Published on Jun 05, 2023

Internal Combustion Engine

The internal combustion engine is an engine in which the burning of a fuel occurs in a confined space called a combustion chamber. This exothermic reaction of a fuel with an oxidizer creates gases of high temperature and pressure, which are permitted to expand. The defining feature of an internal combustion engine is that useful work is performed by the expanding hot gases acting directly to cause movement, for example by acting on pistons, rotors, or even by pressing on and moving the entire engine itself.


The basic way all internal combustion engines work is to suck in a mixture of fuel and air, compress it, ignite it either with a spark plug or by self-igntion (in the case of a diesel engine), allow the explosion of combusting gasses to force the piston back down and then expel the exhaust gas. The vertical movement of the piston is converted into rotary motion in the crank via connecting rods. The crank then goes out to the gearbox via a flywheel and clutch, and the gearbox sends the rotary motion to the wheels, driving the vehicle forwards.

The most common types of internal combustion engine and how they work. It's worth reading this bit first otherwise the whole section on octane later in the page will seem a bit odd. Almost every car sold today has a 4-stroke engine. So do a lot of motorbikes, lawnmowers, snowblowers and other mechanical equipment. But there are still a lot of 2-stroke engines about in smaller motorbikes, smaller lawnmowers, leaf-blowers, snowblowers and such.


A 2-stroke engine is different from a 4-stroke engine in two basic ways. First, the combustion cycle is completed within a single piston stroke as oppose to two piston strokes, and second, the lubricating oil for the engine is mixed in with the petrol or fuel. In some cases, such as lawnmowers, you are expected to pre-mix the oil and petrol yourself in a container, then pour it into the fuel tank. In other cases, such as small motorbikes, the bike has a secondary oil tank that you fill with 2-stroke oil and then the engine has a small pump which mixes the oil and petrol together for you.

The simplicity of a 2-stroke engine lies in the reed valve and the design of the piston itself. The 2-stroke piston is generally taller than the 4-stroke version, and it has two slots cut into one side of it. These slots, combined with the reed valve, are what make a 2-stroke engine work the way it does. As the piston reaches the top of its stroke, the spark plug ignites the fuel-air-oil mixture. The piston begins to retreat. As it does, the slots cut into the piston on the right begin to align with the bypass port in the cylinder wall . The receding piston pressurises the crank case which forces the reed or flapper valve to close, and at the same time forces the fuel-air-oil mixture already in the crankcase out through the piston slots and into the bypass port. This effectively routes the mixture up the side of the cylinder and squirts it into the combustion chamber above the piston, forcing the exhaust gas to expel through the green exhaust port on the left. Once the piston begins to advance again, it generates a vacuum in the crank case. The reed or flapper valve is sucked open and a fresh charge of fuel-air-oil mix is sucked into the crank case. When the piston reaches the top of its travel, the spark plug ignites the mixture and the cycle begins again.