How does a Diesel Engine Work?

A Brief History of the Diesel Engine:

In 1878, Rudolf Diesel was attending collage in Germany where he learnt of the low efficiency of gasoline and steam engines at the time, which in most cases used as little as 10% of the fuel to power the object. The knowledge of the inefficiency drove him to develop a more efficient combustion power engine and by 1892 Diesel had obtained a patent on what is know commonly know as the Diesel Engine.


How it Works

A diesel engine, like a petrol engine used the chemical energy found in fossil fuels to drive a piston up and down as illustrated below. The pistons are connected to a crankshaft which changes the linear motion of the piston into rotary motion needed to power what ever application the engine is needed for. It can be to turn the wheels of a car or to turn the rotor of the alternator within the stator to create power.

Like a petrol engine, a diesel engine works on the notion of extracting the energy from fossil fuels through the process of combustion to move the piston. The difference is how the combustion occurs. A diesel engine works on the premise that air heats when compressed. Hence the air is compressed, the fuel is injected and the fuel ignites from the heat generated by the compressed air.

1. Air Intake (Intake Stroke)
2. Pressurised (Compression Stroke)
3. Fuel Injection and Combustion (Combustion Stroke)
4. Exhaust fumes expelled (Exhaust Stroke)

Like a petrol engine, the engine uses a four stroke principle as eluded to above. This four step cycle happens thousands of times a minute to rotate the crank shaft. Diesel engines use direct fuel injection; this is that fuel is injected directly into the cylinder when third stroke takes place.

The injection of fuel into the cylinder is the most complex aspect of the process and has been interpreted and developed in many different ways. The objective is to ensure that a fine mist of fuel is evenly circulated within the cylinder to ensure that the mini-explosion pushes the piston up at its optimum efficiency to minimize fuel consumption and maximize power. The process has seen developments such as pre-combustion chambers being used, induction valves and air circulation devices. The primary concern is that the injectors can withstand the pressure and heat produced when the engine is running.

In some cases glow plugs are used to ensure that the air can get up to the right temperature. If the compression of air cannot produce enough heat, a glow plug will be used to raise the temperature of the air and ensure that the fuel ignites within the cylinder. The glow plug generally heats up until it glows red as depicted below. Glow plugs are only used when starting the engine after which they are essentially turned off as the engine should be running. This is why Diesel engines require a pre-heating period before they are started.

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