04/03/2025
How do two stroke engines work?
I have given it my all to try an pack as much information as humanly possible and present them in a simple, coherent and understandable manner with the goal of creating what is the only video anyone needs to watch to learn how a four stroke and a two stroke engine work. The video also covers the differences, benefits and drawbacks of 2 stroke and 4 stroke engines and also answers the question of why the four stroke ultimately prevailed over the two stroke in most applications despite being larger, heavier, more expensive and more complex.
Let's start with the four stroke. Why is it called a four stroke? It's because the four stroke engine needs four strokes to complete one combustion cycle. Every time the piston moves from top to bottom............. or vice versa............... that's one stroke. One stroke of the piston equals 180 degrees of crankshaft rotation.
Now the four strokes are: intake... compression..... combustion..... and exhaust
During the intake stroke the piston travels from top to bottom, which are also known as top dead center and bottom dead center. As the piston does this it creates an empty space or vacuum inside the cylinder. This newly created void is essentially a brief absensce of air, and because we have an abscence of air we also have an abscence of air pressure. In other words we have low air pressure inside the cylinder and atmospheric air pressure outside the cylinder. This air pressure difference cannot continue to exist and air naturally seeks to equalize pressure everywhere, and so air together with fuel from the outside rushes into the cylinder and fills it with a fresh air-fuel mixture.
Although cobmustion inside an engine is often described as a bang or explosion, that isn't what's actually happening. An explosion is detonation, which is a rapid uncontrolled process. In contrast to thist, combustion is deflagration, which is a much slower, more even and controlled process.
Combustion spreads out evently outward from the spark plug though heat transfer. The small portion of air fuel mixture initially ingited by the spark plug heats up and ignites the next layer of the air fuel mixture until all of the air fuel mixture is burned.
As the combustion flame front spreads it rapidly raises the tempreature and pressure inside the cylinder. Because the cylinder is sealed this pressure has nowhere else to go so it ends up pushing the piston down the cylinder with great strength....... This is our combustion stroke and of the four strokes this is the only one that actually generates power and it does so by converting the energy released by combustion into motion of the piston which then turns the crankshaft and ultimately the wheels.
By the time the piston reaches bottom dead center again all the air fuel mixture has been burned and we now have ehxuast gas, or the remains of combustion inside the cylinder.
Now in the case of the two stroke we have a cylinder head together with the valve cover whereas in the four stroke we really have just a cylinder cover or cap. There are zero moving parts in the 2 stroke head. No valves, no chains, no cams, no springs and therefore less weight, less complexity, less cost and less potential for failure.
Each time the 2 stroke piston is at top dead center a combustion event occurs. This also explains the name „two stroke“. The engine only needs two piston strokes to complete its combustion cycle. In other words each 360 degrees or one full engine revolution results in a combustion event. Whereas in a four stroke combustion occurs only every other time the piston reaches top dead center, combustion occurs only every 720 degrees of engine rotation. This means that the two stroke produces twice as many combustion events or power pulses for the same rpm, which means that, at least in theory, the two stroke engine can make twice as much power from the same displacement compared to a four stroke.
The other important thing we can note is that the strokes are very clearly defined and separated in a four stroke. Each completed stroke of the piston marks the beggining of one and the end of another stroke of the combustion cycle. But the two stroke sort of lumps the strokes together, they overlap and occur simultanously. The two stroke is actually multi-tasking which enables it to squeeze more action into the same time-frame. But as with everything, there is a price to be paid.
