Two Stroke Vs. Four Stroke Motorcycle Engines
There are four sequences that need to be cleared out for that matter: intake, compression, power and exhaust. As you might have guessed, there are a few mechanical differences between the two, which we will try to point out next.
Four Stroke Engine
With both intake and exhaust valves closed, the piston goes back up to the top of the cylinder compressing the fuel-air mixture. This is what happens during the compression stroke.
The compressed air–fuel mixture is then ignited by a spark. The pressure from the fuel-air mixture combustion drives the piston back down with humongous force, keeping the crankshaft turning. This is the power stroke phase, which is the main source of the engine's torque and power.
Finally, during the exhaust stroke, the piston again goes up and pushes the burned gas from the cylinder out the exhaust valve. Another thing worth mentioning is that the spark plug only fires once every two revolutions.
Two Stroke Engine
As the piston goes down from combustion, spent gasses are allowed to exit the chamber through the exhaust port. The air–fuel mixture is drawn through an inlet located lower in the chamber. As the piston rises again, it blocks off the inlet and exhaust ports, compressing the gasses at the top of the chamber. The spark plug fires and the process starts over. The engine fires on every revolution.
Let the fight begin!
A two stroke engine can produce twice the amount of power (and makes twice as much noise) than a four stroke engine of the same size. This is because it fires once every revolution, giving it twice the power of a four stroke, which only fires once every other revolution. Significantly, it also has a higher weight-to-power ratio because it is much lighter.
Two stroke engines are simpler and cheaper to manufacture compared to four stroke engines because of their simpler design. Four stroke engines are longer lasting than two stroke engines that don't have a dedicated lubricating system. However, the spark plugs in a two stroke engine last longer than those in a four stroke engine.
Four stroke engines are more fuel efficient and environmentally friendly when compared to two stroke engines that also create an unpleasant smell. Two stroke engines are responsible for much more pollution due to the combustion of oil.
Two stroke units may also exceed legal noise limits in some areas. That is why you should consider checking to see if any restrictions apply in your area before purchasing one.
comments written so far
For me, 4 stroke is more challenging to modified.
Love that smell, can't say I have a favourite between 2/4 strokers, I run a gsx1200 inazuma (4s) and a KR1-S 250 (2s) they are both lots of fun in their own right.
Consider 100cc of 2stroke vs 125cc of 4stroke, is that fair enough?? When you have doubled power (by doubled combustion per revolution), i can't say its fair. To be fair, how do you think to chalengge 200cc of four stroke?
what????? the smell of pre-mix in the morning gets me going. its the best smell. it is almost hypnotic.