engines

http://en.wikipedia.org/wiki/2-stroke_engine

http://en.wikipedia.org/wiki/4-stroke_engine

Smoke. A 2-stroke emits smoke as the oil in the fuel (1 part in 100?) burns.

Follow a Wartburg or a Trabant car along the road and all will become clear rather smoky.

And rather smelly, too (phew, yuk! ).

And sounds funny too. (prrra prrra prrra prrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrra! pam!pam!pam!). Even with high end high tech engines, it sounds much more like a toy engine. Quite far from the usual more uniform "vrum vrum".

The cylinder head is the biggest indicator.
A 4 stroke has a load of valve gear in the cylinder head, a 2 stroke does not.
Also the position of the exhaust port/manifold. A 4 stroke has the exhaust coming through the ports in the cylinder head, in a 2 stroke the exhaust comes out through the cylinder wall.

(The above are generalisations, I'm sure someone can find engines which do not meet these critera, so don't sue me...)

Del

by the strokes? or by the sound?

easiest way would be that simple 2 stroke engines don't have inlet valves like in cars and a simple carburator that operates by gravity

2-stroke engines have 1 power stroke per revolution, 4-stroke engines have 1 power stroke every 2 revolutions .

Many outboard motors for small boats are 2-stroke as the engine is simpler and there's not so much to go wrong go wrong go wrong go wrong go wrong.....

If you mix oil and gas for the fuel tank, it is 2 stroke.

If you have a remote oil tank using 2 stroke oil and a remote fuel tank, it is 2 stroke.

If you put gas in the tank and oil in the motor, it's 4 stroke.

Well in my youth my Honda MT-5 motor bike was a 2 stroke with a seperate oil tank

If you have a remote oil tank using 2 stroke oil and a remote fuel tank, it is 2 stroke.

While most 2 strokes don't have valves in the head, some do, especially 2 stroke diesels.

2 stroke diesel is almost always supercharged or turbo charged as this scavenges exhaust air out of the cylinder, allowing compression of a clean charge.

With a diesel, remove the injectors and turn the motor over by hand.

If it tries to inject every revolution, it is 2 stroke. If every 2nd rev it is 4 stroke.

For a petrol engine, do the same thing. Remove the spark plugs, rotate engine and check if the plug sparks every rev or every 2nd rev.

The characteristics mentioned above are quite common with 2 stroke engines because they can't fully clear the exhaust gases before a fresh air charge is taken in. This mixture of exhaust with intake gases causes poor combustion and significantly reduces the range of fuel/air mixtures which can be effectively burnt, consequently they usually have quite a bit of unburnt fuel in the exhaust stream, and tend to have a poor operating rev range (although not always).

The main advantage is that they are usually simpler and lighter for the same power output, hence their use in chain saws, small motor mowers and outboard motors.

There seems a trend in outboard motors to 4 stroke motors because of the improved efficiency.