What's the difference between a 2 stroke
and a 4 stroke engine?

The cool page with moving graphics on Howstuffworks.com and this even better one at carbibles.com.

A 4 stroke engine usually has a distributor that supplies a spark to the cylinder only when its piston is near TDC on the compression stroke, ie. one spark every two turns of the crank shaft. Some 4 stroke engines do away with the distributor and make sparks every turn of the crank. This means a spark happens in a cylinder that just has burnt gasses in it. This just means the sparkplug wears out faster.

This from Michael from BIGENZ:
Most of what is written below on advantages and disadvantages of 2 strokes Vs 4 strokes is not actually correct! Take for example the lubrication issue of 2 stroke engines, sure small chainsaw engines may have the oil mixed with the fuel but this is not a direct result of the engine being a 2 stroke, this is just a result of someone designing a very simple engine. look at any large Caterpillar, or Detroit 2 stroke they have conventional oil sumps, oil pumps and full pressure fed lubrication systems and they are 2 stroke!!! also the argument about valves of 4 strokes Vs reeds and ports of 2 strokes is also incorrect. Sure some simple 2 strokes may use very primative systems to achieve the conrol of fuel/air mixture into the engine and exhaust out of the engine but again this is not a function of them being 2 stroke! I've worked on 2 stroke engines that feature poppet valves in the head (like a standard 4 stroke) - but they are definately 2 stroke - it's just that engines like this are not so much in the public eye - next time an ocean liner (ship) pulls into port check out its 2 stroke, turbo charged, direct injected diesel engine!! Finally the arguments of simplicity, weight, power to weight, and cost of manufacturing are not a funtion as such of 2 stroke Vs 4 stroke engines. The mistake of most of these commentaries is that they are comparing a simple chainsaw 2 stroke engine Vs a complex 4 stroke engine from a automobile - not a very fair comparision. As far as emmisions of 2 strokes - check out the Surrich/Orbital 2 stroke design that Mercury outboards are using - this is as clean burning as any 4 stroke.

The ONLY correct comparison below of 2 strokes Vs 4 strokes is that a 2 stroke can (in theory) produce twice the power of a 4 stroke for the same sized engine and the same revs.

As for the diagram [in the Snowmobiles episode of Scrapheap Challenge] - sure small 4 stroke engines do tend to "waste fire" the spark plug at the end of the exhaust stroke but this would not cause an explosion as depicted in the diagrams, but again here the idea of "waste firing"has noting to do with the 4 stroke cycle, rather it is a result of the simple design of a lawnmower motor. The diagrams should depict the inherant differances of a 2 stroke and 4 stroke engine. Hence one should show a firing every revolution and the other a firing every 2 revolutions.

TWO STROKE ENGINES - THE DISPUTED LIST
Advantages:
- Two-stroke engines do not have valves, simplifying their construction.
- Two-stroke engines fire once every revolution (four-stroke engines fire once every other revolution). This gives two-stroke engines a significant power boost.
- Two-stroke engines are lighter, and cost less to manufacture.
- Two-stroke engines have the potential for about twice the power in the same size because there are twice as many power strokes per revolution.

Disadvantages:
- Two-stroke engines don't live as long as four-stroke engines. The lack of a dedicated lubrication system means that the parts of a two-stroke engine wear-out faster. Two-stroke engines require a mix of oil in with the gas to lubricate the crankshaft, connecting rod and cylinder walls.
- Two-stroke oil can be expensive. Mixing ratio is about 4 ounces per gallon of gas: burning about a gallon of oil every 1,000 miles.
- Two-stroke engines do not use fuel efficiently, yielding fewer miles per gallon.
- Two-stroke engines produce more pollution.
From:
-- The combustion of the oil in the gas. The oil makes all two-stroke engines smoky to some extent, and a badly worn two-stroke engine can emit more oily smoke.
-- Each time a new mix of air/fuel is loaded into the combustion chamber, part of it leaks out through the exhaust port.

And from Jonne's page: Why 4-stroke beats 2-stroke

First some explanation on how the 4-stroke engine works:
When the piston moves down fresh air and gas is sucked inside the cilinder from the carburettor. When the piston moves up again the valves are closed and the air is being compressed. When the piston reaches it's highest position the sparkplug produces a spark that ignites the fumes and causes an explosion. Because of the power of the explosion the piston is pushed back. When the piston moves back up again the exhaustvalve is open and the fumes are pushed out the cilinder. Now the whole process restarts (exactly after 4 strokes. That's why we call it a 4-stroke engine...Duh!).

The 2-stroke engine works a little different. It performs 2 strokes at the same time (twice) so after only 2 strokes the whole process restarts. A two stroke engine uses the space above and below the piston. Below the piston are fresh gasses, above the piston these gasses are ignited. Assume the piston is in the lowest position with fresh gasses in the cilinder above it. When the piston moves up, these gasses are compressed, but at the same time teh airpressure below the piston drops and fresh air is sucked inside. When the piston is in it's highest position the sparkplug ignites the gasses and the piston is pushed down. About halfway down a channel from the fresh gasses to the exhaust gasses opens and a channel to the exhaust. Because of airpressure differences the fresh gasses flow inside the cilinder pushing the exhaust fumes out. Now the process starts again. As you can see this only took 2 strokes.

But which one is better?!? Of coarse "we" 4-stroke people say 4-stroke, but do you know why?? Probably not, so I'll explain. First I'll name some of the advantages of the 2-stroke engine over the 4-stroke engine.

• When the engine is reving at 10000rpm the 2-stroke engine ignites gasses 10000 times per minute while 4-stroke engines ignite only half. So it should have twice as much power, right?
• The 2-stroke engine does have channels for air transport but the piston opens and closes the channels, in a 4-stroke engine there are seperate valves and a camshaft that do this. This means extra moving parts and a far more difficult cilinderhead design. Because of this maintenance is more difficult.
• 2-stroke engines give more power. At 50cc races 2-stroke bikes can reach speeds of over 130km/h while 4-stroke bikes can hardly reach those speeds with 70cc!! The same applies to everyday use. When you remove some restrictors from a scooter it will go 80km/h. You won't reach these speeds with you're 4-stroke bike by just removing some restrictors.

What about the advantages of 4-stroke bikes? Are there any? OF COURSE! LOTS!!

• So what does a stock 2-stroke bike use? About 1 liter fuel per 35km. What does a stock 4-stroke bike use? About 1 liter fuel per 50km!
• 2-stroke bikes burn oil!! This absolutely doesn't give them more power and is of no use at all (for ignition pusposes), all it does is clogg up the exhaust and smell bad! So why they do it? They have to lubricate the cilinder and cilinderchannels big time or the engine will be wrecked...
• Look at the cilinder(head) of both engines. Notice the HUGH cooling fins on the 2-stroke engine. Heck, some of them even have forced air cooling or even water cooling.Did you burn the fuel for heat!?! I don't think so! 2-stroke bikes waste much more energy on heat than a 4-stroke bike does so the 4-stroke engine is much more energy efficient.
• Did you read the part on how the 2-stroke engine works?? Did you notice fresh fumes are used to push out the exhaust fumes?? Of course these two gasses will mix in that process, so or fresh fumes are pushed out too or exhaust fumes stay in, either way, it's bad and it sucks.
• Ever tried to speed up a 2-stroke bike?? So fast that it could do speeds of over 100km/h. If you have you know you have to add lots of extra oil to the fuel and even than it's a big problem to keep the engine running for let's say a couple of months without mayor engine damage, wasted cilinders and screwed up pistons. Take a 4-stroke engine on the other hand and it will keep running fine for months and months and months... without extra oil ;) The reason is that because of those channels in the cilinder it's much harder to keep it lubricated enough all the time. The 4-stroke engine doesn't have this problem because it has no channel in the cilinder.
• Have you ever drove on a Honda MT/MB or anything comparable with a fast cilinder? Did you notice it only had power between about 9000 to 12000 rpm?? Take a 4-stroke engine and it will have power from 1000 to 12000rpm. No problem! The reason is that such a 2-stroke cilinder has optimized airflow channels for that rpm range, outside that range the whole gas mixture gets screwed up big time. That's why those scooter engines always run at almost the same rpm. To optimize airflow, without that a 2-stroke engine looses a lot of it's power.
• Because of technical aspects the 2-stroke engine can only have it's intake channel open only half as long as the 4-stroke engine...What a pitty :)
• Listen to the sound of both engines...Doesn't need further explanation I suppose.

Yeah, yeah, I know, 2-stroke bikes with the same displacement are faster, but look at it in another way. A 50cc racing 2-stroke engine will use over a liter per 10km. Now take a 4-stroke engine that uses the same amount of fuel. I guess you would end up with a 90cc or even a 106cc with a 26mm carb or something. No try beating such a machine with that crappy 50cc racebike thing :)