What is the difference between 2 stroke and 4 stroke engines?

The technical answer

A 4 stroke engine has four strokes of the Piston to a cycle.
1st stroke is the intake when the piston sucks in the fuel and air mix from the carburetor into the cylinder.
2nd stroke is the compression of the fuel mix.
3rd stroke is the combustion when the spark ignites the fuel mix
4th stroke is the exhaust stroke where the exhaust gas is expelled from the cylinder.
The cycle then starts again.

A 2 stroke engine has two strokes of the piston per cycle.
1st stroke The compressed fuel-air mix ignites. At the same time the intake port is covered by the piston. The new mix in the crankcase becomes pre-compressed. Shortly before the piston approaches the lower dead centre, the exhaust port is uncovered. Being pressurised in the crankcase the mixture rushes into the cylinder displacing the consumed mixture (exhaust now).

2nd stroke The piston is moving up. The exhaust port are covered, the mixture in the cylinder is compressed. At the same time new fuel-air mixture is sucked into the crankcase.

In practice

A four stroke engine requires engine oil to be put into the engine and a dip stick will show if the oil level is correct or not. Unleaded petrol is then used to fill the fuel tank.

A two stroke engine requires the oil and unleaded fuel to be mixed together and then added as a pre mix to the fuel tank. It is very important to get this mix right as the oil that you mix into the unleaded petrol is used to lubricate the engine. Should you forget to mix the oil or do not mix enough the engine will seize.

Pros and Cons

2 stroke engines are smaller and lighter engines so are ideal for hand held machines
2 stroke machines are very high revving enabling faster cutting.
2 stroke machines are ideal for machines that are moved through 360 degrees and used upside down. This is because the lubricating oil is fed through the engine in the fuel mix under pressure ensuring proper lubrication all of the time.
4 strokes are more powerful engines
4 stroke engines are more fuel efficient and have better emissions.
4 stroke engines do not require the manual mixing of oil and fuel.

The Future

Due to the current focus on reducing emissions it is possible that 2 stroke engines will become a thing of the past. Already one or two 2 stroke machines have been taken out of production for this very reason. It is likely that 4 stroke engines will begin to replace 2 stroke.

However, to do this effectively manufactures need to make the 4 stroke engines lighter and need to pressurize the oil lubrication to allow the machine to be used at any angle.