HOW can a 4 STROKE - use PRE MIX?

[hammersley]

In another thread I asked about string trimmers and I was referenced to the Stihl http://www.stihlusa.com/news_fourmix.html. I noticed that others are calling themselves 4 strokes as well - BUT - you still use a PRE-MIX - i. e. - no oil sump!

 

[TowPro]

You can't tell by the pictures, but I bet its still sucks the air/fuel mix from the carb to under the piston on the "up" movement of the piston, then the "down" movement forces this charge into an intake area under pressure, when the intake valve moves open, as the piston is going down, the downward movement of the piston "forces" the fuel mix into the combustion chamber, kind of like a super charger does, thus you have darn near the same HP as a 2 smoker.



the problem with 2 smokers is how they use incoming air/fuel mix from under the piston, through the transfer ports to force the exhaust mix out. It does the job, but some of the unburnt fuel mix goes out the exhaust with it, lowering efficiency and raising pollution output.



But the oil mixed with the fuel in both engines is not something the "green meanies" think to highly about either.

 

[parcher]

TowPro got it right. I have a video I got when I went to Stihl school and that's exactky how it works. The crankcase must be sealed (NO air leaks) thereis a hole in the top of the intake port before the valve. this is open to the valve cover and it in turn is open to the crankcase. The Stihl unit uses a single lobe cam and will rev out to 10,500 rpm!