Originally Posted by Celestialsailor
Also...I will say while 2 strokes are lighter and simpler in design, they are somewhat inefficient. They have to spin and work harder than a 4 stroke
. I know on motorcycles, they only last half as long as their 4 stroke
On motorcycles, you're right -- two-strokes definitely don't last as long. I am guessing that this is a function of cooling
, however, on those high specific-output motors.
I don't think you can say this about two-stroke outboards, however, which tend to be slow-running and low stress engines. My 25 horsepower Mariner is 430cc -- so the specific output is only slightly more than my main diesel engine
(100 hp out of 2000cc). It's raw water
cooled like all outboards, so does not have cooling
issues of motorcycles. It redlines at all of 5500rpm. Two stroke engines don't usually rev as high as four-strokes, because they don't need to -- they have twice the power strokes at a given RPM
as a four-stroke.
My snowmobile has a very high specific-output two-stroke made by Rotax -- 120 horsepower out of 600cc. It is water
cooled, with a snow-to-water heat exchanger
in the tunnel. But it only revs to 8000 rpm
, and pulls like a freight train from 2000 rpm. The equivalent four-stroke Yamaha revs to 13,000rpm, is I think 900cc, and is nearly double the weight, has an extremely narrow power band, etc., etc. Those Rotax two strokes are extremely long-lived for such a high output motor
-- they easily go a couple thousand hours without any service
at all other than plugs and cleaning
the RAVE valves. Mine is semi-direct injected which overcomes the main disadvantage of two-strokes (see below), and so fuel
economy is similar to that of a four-stroke. It is a fantastic engine
Two-strokes are inherently less fuel
efficient than four-strokes, because (a) they need to run richer for cooling; and (b) some part of the fuel-air mixture is wasted out the exhaust
during the overlap period of the cycle (unless they are direct or semi-direct injected). This makes them more polluting. But these are just about the only disadvantages I can see -- otherwise, two-strokes are fantastic. The main advantage of them is that a 430cc two-stroke, say, works like an 860cc four stroke, in terms of the volume of fuel-air mixture you can burn on every rotation. Yet they are smaller and lighter than even a 430cc four-stroke.
I guess you can tell that I love two-strokes. I dream about a two-stroke diesel
as a main engine -- if someone would modernize the old Detroit Diesel concept