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I just wanted to know that what are the major differences in 4 stroke engine or in 2 stroke engine.

The only difference I know is that a 4-stroke engine goes through four stages, or two complete revolutions, to complete one power stroke. A 2-stroke engine goes through 2 stages, or one complete revolution, to complete one power stroke.

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A 4-stroke engine will pass smog LOL.

Engines take a volume of air, combine it with fuel, put it into a cylinder and then do two strokes: compression stroke and power stroke. The cylinder is now full of useless exhaust (CO2 and H2O and some other crud). So you have a problem called "scavenging". Scavenging is removing the useless exhaust gases from the cylinder, and charging the cylinder with fresh air.

In a 4-cycle engine, we add two more cycles: the exhaust cycle ejects and the intake cycle refills, and there's good separation of the two. This is so straightforward that 4-stroke minded people don't even think about it and don't give it a name.

In a 2-cycle engine, you have to do ??? to make that scavenging happen. Typically both exhaust and intake ports or valves open at the same time, and ??? pushes fresh air in, and in doing so pushes exhaust out.

This is typically quite inefficient, i.e. either you're failing to push all the exhaust out, or you're pushing some fresh air (or worse, air+fuel) through the engine and straight into the exhaust system. Or both depending on throttle and RPM.

  • The smallest chainsaw, go-kart and such engines use the crankcase as a compressor: the piston being at the bottom means the crankcase has the least air volume in it, so it pressurizes the air somewhat which is now forced into the cylinder, pushing out exhaust. Typically ports are both at the bottom of the cylinder, and there's an air deflector on the piston to help direct fresh air down the side of the cylinder to push exhaust up the other side. However, having air blasting through the crankcase makes engine lubrication "rather interesting". Hence having to mix fuel and 2-cycle oil for many such engines.
  • GMC and Electro-Motive engines have intake ports at the bottom of the cylinder, and four exhaust valves like a normal 4-valve engine, except they're ALL exhaust valves. An external "blower" (literally the thing on Mad Max's car; that's off a common GMC 6-71 bus engine) is compressing fresh air so it forcibly blows air through the cylinder when the port and valves open. Some turbocharged engines work at low power levels by gear-driving the turbo through a monstrous gear-up and an over-running clutch, which allows exhaust gases to run the turbo even faster when flow allows.
  • Fairbanks-Morse and Deltic opposed-piston engines do the same, but have intake ports at the bottom of one piston, and exhaust ports at the bottom of the other piston. This gets rid of valves, but requires 2 crankshafts with elaborate gearing to synchronize them.
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  • I was wondering if you were going to follow up :o) I worked on 8V-71 engines in the Army which had a blower and a turbocharger on top of that. Wild. Commented May 14 at 0:37
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When it comes down to the practical purpose of putting an engine to work, torque and rpm are two important numbers that can be used to contrast 2-stroke and 4-stroke engines.

Typically, engines are designed to provide a certain amount of torque at a given rpm, for the most part, 2-stroke provide less torque and more rpm than 4-strokes.

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I just wanted to know that what are the major differences in 4 stroke engine or in 2 stroke engine.

The differences depend on the type of engine. You could be talking about small gasoline engines. Or you could be talking about large marine diesels for example.

For small gasoline engines, the two stroke engine is a cost and weight saving method. Since the two stroke engine has a combustion cycle twice as often, so for the same displacement, it has twice as much power. Therefore, an engine of a given power costs and weighs less. Furthermore, the reduction in moving parts results in more cost saving. For example, a two-stroke gasoline engine can work without a liquid lubrication system since it uses a special kind of fuel with oil mixed in it. It doesn't also need valves since the engine has two ports which are obscured by the piston and work like valves. All you need is one reed valve.

So it looks like a two stroke engine is very useful, lots of power for small weight and low cost. However, it requires a special kind of fuel with oil mixed in which adds to the cost and the oil is blown through, polluting the environment. Also fuel usage is very inefficient, since the fuel/air mixture can blow right through the engine without all of it burning inside the cylinder. So it pollutes and uses fuel very inefficiently. The lubrication system is probably far from ideal so a two stroke gasoline engine doesn't last as long as a good four-stroke one would.

Two strokes is mainly useful for string trimmers (although today electricity is a better choice) and chainsaws (today electricity is getting competitive here too). Slightly heavierweight engines like in generators, lawnmowers or mopeds/motorcycles almost exclusively today use four stroke engines.

However, for large engines the situation could be very different. If you have the money, you can install a direct injection system into a two stroke engine. Then the fuel is injected only when the ports/valves are closed, meaning fuel isn't blown right through anymore. This also frees the crankcase from a fuel/air mixture pump to a lubrication system. So you can have normal liquid oil used as a lubricant which no longer needs to be mixed to the fuel/air mixture.

Large marine diesels actually sometimes use two strokes. Here the strokes are twice as often, so for the same friction losses, twice as much power is generated. Minimizing friction to maximize fuel efficiency is important here.

What's a mystery to me is why cars don't use direct injection two stroke engines. They could, and it would probably work extremely well. It would allow more power for the same size/cost engine, or same power with a cheaper and smaller engine.

The only difference I know is that a 4-stroke engine goes through four stages, or two complete revolutions, to complete one power stroke. A 2-stroke engine goes through 2 stages, or one complete revolution, to complete one power stroke.

This is the main difference. However, this results in several design problems that need to be solved, and are solved differently in small gasoline engines and in large marine diesels.

I would say that although small gasoline 2-strokes and large marine 2-strokes are both 2-stroke engines, they are very different kinds of beasts. You cannot compare these to each other. Small gasoline 2-stroke is a cost and weight reduction thing, at the cost of pollution, fuel usage, short engine lifetime etc. Large marine 2-stroke is a fuel efficiency improvement thing.

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