Diesel engine

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A Diesel engine built by MAN AG in 1906

The diesel engine is a type of internal combustion engine; more specifically, it is a compression ignition engine, in which the fuel is ignited by being suddenly exposed to the high temperature and pressure of a compressed gas, rather than by a separate source of ignition, such as a spark plug, as is the case in the gasoline engine.

This is known as the diesel cycle, after German engineer Rudolf Diesel, who invented it in 1892 based on the hot bulb engine and received the patent on February 23, 1893. Diesel intended the engine to use a variety of fuels including coal dust. He demonstrated it in the 1900 Exposition Universelle Exposition Universelle (World's Fair) using peanut oil (see biodiesel).

How diesel engines work

When a gas is compressed, its temperature rises (see the combined gas law); a diesel engine uses this property to ignite the fuel. Air is drawn into the cylinder of a diesel engine and compressed by the rising piston at a much higher compression ratio than for a spark-ignition engine, up to 25:1. The air temperature reaches 700–900 Celsius °C, or 1300–1650 Fahrenheit°F. At the top of the piston stroke, diesel fuel is injected into the combustion chamber at high pressure, through an atomising nozzle, mixing with the hot, high-pressure air. The resulting mixture ignites and burns very rapidly. This contained combustion causes the gas in the chamber to heat up rapidly, which increases its pressure, which in turn forces the piston downwards. The connecting rod transmits this motion to the crankshaft, which is forced to turn, delivering rotary power at the output end of the crankshaft. 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 intercooler to cool the intake air after compression by the turbocharger further increases efficiency.

In very cold weather, diesel fuel thickens and increases in viscosity and forms wax crystals or a gel. This can make it difficult for the fuel injector to get fuel into the cylinder in an effective manner, making cold weather starts difficult at times, though 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 utilize 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. Engine block heaters (electric resistive heaters in the engine block) plugged into the utility grid are often used when an engine is shut down for extended periods (more than an hour) in cold weather to reduce startup time and engine wear.

A vital component of older diesel engine systems was the governor, which limited the speed of the engine by controlling the rate of fuel delivery. Unlike a petrol (gasoline) engine, the incoming air is not throttled, so the engine would overspeed if this was not done. Older injection systems were driven by a gear system from the engine (and thus supplied fuel only linearly with engine speed). Modern electronically-controlled engines apply similar control to petrol engines and limit the maximum RPM through the engine control module (ECM) or engine control unit (ECU) - the engine-mounted "computer". The ECM/ECU receives an engine speed signal from a sensor and then using its algorithms and look-up calibration tables stored in the ECM/ECU , it controls the amount of fuel and its timing (the "start of injection") through electric or hydraulic actuators to maintain engine speed.

Controlling the timing of the start of injection of fuel into the cylinder is key to minimising their emissions and maximising the fuel economy (efficiency) of the engine. The exact timing of starting this fuel injection into the cylinder is controlled electronically in most of today's modern engines. The timing is usually measured in units of crank angle of the piston before Top Dead Center (TDC). For example, if the ECM/ECU initiates fuel injection when the piston is 10 degrees before TDC, the start of injection or "timing" is said to be 10 deg BTDC. The optimal timing will depend on both the engine design as well as its speed and load.

Advancing (injecting when the piston is further away from TDC) the start of injection results in higher in-cylinder pressure, temperature, and higher efficiency but also results in higher emissions of Oxides of Nitrogen (NOx) due to the higher temperatures. At the other extreme, very retarded start of injection or timing causes incomplete combustion. This results in higher Particulate Matter (PM) and unburned hydrocarbon (HC) emissions and more smoke.