Originally Posted by
jaka
Hi!
Ofcourse the fuel pressure will change from full tank to empty! That's why we modelers set our engines a little rich at the beginning of the flight so that the engine doesn't stop when the fuel level is near empty.
Our engines are airpumps! They suck air through the carb and blows out air combined with fuel vapor trough the exhaust.
If they don't have enough piston to cylinder fit or...if the fuel level changes too much they cannot pump air as good as they should and the result is that they (The engine) stop!
This is the reason why we set our engines a little rich at the beginning of the flight, to help the engine from fuel starvation when the fuel level (pressure inside the tank) gets lower as the tank empties.
And that's why a suitable tank size for a .40-.46 two stroke sport engine is 8-10oz. Not a 14oz tank!
It seems you may be a little confused on a few things.
Tank size WILL NOT alter tank pressure. Muffler pressure or crankcase pressure is used to pressurise the fuel tank in order to force the fuel to the carby as atmospheric pressure is not enough to provide enough fuel to the engine. Engine speed will alter pressure so when the engine is at low throttle the tank pressure will be less and at full throttle the tank pressure will be higher.
We set the engines a little rich for 2 reasons. 1. The engine unloads once in the air, which leans the mixture. 2. When the tank is full, there is head pressure which assists the carby to draw fuel in, which will give a richer mixture. When the fuel level is getting low, it's harder for the carby to draw fuel in and will result in a leaner mixture.
Our engines are not air pumps. They are internal combustion engines which convert energy created by the combustion process, to torque which tuns the prop. The air/fuel mixture is drawn into the combustion chamber and then the unburnt oil/fuel is expelled through the exhaust.
Piston/cylinder fit will affect the engine performance. As fuel level changes the mixture will change slightly and if it's too lean the engine will stop.
This is a basic over view. Hope it helps your understanding.