Ok everyone,
After my recent problems with mogas I digged into this subject and it
looks like that with some minor changes the Rotax 914 fuel system can be
made more suitable for mogas.
There are two problems with the use of mogas concerning the Rotax 914:
1) Vapor pressure.
The boiling point of mogas is often lower than that of avgas. With
higher temperatures and lower pressures (altitude) the fuel vaporizes
inside the fuel system. It starts with small bubbles, but eventually it
leads to a complete stoppage of the fuel flow (vapor lock).
2) Ethanol.
Ethanol in the fuel can bind water and keep it in suspension. At lower
temperatures (like after climbing to altitude) the water can precipitate
out of the fuel. Eventually this can lead to freezing or other water
related problems.
For both problems there are solutions:
1) In the 914, the fuel system is pressurised, and there is a high
return flow of fuel. The pumps are located outside the engine bay.
Consequently, there is almost no heating of the fuel, and bubbles are
transferred back to the fuel tank because of the return flow and
pressure regulator.
There is one weak point, and that is the fuel pump inlet. The suction of
the fuel pump causes a pressure drop in the inlet, especially if the
inlet system contains a fuel filter. The drop of pressure invites the
fuel to vaporize instantly. Once vaporized, it will not easily revert
back to a liquid state, even if the pressure is increased again.
In cars, to minimeze vapor lock problems, fuel pumps are often located
inside the tank, to eliminate/minimize the suction (low pressure) area.
In the 914, I think a good solution would be to install a high flow/low
pressure pump at the main tank outlet, combined with a bypass check
valve. Such a pump will eliminate the flow resistance of the fuel valve,
the filters, the hoses, the various joints, and thus eliminate the low
pressure area. If the pressure in the entire fuel system is kept higher
than in the tank, the fuel will not vaporize.
Should the pump fail, the bypass check valve will allow the system to
operate as before.
I found this on ebay:
http://www.ebay.com/itm/320954771747
"THIS PUMP IS A FREE FLOW the solenoid pump uses a piston actuated by
an electromagnetic coil to generate fuel pressure and flow. By using
only short pulses of electricity, the solenoid pump is very
energy-efficient. It is also long-lasting, with no internal rubber parts
or bellows to wear out."
2) The precipitation of water out of the fuel can be prevented by
keeping the fuel on its initial temperature. If, before the flight, no
water can be found at the bottom of the tank, no water will form if the
fuel is kept at the same temperature.
For the Rotax 914, the fuel return line flows at least 30 liters per
hour. Heating this fuel should be sufficient to prevent the fuel from
cooling down at altitude, especially if the return line enters the tank
at the bottom. The easiest way is to install an electrical fuel heater
with thermostat as often found on diesel engines. Another way would be
to use the coolant as a heat source, via a thermostatic valve and heat
exchanger.
Heating the fuel may look a contradiction to 1) where we want to prevent
vapor lock. However if the fuel is not heated but merely kept on the
take off temperature, vapor lock should not be an issue. It the fuel
temperature was good enough for a full power take off, it certainly is
enough for maintaining cruise power.
On Ebay I found this:
http://www.ebay.com/itm/120928913405
Any comments or further ideas on this subject?
Frans
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