Why not a pressurized fuel tank to cure fuel starvation?

by Had Robinson
updated April 1, 2023

There are many pilots, myself included, who have toyed with this idea for years.  In many ways, it is a simpler solution – but with serious issues.  They are:

• The tank must have a regulated, constant pressure regardless of the height of the fuel in the tank – an engineering challenge.
• It should not be electrically operated because of the danger of fire/explsion.
• There must be some sort of relief valve, for safety, that cannot fail.

What if the relief valve fails? Clogs? If the air pump is pulse/port powered, what happens if the muffler gets blocked with the fiberglass packing getting loose? (It happened here a month ago with an FSM installed, pump just slowed down.) The pressure inside the crankcase will dramatically increase to many PSI.

What if the diaphragm inside the pump fails (and also the tank relief valve) which will suddenly put 7 or 10 psi into the tank? You are flying along with a ¼ tank of fuel and a fuel line fails or pops off. You stop the engine but you will still have a pressurized bomb spewing gasoline until pressure in the tank is zero. This is what gave me the willies…. 

If the FSM fails, only a few milliliters of fuel will escape before the engine shuts down from a lack of fuel. Furthermore, the fuel pressure at the carb is barely positive, about 0.1 PSI.

If the pressure is not constant, there will be changes in the air/fuel ratio across the throttle range.

The pressure control has to be simple, reliable, instantly decommissioned, if necessary, when in the air.  Paramotor HDPE tanks are not engineered to be pressurized is another issue.

The Italians look at out stuff but they won’t touch something like the FSM because of the risk – one hose clamp fails and the fuel line to the carb pops off, etc. The FSM has been operating here and in tropical Australia without a hitch – thankfully and that, alone, is a plus. Anybody can duplicate the FSM but it will cost more and you will have to accept a lot of risk that we have taken years to eliminate.  The installation and testing instructions are thorough.  The VLCP’s are not obtainable in small quantities at a reasonable cost and are a critical part of the system because they must have a life equal to the carburetor soft parts and a very low crack pressure.  They are available from us at less than the cost from typical Internet suppliers.

Sealing any container filled with gasoline is dicey, maybe more so when the paramotor is on the ground, being transported, stored.

That is why we did not explore this further.  I do not recommend it, probably because of the risk to myself in developing and testing it.  It has to be 100% bulletproof against any failure of the components, the “what if” scenario.  I’ve thought and thought about it.  I launch and then find out in the air that I missed something?  That was when I got cold feet and don’t even trust my own fuel storage tanks – I store them outside.  I’ve had just (1) fuel tank split open at the special area that manufacturers create in case of excessive pressure.  Only vapor escaped, it was outside, but what if it was inside and a pet knocked the tank over and THEN liquid gasoline came out?  The “what ifs” are daunting in dealing with gasoline and why it is not a good idea to do it yourself.

The best solution would be an in-tank pump of some kind which would be simple but it would require electricity and that raises other issues e.g., TWA flight 800.  Electrical parts inside a tank of gasoline have to be perfect.  Then there is the added weight.

The Germans are brilliant with this stuff and what they did was put the tank above the engine – the fuel starvation problem was solved.  But why FreshBreeze put a float carb on the engine, I cannot figure out.  It is superior to a diaphragm carb but difficult and tedious to tune.  With all the gasoline above the engine they did not miss a beat and added a simple safety release that drops the entire engine off the frame in a few seconds.  You’re flying along and a leak begins, hits the hot exhaust manifold and ignites.  Now what?  You pull two pins and the entire engine drops away and down through the roof of some home or office building, burning it all to the ground.  I wonder what happens next?

But they did not go with a pressurized system and that stuck with me.  I would not fly with the FSM unless I thought it was safe.