What is the difference between two and four stroke engines?

Answer 1

In a 4-stroke engine, the steps (strokes) are:

1. Intake stroke: During the intake stroke, the piston goes down from the top of the cylinder to the bottom, reducing the pressure inside the cylinder. It then draws a mixture of fuel and air into the cylinder through the intake port, ready for the compression stroke.
2. Compression stroke: 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.
3. Power 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.
4. Exhaust stroke: Finally, during the exhaust stroke, the piston again goes up and pushes the burned gas from the cylinder out the exhaust valve.

Something worth mentioning is that the spark plug only fires once every two revolutions in a 4-stroke engine.

Unlike the four stroke one, a two stroke engine has three strokes combined into one action, meaning the intake and exhaust are both integrated into the compression movement of the piston, therefore eliminating the need for valves. This is thanks to an inlet and exhaust port integrated into the wall of the combustion chamber.

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 spark plug fires on every revolution.

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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 the oil that has to be added to the fuel to lubricate the piston.