The automotive engines have seen a major transformation over the years. However, the gasoline-powered combustion engine designs still stay the same. These are two-stroke and four-stroke engines.
These two are commonly used and are hard to miss. But do you know the differences between Two Stroke Engine vs Four Stroke Engines? Read below to understand their work and what makes one better than the other.
However, before that, you need to understand the meaning of stroke and the working of the combustion engine.
Combustion engine – How do they work?
When the engine is in a combustion cycle, then within the cylinder, the piston starts to move up and down. These are top dead centre or TDC and bottom dead centre or BDC. They refer to the position of the piston in the cylinder. TDC is the position of the piston near the valve, and BDC is the piston that is further from the valve.
When the piston moves from the TDC to the BDC position or vice versa, this is called a stroke. A combustion cycle or a combustion revolution refers to the entire process of the gas and the air to be sucked into the piston, igniting and then expelling it as an exhaust.
- Intake is when the piston moves down in the cylinder, allowing a mix of air and fuel into the combustion chamber.
- Compression is when the piston moves back up in the cylinder, and the intake valve closes to compress the gas within.
- Combustion is a spark generated from the spark plug which ignites the gas.
- Exhaust is when the piston goes back up in the cylinder and causes the exhaust valve to open.
Two-stroke and the four-stroke engine
The difference between the two-stroke and the four-stroke engine lies in how fast the combustion cycle process occurs. This is based on the number of times it takes for the piston to move up and down in each cycle.
In the case of a two-stroke engine, the complete combustion cycle will be completed in just a single piston stroke. This is a compression stroke that is followed by an explosion in the compressed fuel. In the return stroke, the exhaust is let out, and this causes a fresh mixture of fuel to enter the cylinder.
The spark plug will first spark once in every single revolution, which causes power to be produced once in every two-stroke of the piston. It is important that in a two-stroke engine, the oil be pre-mixed with fuel.
In the case of a four-stroke engine, the piston will complete two strokes in each revolution. This is one compression stroke, and a return stroke follows one exhaust stroke and each of these.
The spark plug will fire just once in every other revolution, and power gets produced in every four-stroke of the piston. There is no need for pre-mixing the oil and fuel here, and there is a separate compartment for oil.
Difference between two-stroke and four-stroke engine
Let us now list down the important differences between the two-stroke and the four-stroke engines:
- In the two-stroke engines, a port is used for the inlet and outlet of fuel. In the case of four-stroke engines, it uses valves for the fuel inlet and outlet.
- Two-stroke engines create a high torque which is less in the case of a four-stroke engine.
- There is only one revolution of the crankshaft in a single power stroke in a two-stroke engine. In the case of a four-stroke engine, the number of crankshaft revolutions in a single power stroke is two.
- The two-stroke engines have less thermal efficiency. The thermal efficiency is high in the case of a four-stroke engine.
- Two-stroke engines are light but noisy. The four-stroke engines have flywheels which make them heavier but less noisy.
- The power to weight ratio is larger in the case of a two-stroke engine and less in the four-stroke engine.
- The two-stroke engine is cheaper because of its simple method of manufacturing. The four-stroke engine uses values and lubrication, which makes manufacturing difficult and thus more expensive.
- The two-stroke engine generates more smoke and is less efficient. The four-stroke engine generates less smoke and is more efficient.
- Two-stroke engines need more lubricating oil because the oil starts to burn with the fuel. The four-stroke engine needs less lubricating oil.
- Because of poor lubrication, two-stroke engines go through more wear and tear. The wear and tear is less in the case of a four-stroke engine.
So now that you are aware of the differences between a two-stroke and a four-stroke engine, let us understand which one is better?
Pros and cons
Listed below are some pros and cons of each of the engine types.
- With respect to efficiency, it is the four-stroke engine that is a clear winner. This is because the fuel is consumed only once in each four-stroke.
- Four-stroke engines, however, are heavy and weigh 50% more than a two-stroke engine.
- A two-stroke engine will create higher torque at a high RPM, but a four-stroke engine creates higher torque at a low RPM.
- The buzzing high pitched sound made by the two-stroke engine makes it fall behind the quieter four-stroke engine.
- The four-stroke engines are more durable since they do not run a high RPM like the two-stroke engine. But the two-stroke engines are more powerful.
- It is easy to fix a two-stroke engine because of its simple design. There are more parts in a four-stroke engine which makes the design complex, making repairs costly.
- You need to pre-mix fuel and oil in a two-stroke engine which is not the case in a four-stroke engine.
- Four-stroke engines are environment-friendly. The two-stroke engine burns oil and releases it into the air and exhaust, which causes pollutants to be introduced into the air.
Two-stroke engines are usually used in small appliances like lawn tools, dirt bikes, and boat motors. Four-stroke engines are used in lawnmowers, go-karts, and the internal combustion engine of your vehicle. It is for you to decide which engine you prefer based on the purpose of use.