The Difference between 2 Stroke and 4 Stroke Engines

Small engines from many different manufactures are made as a 2 stroke or 4 stroke engine, but you may be wondering what the difference is between these two kinds of small engines.

2-Stroke Engines

A two stroke engine is an engine that does not have valves, or a specified lubrication system. A two stroke engine is very simple, much simpler than the 4-stroke engine.  It has very few working parts and is very light. This means that there is very little maintenance required and it is much cheaper to manufacture.

2-stroke engines are generally used in handheld lawn equipment because the power per weight is greater than 4-stroke engines; this means that a smaller engine will pack more power than a smaller 4-stroke engine. Engine life isn’t really a concern with a chainsaw like it would be with a vehicle, because they aren’t being used constantly so they will last long enough with the way they are generally used.  They are also used in shorter time increments, so the problem of pollution is not as serious.

In a 2-stroke engine, it is required that oil be mixed in with the fuel, at a very precise ratio, in order to determine the ratio, you should check the owner’s manual of your equipment. The fuel and oil mixture running through the engine is what lubricates the parts, however, because it is not a specified lubrication system, these engines usually wear down quicker.

How it works:

• Fuel and air compress in the main cylinder (compression stroke)
• The spark plug fires, causing an explosion that drives the piston downwards (power stroke)
• When the piston is nearing the end of its stroke the exhaust port is uncovered (power stroke)
• The pressure in the cylinder drives most of the exhaust gasses out of the cylinder through the exhaust port (power stroke)
• As the piston hits the bottom, the intake port is uncovered and more fuel and air and pulled into the main cylinder to start the process over again (compression stroke)

This process can also be known by 2 kinds of strokes, the compression stroke and the power stroke. The compression stroke is when the piston moves upwards and compresses the fuel and air, or step 1/5. The power stroke is all of the other steps, the steps that actually produce the explosion and create the power.

Pros of a 2-Stroke Engine:

• No valves, simpler construction
• Fire once per revolution which means that I has a higher power output than a 4-stroke engine
• It is lighter, thus making it cheaper to manufacture
• They have potential for twice the power than a 4 stroke engine of the same size
• Less maintenance

Cons of a 2-Stroke Engine:

• Don’t live as long as a 4-stroke engine because of not having a specified lubrication system
• Having to mix fuel and oil to lubricate engine, can get expensive
• They are not fuel efficient engines
• They produce more pollution
• They are very smoky engines
• The air/fuel mix leaks out through the exhaust port

4-Stroke Engines

A 4-Stroke engine is the kind of engine that is common on lawn mowers. It is heavy and has to remain in an upright position to be used, so it doesn’t work so well on lawn equipment such as weed eaters or chainsaws. They also are more expensive due to the complexity of the engine and the repairs that would be necessary to fix them if something went wrong.

A 4-stroke engine has a cycle that consists of 4 strokes, in those 4-strokes, the piston goes around twice in the engine, so the spark plug fires every other rotation. There are two separate valves in this kind of engine, one for intake and one for exhaust, unlike the 2-stroke engine.

In a 4-stroke engine, the oil is contained in the crankcase, and there is usually a splash lubrication system where the oil is released and the only job of the system is to keep everything lubricated. Because of this, the oil needs changed after a certain amount of hours and the level needs to be checked frequently. Since the only function is to keep everything moving smoothly, wear happens much slower on a 4-stroke engine than a 2-stroke engine.

 

How they work:

1)  The piston moves up and compresses a fuel and air mixture in the chamber (compression stroke)

2)  Fuel is ignited by the spark plug and it drives the piston down providing the necessary   momentum to keep the crankshaft turning (power stroke)

3)  The piston moves back up and pushes the used/burned gasses out of the exhaust valve ( exhaust stroke)

4)   The piston goes back down and draws the fresh air and fuel in preparation to move back up and compress the air (intake stroke)

 

Pros of a 4-stroke engine:

• More torque, the engine is quieter and more reliable
• Last longer than a 2 stroke engine
• You don’t have to mix the oil and fuel
• Runs cleaner than a 2 stroke, less pollution
• Much more fuel efficient

Cons of a 4-stroke engine:

• More complicated design
• Valves, complex design, harder to repair/troubleshoot
• Half as powerful as a 2-stroke engine of the same size
• 4-stroke fires 1 time per revolution
• 2-stroke fires every revolution
• More expensive than a 2-stroke engine
• More parts, more expensive to manufacture, more expensive to repair
• Because of the more complex design, these engines are much heavier