RE: Pump pressure setting?
 

GN:

There is too much confusion concerning what a pumped fuel system does, and what a regulated pressure system does, and how they both work.

First, the pumped systems.

The only aftermarket pumps currently available that I know of are the “Perry” VP-20 and VP-30, made by Conley Precision Products. These are both regulated pumps, the VP-20 being operated by the vibration of the engine, and the VP-30 by pressure pulses either from the engine crankcase or a tap in the exhaust. Other than this one difference, their operation is the same.

Both are diaphragm pumps with check valves to control flow. The regulator sets the delivered pressure, but otherwise has no effect on the fuel quantity delivered.

Since these pumps use check valves there is effectively no restriction to gravity flow, a tank mounted too high will tend to drain through, flooding a stopped engine and spilling the fuel on the ground.

The Perry pumps are best used for remote tanks, such as one mounted on the airplane CG.

Another advantage to the positive fuel feed given by the Perry pumps is that by not relying on the carb suction for fuel fees, a much larger carb can be fitted to a given engine and still have reliable operation with a corresponding increase in power.

It is difficult to the point of impossible to adjust an air bleed type carb to work with a pump.

And now the regulated pressure systems.

The Cline and Iron Bay regulated pressure systems are entirely different, allowing no fuel flow except when the carb “Sucks” the fuel through. They are what is known as “Demand” regulators, allowing flow only when the draw from the carb, acting on a diaphragm, pulls the fuel delivery valve open. The biggest difference in the two is the Cline using a plastic body, while the Iron Bay uses an all metal body.

These two require a pressurized fuel tank, they get this pressure by using a check valve in either a crankcase port or the exhaust. The check valve causes the pressure to build in the tank, forcing the fuel to the regulator.

Since these two systems require the carb to draw the fuel into the engine, some engines that will not run well without muffler pressure wont work well with them, but these almost always work well using one of the Perry pumps.

If I’ve left any questions, please ask.

Bill.

RE: Pump pressure setting?
 

Bill:

What I was trying to do with the last post was to see if you, W8 and I could agree on the 3 things I stated there. I tried to distill information from a number of sources including this thread: http://www.rcuniverse.com/forum/m_26.../tm.htm#267362
I think you have made additional points, but I still wonder if you agree with me.

One of the conclusions I came to was that the words "regulator" (ala Cline) and "regulation" as pertains to the VP20 and 30, while they suggest simularity, hide significant differences in terms of, among other things, the type of carburetor which can successfully be used.

I think that is part of the problem with the Webra/UAV 120s with the TNII carb when a pump is added. It may be a "regulating" pump, but it still delivers a pressure at the carb, that is outside the design parameters of that carb.

I have used the Cline "regulator", with crankcase pressure and a closed system, with the 120 (without pump) with the TNII with no problem, indeed it made the engine run more reliably. But, I believe that this experience outline in this thread suggests that the TNII does not work well with the pump that comes with those engines. I suppose the proof will be in the pudding, when I try the Promix II.

If I am correct, and given the email from Webra Austria mentioned above, I do not understand why Webra distributes pumped Webra 120s with the TNII carb. They sell another version of the 120, with pump, with the Promix II carb, which they indicated is the carb to use with a pump.