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Rotax engines and fuel pumps


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ROTAX engines and fuel pumps - series and parallel – it has been discussed before but I don’t see my answer


On a ’24’ registered aircraft you have to keep things the way the factory does it – no modification


I know different makes of ultralights that have an electric pump plus the mechanical pump (engine driven) that are all plumbed in series. The aircraft I am familiar with then has excess fuel returning to the fuel tank


If the electric pump failed it will fail in the open position (no written guarantee can be found for this statement). If the mechanical pump fails - I am told that fuel will still be delivered through an internal by-pass


Also, I am told that if plumbed in parallel the fuel is unregulated and could cause over pressuring and possible flooding. Also, in series during high demand situations with the electric boost pump on – the required head of pressure is maintained thus preventing cavitation and possible vapour locks


Question is – do fuel pumps plumbed in series – an electric boost pump then the mechanical pump offer redundancy – if either pump fails – will the other pump do the job. I’d like to be assured that plumbed in series is the correct way







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Guest rocketdriver

As a generalisation,two identical pumps in series will double the fuel pressure at the carby with little increase in volume. Pumps in parallel will mostly double the available fuel flow with a small increase in fuel pressure at the carby.


The one thing to keep in mind however is that I believe some mechanical fuel pumps are designed so that, if the electric pump in series is switched on, the increased inlet pressure at the mechanical fuel pump moves the pump diaphragm away from the cam operated pump lever, so that you effectively only have one pump operational and if either fails the other automatically takes over ....


So if your mechanical pump operates like this, pumps in series would be the way to go. Otherwise, pumps in parallel, because variation in fuel pressure at the carby inlet has a big effect on mixture.







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Like to back up that RD, especially with the mechanical pump on the engine of the type where the delivery pressure is determined by the spring under the diaphragm. If the aux pump delivers more pressure than the engine driven one, then THAT pressure would apply. You don't add them. IF they are centrifugal pumps in series you DO add them for the pressure determination. You are better to keep the pressure at the float needle at the lower side of the recommended range.


Flow can be checked by running the fuel into a measured container, over a fixed time & should be quite a bit above, ( 1.5 x?) of the max required at WOT for take-off. Nev



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My interpretation of pump capabilities is that the engine driven pump delivers fuel which is pressurised by the diaphragm spring. Say for example 4 lbs sq. inch. The electric boost pump delivers fuel pressurised by the plunger which is solenoid activated. If the electric pump delivers more pressure than the engine driven pump, then that will be the pressure at the carby. To have them in parallel means that if the engine driven pump suffers from a leaking diaphragm, the electric pump will pump fuel through the diaphragm, unless there are non return valves.



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