2-stoke engine questions

[George]

1. A 'classic' 2-stroke engine has the carb feeding into the crankcase, where the fuel/oil mix lubes the bearings. What happens to that fuel? It seems it must end up getting burned? If so, does it need to re-vaporize?

1. Our Stihl trimmer (FS36) has a membrane carburator. AIUI, these are supposed to operate in any position. So, what drives the fuel from the tank to the carb? I'd thought that the fuel tank was (slightly) pressurized for this. (There's a 2nd hose between them, running through the primer bulb.) But, when I remove the carb fuel hose, with the engine running, no gas comes out of the hose. Is fuel just pulled by the venturi vacuum?

Thanks, George

[Steve Barker]

1. the fuel/oil mix is transferred to the cylinder and burned

1. The diaphragm in the carb IS the fuel pump. There are two little check valves as a part of the assembly, and the motion of the diaphragm along with the needle and seat cause fuel movement. The diaphragm is driven by crankcase pulsations via a small hole you will see next to the main port. In the case of some chain saws with isolated carbs, you will see a "pulse" hose connecting the crankcase to the carb.

[Hank]

The gas/oil mixture is sucked in thru the carb when the piston goes from bottom to top. When the piston is coming down, it forces the gas/ oil thru a port in the cyclinder walls and into the combustion chamber and then the spark plug ignites it. For more info, google it.

Removing a fuel line while the engine is running is asking for disaster. If fuel got on the spark plug, muffler or close to any ignition source, you'll have a fire. You were lucky fuel didn't run out, get on you and ignite.

Hank

[LSMFT]

1, It flows from the crankcase into the intake port and burned after lubricating the crank. 2 Vacuum. Sometimes when the engine becomes too worn there is insufficient vacuum to operate the diaphragm and suck gas into it. I had a leaf blower that went out that way.

[A. Baum]

Swirls up around the crank and piston. Intake is into the crankcase sometimes in large 2 stroke a reed valve is used. Also to increase power by sucking in more air/fuel and 'expansion chamber' muffler is used. It's tuned to the exhaust/intake cycle to help suck out more exhaust and by virtue of that, suck in more air/fuel.

The exhaust cycle creates a vacuum that pulls fuel into the motor through a set of two jets. One for low rpm and one for full throttle. The membrane takes place of a bowl float in an engine not designed to operate in any position. The membrane also allows that little rubber bulb to be pushed and offers a one way fuel flow into the carb for priming. That bulb in conjunction with the membrane is the replacement for the bowel and float on a carb designed to operate on a level surface. The other hose may be a pressure relief from the gas tank to not allow pressure build up in the gas tank. Not sure about that one.

[clare]

The carb has a "pulse pump" in it that pulls the gas from the tank. The fuel mix in the crankcase is vapourized by engine heat, as well as being thrashed to a fine mist by the moving parts inside the crank-case. It is lightl compressed in the crankcase when the piston comes down, and then flows through transfer ports into the cyl, where it is compressed, burned, and exhausted.

[clare]

GENERALLY there are two diaphragms. One is the pump, the other is the "regulator" which takes the place of the float and float-bowl.

[George]

That's the part I was wondering about - it seems like some of it would precipitate/condense on contact with the crank, etc. If it didn't re-vaporize, ... you'd end up with a puddle of gas in the crankcase. Which you don't.

So, OK. Thanks.