How does an internal combustion engine work?
An internal combustion engine is a device which burns fuel inside a small space known as an internal combustion chamber. This produces an exothermic reaction - a reaction that produces heat and pressure and in turn pushes moving parts such as pistons.
This process relies on fuel, which upon being mixed with oxygen must be ignited. This is normally achieved by the use of an electric battery which produces a spark. The battery is recharged by the alternator when the engine is running.
Once ignited the air-fuel mixture has more energy than it did previously. The energy manifests itself as heat and pressure which expands to push the engine’s pistons.
After the pistons have been moved, the excess gases are removed from the engine using some sort of ventilation system so the pistons can return to their original positions and the cycle can continue.
4 - Stroke Engine
The easiest way to remember how a 4 stroke engine works is by remembering the phrase ‘suck, squeeze, bang and blow’. This refers to a cycle of 4-strokes known as the OTTO cycle which was first patented in 1854.
Stroke 1 – Induction (Intake)
- The inlet valve opens as the piston moves down the cylinder
- Air is drawn into the cylinder
- The inlet valve closes and compresses the fuel-air mix
- Both valves are closed and the induction stroke ends
Stroke 2 – Compression
- The piston moves up the cylinder causing compression
- The fuel is injected in an atomised form and ignited by a spark plug (petrol engine) or by compression of the fuel – air mix (diesel engine)
Stroke 3 – Power
- The temperature in the cylinder causes the diesel to ignite
- This causes a huge increase in pressure forcing the pistons down
- The exhaust valve opens to release the gases and the ignition phase ends
Stroke 4 – Exhaust
- The exhaust gases are forced out of the exhaust
- The inlet valve opens and the cycle starts again
What’s the difference between a diesel and a petrol engine?
In a petrol engine the air and fuel are premixed before compression usually by electronically controlled fuel injection. A spark plug is then used for ignition.
In a diesel engine the air is compressed to cause an increase in temperature to circa 600 degrees C, fuel is then injected at the end of the compression stroke and ignition takes place.
2 stroke engine
The 2 stroke engine differs from the more common 4 stroke engine by completing the same four processes in two strokes of the piston rather than 4. This is achieved by using the beginning of the compression stroke and the end of it to perform the intake and exhaust functions. This allows a power stroke for every revolution of the crank instead of every second revolution in a four stroke engine.
Stroke 1 - Intake and compression
Stroke 2 - Power and exhaust
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