Originally posted by DarZeelon
Sport_pilot
...And a 4 stroke's exhaust valve begins to open 40-70 degrees before BDC and its intake begins to close about the same duration after BDC. And there is an overlap period when both intake and exhaust are open, around TDC...
A sport two stroke has about 140 degrees of exhaust timing and about 120 degrees of intake timing. Sounds similar?
I wrote "theoretical", didn't I?
Sincerely,
Good point. But the exhaust valve timing of even 40 degrees is on the high side. 70 degrees would be a lopey high perf engine. And 150 degrees of exhaust would probably be more common on a two stroke. I don't know about our model engines but I suspect they are around 35 degrees. And the exhaust valve opens very slowly at first, the pressure in the cylinder is probably providing usable power for about 10 to 20 degrees of that, don't know just a guess. The ports of a two stroke open suddenly all pressure is relieved in just a few degrees. Good point however, hadn't thought of that. Of course you are right, the poorer exhaust scavenging of the two stroke is part of the reason it doesn't produce twice the power also.