Fuel line system

Donnier

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Jan 25, 2018
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Carrollton, Ohio
I have a gyrocopter with a Rotax 582. The fuel system is using the standard mechanical pulse pump and an electrical pump connected in parallel to the carburetors. There are 2 check valves to prevent backup into the pulse pump and 1 check valve for the electric pump. Is this the correct setup or should it be configured as an inline series fuel delivery system?
 
This is quickly going to become a Ford vs Chevy kind of argument.

The Two pumps in series are going to deliver approximately the same amount of fuel at the same pressure if either one or both are operating.
The Two pumps in series will deliver the same pressure at potentially twice the amount of flow. This could be bad or it could be benign.

The real benefit is if you place the electric pump in series and far upstream from the Hot Engine compartment.
In the event of the fuel lines getting hot and produce a vapor lock and the engine stalling,
energizing the electric pump will quickly purge the vapor out of the lines allowing a rapid restart.

This commonly occurs after heat soaking the engine compartment during cruise flight and then reducing the throttle to idle for the descent.
The high fuel flow that was cooling the fuel line at cruise reduces to a trickle at idle and allows the lines to heat up permitting the fuel in the line to vaporize.
You find yourself turning base and being faced with an engine failure at low altitude with no quick means of a restart.

If the electric pump is near the fuel tank it will be able to quickly purge the fuel lines and make a restart possible.

All this become moot if you just adopt the procedure of turning ON the electric pump during any dramatic power reductions and at all times when in the pattern.
Any vapor in the fuel lines will be promptly purged without you ever knowing they happened.
 
Thanks for the reply. In this case there is no engine compartment and my concern was that with the pumps in parallel, do the check valves fight against each other. In other words, will the fuel flow pressure from one pump act against the check valve or valves of the the other to hinder fuel flow through them?
 
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I must disagree with one of Uncle Willie's points. When pumps are in series, the fuels pressures will be additive and will increase pressure and thus will increase fuel flow. You have to be careful with total fuel pressure which must remain below the carburetors manufacturers recommendations for input pressures. It is possible to over power the needle valves in the fuel bowls and cause engine flooding.

Paralllel plumbing of 2 fuel pumps will only pressurize the fuel system to what ever the highest single fuel pump can produce. This parallel arrangement must have a check valve at each pump or else the higher pressure pump will back feed through the lower pressure pump, probably not a real problem unless the fuel pump fails while open, allowing fuel to flow back to the tank and not to the carburetors.

I fly an open gyro with no engine enclosure. I don't believe that vapor lock would be an issue.

My crude schematic of my fuel system shows the 2 completely redundant parallel arrangement on my gyro with a 2 carb MZ202 engine. In the event of a failure of either pump, fuel will still be provided to the carburetors.

When I preflight, I verify the Facet electric pump by feeling the fuel pulse through the plastic fuel line. With the engine running, I momentarily shut of the Facet pump off and wait for a minute to check the Mikuni pump operation. If the Mikuni was't working, the engine would stall. I fly with the Facet pump running continuously.

You may have notice that I have only one mechanical fuel check valve after the Facet pump but not after the Mikuni. The Mikuni is designed in such a way that it has 2 diaphragm valves which is essentially 2 check valves- fuel can only flow in one direction.
 

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When two pumps are in series, they both deliver equal flows. Obviously only one flow will go through a line. The pressure and flow produced by the two pumps in series will be greater that the p&f of either single pump but will not be an additive of the two because the friction rate (resistance) in the line increases with the square of the flow. Two pumps in series will pump “roughly” about 130% of what a single pump will do (assuming both pumps have the same pumping characteristics (pump curve)). Of course with pumps feeding an engine, the flow will be limited to what is introduced into the carburetor and the additional p&ff produced by the second pump may not be beneficial so the second pump is only a safety backup in case of pump failure.

when two pumps are in series, the flow will be the direct sum of the flow generated by each pump. However, where both pump into a common line, like in the series described above, the friction rate increases at that point. Pressure impact is a little more complex if the pumps are not the same as the pump producing the greater pressure will “back feed” pressure (to put it in simple terms) into the other pump at reduce the flow that might otherwise be developed by the lesser pump.

in either event, I don’t See where a check valve will have any impact with pumps in series. With pumps in parallel, a check valve will prevent an operating pump from back feeding into a non-operable pump in the event of failure. However, by nature of design, don’t most fuel pumps incorporate check valves as part of the mechanics?
 
Let us not overlook the pump designs when determining the pressures.

If we were talking about the 912is where the fuel rail is at ~45psi and being fed by two positive displacement pumps in series,
a blocked fuel line would approach 200 psi as each pump is capable of over 90 psi before their pressure relief valves open.

This is not the case here.
We have an electric impeller pump feeding a diaphragm pump on the engine.
The impeller pump will add 3-5 psi to its output but the diaphragm pump will add nothing to the pressure.
On each impulse to the diaphragm, the spring behind the diaphragm gets compressed and pushes back at ~4psi.
If the impeller pump's flow can keep the pressure at or slightly above 4psi, the diaphragm does not move and the next impulse does nothing.
The spring is already compressed. A diaphragm pump will not add to the pressure. and the electric pump just pumps harmlessly through the diaphragm pump.
No check valves needed.

There are electric diaphragm/piston fuel pumps available that operate using electric solenoids to compress the spring.
These can be identified by their "Ticka-Ticka" sound when they run.
You could plumb a dozen 4psi Diaphragm pumps in series and the output pressure would still only be 4psi. They will not add pressures.

Placing multiple impeller pumps in series would be a different story, but that is not what we have here.
 
Donnier;n1129438 said:
Thanks for the reply. In this case there is no engine compartment and my concern was that with the pumps in parallel, do the check valves fight against each other. In other words, will the fuel flow pressure from one pump act against the check valve or valves of the the other to hinder fuel flow through them?

In your case the pressure and flow will be whatever pressure is delivered by the stronger pump.
The engines diaphragm pump already has internal check valves, so the external one is redundant.
The Electric pump is capable of backflow so the check valve will be needed.

If the Electric pump is the stronger of the two, The diaphragm pump will idle harmlessly without any consequences.

In the event that the diaphragm pump is slightly stronger than the electric pump,
the check valve after the electric pump will never open due to the higher pressure after the valve.
This also means that there might not be any actual fuel flow through the electric pump.
This could lead to problems when there are no pump failure and both pumps are running normally.
The fuel flow through the electric pump is what cools the pump.
With no cooling flow flow the pump will run warmer/hotter than normal.

Another check mark to the Series side of the argument.
 
Some machines with in series pumps use a fuel return tube to relieve pressure. I have seen a Mikuni running in series downstream of a Facet with a capillary restrictor in a tube returning fuel to the tank. Both were diaghragm. It was a certified aircraft. Fuel pressure regulators are also available.
 
Remember that a diaphragm failure of a pulse type pump will allow fuel to be pumped into the engine crank case by an electric pump on the same line so it needs to be able to be isolated if needed to continue the flight.
 
phantom;n1129524 said:
Remember that a diaphragm failure of a pulse type pump will allow fuel to be pumped into the engine crank case by an electric pump on the same line so it needs to be able to be isolated if needed to continue the flight.

Very nice piece of forgotten advise. I had a Mustang mechanical pump fail and water down the crankcase oil once many years ago. Those mechanical pumps were so reliable the we never thought of them.
 
Vapor lock is quite possible with the stock Rotax pressure-driven diaphragm pump. It must be mounted OFF the block and away from hot components. My early Air Command arrived with the pump screwed directly to the side of the crankcase. Lo an' behold -- it would vapor-lock and quit on hot days. Moving it to the frame cured the problem instantly.

Replace the diaphragm, or the whole pump, frequently. Mount and plumb the pump so that any goo that blows up the line toward the pump from the case pressure line runs downhill back to the case.
 
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