine reaches operating temperature.

A vital component of older diesel engine systems is the governor, which limits the speed of the engine by controlling the rate of fuel delivery. Unlike in petrol (gasoline) engines, incoming air is not throttled and an engine without a governor can over speed.

Older injection systems were driven by a gear system from the engine and thus supplied fuel in proportion with engine speed. Modern, electronically controlled engines apply controls similar to those of petrol engines and limit the maximum RPM through an electronic control module (ECM) or electronic control unit (ECU)—the engine-mounted computer. The ECM/ECU receives an engine speed signal from a sensor and controls the amount of fuel and timing through electric or hydraulic actuators.

Controlling the timing of the start of injection of fuel into the cylinder is a key to minimizing emissions, and maximizing fuel economy (efficiency), of the engine. The timing is usually measured in units of crank angle of the piston before .

Advancing the start of injection results in higher in-cylinder pressure and temperature, and higher efficiency, but also results in higher emissions of oxides of nitrogen because of the higher temperatures. At the other extreme, delayed start of injection causes incomplete combustion.

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