28th Jul 2009
What’s The Difference Between 2-Stroke and 4-Stroke Engines?
The basics of the Internal Combustion Engine
The internal combustion engine is actually fairly simple in principal. Essentially a mixture of fuel and air is sucked into a cylinder where it is compressed by a piston and then ignited by a spark plug at the top. The resulting explosion pushes the piston back down and out of the way making space for a new batch of fuel and air to be sucked in and the whole cycle repeats.
This would all be fine apart from three things. Firstly the explosion produces a fair amount of gases which need to go somewhere before new fuel can be sucked in and secondly metal doesn’t slide very easily across a metal surface. It needs to be lubricated.
The 2 Stroke Solution
Putting a hole in the side wall of the cylinder gives the exhaust gases an easy exit point which is closed by the piston on its way back up. Theoretically all you need to do then is add some oil to the fuel/air mixture and it will lubricate the metal before being burnt up along with the rest of the gas. The only problem then is how to get the new fuel mix back into the space which is left.
So we have a second hole in the opposite wall of the cylinder called an inlet. At the mouth of this inlet is a valve which allows the fuel into the engine but doesn’t let it back out. In some engines this is a simple vacuum operated valve and in others it just looks like a plastic disc with a cut-out window. This disc spins in time with the crank – the bit at the base of the piston – so that once every revolution the cut-out lines up with the inlet and allows a measure of fuel mix into the crankcase.
Once the fuel mix is in the crankcase two things happen. Firstly the oil does its job and lubricates, secondly the whole lot gets pressurised by the piston on its downward stroke. This is whilst the exhaust gases are escaping out the top, which creates a bit of a vacuum so the pressurised mixture floods upwards to fill the gap.
The reciprocal mass of the crank acts like a flywheel and because of momentum it keeps on spinning, pushing the piston back up the cylinder shutting off the exhaust hole and once again compresses the fuel mix ready for the next spark to ignite it so the whole cycle can start once again.
The 4 Stroke Solution
The 4 stroke engine does things with a little more finesse. It takes it’s time and does one things at a time but is still all based around a tiny spark igniting a small but highly compressed volume of fuel and air.
As with the 2 stroke, the power of the explosion forces the piston down the cylinder, spinning the crank which acts like a flywheel as well as turning the gearbox of course. This is called the power stroke. As the piston reaches the bottom of the cylinder the momentum begins to push the piston back upward and starts the exhaust stroke.
Valves at the top of the engine open to allow the release of exhaust gases which are pushed out by the upwardly mobile piston. They then close as the piston reaches top dead centre and a second set on the opposite side of the cylinder head open up to allow the entry of fuel and air which is sucked in by the piston on it’s downward stroke. No oil in the mix with a 4 stroke though. All the moving parts sit instead in a permanent pool of oil which gets pushed around the engine by all this movement in much the same way as the fuel/oil mix gets pushed around the 2 stroke but this clever opening and shutting of valves keeps the fuel and oil entirely separate…
Finally, on the fourth stroke as the piston travels once again up the cylinder still being driven by momentum the valves close once again, compressing the fuel ready for the spark.
Summary
2 strokes are rough and ready simple little engines that have twice as many power strokes at the same speed as a their more sedate cousin, the 4 stroke. However due to the lack of complicated valves they also waste a certain amount of fuel mix as it rushes over the piston and escapes with the exhaust gases. So they are less efficient as well as being noisier and smellier. They are also less efficient at lubrication and consequently need rebuilding quite a bit more often as they wear out easily.
That said they are far easier *to* rebuild and the extra power form those extra strokes is arguably worth it.
The basics of the Internal Combustion Engine
The internal combustion engine is actually fairly simple in principal. Essentially a mixture of fuel and air is sucked into a cylinder where it is compressed by a piston and then ignited by a spark plug at the top. The resulting explosion pushes the piston back down and out of the way making space for a new batch of fuel and air to be sucked in and the whole cycle repeats.
This would all be fine apart from three things. Firstly the explosion produces a fair amount of gases which need to go somewhere before new fuel can be sucked in and secondly metal doesn’t slide very easily across a metal surface. It needs to be lubricated.
The 2 Stroke Solution
Putting a hole in the side wall of the cylinder gives the exhaust gases an easy exit point which is closed by the piston on its way back up. Theoretically all you need to do then is add some oil to the fuel/air mixture and it will lubricate the metal before being burnt up along with the rest of the gas. The only problem then is how to get the new fuel mix back into the space which is left.
So we have a second hole in the opposite wall of the cylinder called an inlet. At the mouth of this inlet is a valve which allows the fuel into the engine but doesn’t let it back out. In some engines this is a simple vacuum operated valve and in others it just looks like a plastic disc with a cut-out window. This disc spins in time with the crank – the bit at the base of the piston – so that once every revolution the cut-out lines up with the inlet and allows a measure of fuel mix into the crankcase.
Once the fuel mix is in the crankcase two things happen. Firstly the oil does its job and lubricates, secondly the whole lot gets pressurised by the piston on its downward stroke. This is whilst the exhaust gases are escaping out the top, which creates a bit of a vacuum so the pressurised mixture floods upwards to fill the gap.
The reciprocal mass of the crank acts like a flywheel and because of momentum it keeps on spinning, pushing the piston back up the cylinder shutting off the exhaust hole and once again compresses the fuel mix ready for the next spark to ignite it so the whole cycle can start once again.
The 4 Stroke Solution
The 4 stroke engine does things with a little more finesse. It takes it’s time and does one things at a time but is still all based around a tiny spark igniting a small but highly compressed volume of fuel and air.
As with the 2 stroke, the power of the explosion forces the piston down the cylinder, spinning the crank which acts like a flywheel as well as turning the gearbox of course. This is called the power stroke. As the piston reaches the bottom of the cylinder the momentum begins to push the piston back upward and starts the exhaust stroke.
Valves at the top of the engine open to allow the release of exhaust gases which are pushed out by the upwardly mobile piston. They then close as the piston reaches top dead centre and a second set on the opposite side of the cylinder head open up to allow the entry of fuel and air which is sucked in by the piston on it’s downward stroke. No oil in the mix with a 4 stroke though. All the moving parts sit instead in a permanent pool of oil which gets pushed around the engine by all this movement in much the same way as the fuel/oil mix gets pushed around the 2 stroke but this clever opening and shutting of valves keeps the fuel and oil entirely separate…
Finally, on the fourth stroke as the piston travels once again up the cylinder still being driven by momentum the valves close once again, compressing the fuel ready for the spark.
Summary
2 strokes are rough and ready simple little engines that have twice as many power strokes at the same speed as a their more sedate cousin, the 4 stroke. However due to the lack of complicated valves they also waste a certain amount of fuel mix as it rushes over the piston and escapes with the exhaust gases. So they are less efficient as well as being noisier and smellier. They are also less efficient at lubrication and consequently need rebuilding quite a bit more often as they wear out easily.
That said they are far easier *to* rebuild and the extra power form those extra strokes is arguably worth it.
Posted by sarafiel under 
General Info 
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