Quote:
Originally Posted by renault_megane_dci
Two strokes gets very poor MPG capacity wise because of therir very low CR.
No if the MPG is related to output rather than capacity, maybe it is a little better ?
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I think you are right.
My KTM125EXC motorbike gets 55mpg. Not very good for a 125 motorcycle but this is a full sized dirtbike with about 35-40hp. A similar 35hp fourstroke 230 lbs dirtbike would get about 55mpg too, in the manner I drive it.
Snowmobiles cannot be given MPG figures because conditions vary so wildly. When I go snowmobiling with a crowd, the 4 strokes typically take less fuel at the pumps, but often not by much. Especially considering the performance advantage most of the 2 strokes have. Modern fuel injected 2 stroke snowmobiles are remarkably frugal on fuel compared to the snowmobiles of yesterday.
Again in outboard motors. Modern 2 stroke outboards, even carbed models, are very good on fuel, almost as good as 4 stroke models and much, much lighter.
I think there is a future for 2 stroke advancement, with lean mixtures, ecological oils and efficient designs. 2 stroke diesels make a lot of sense, oil injection targeted at the bearings, and other innovations.
I am actually doing a lot of work on a Yamaha 200cc air cooled quad engine, trying to increase power over stock. This is a 17hp motor that my son and I have increased to over 30hp without a noticeable increase in fuel use (as determined by range and running time on the small tank).
There was mention of the low compression ratio typical in modern 2 strokes.
Not really a fair comparison. A 2 stroke can use 8:1 compression to make high HP because the exhaust pipe has a supercharging effect.
A modern 4 stroke bike engine uses up to 12:1 compression because the wild high rpm cam timing bleeds off so much of the compression pressure.
So much work as gone into weight reduction in modern cars.
A 2 stroke already meets that mark. Just think if we could be driving 500cc cars with variable port timing and exhaust length to give both power and economy. That 500cc motor could weigh less than 30kg (70 lbs) and put out more than 100hp with very few moving parts.