Nigel Charles wrote:
>
> Having proved my system works,
> your suggestion, using two electric pumps is my next step. Provided the
> electrical system is set up correctly there should be very little risk of
> both pumps failing simultaneously enabling the removal of the mechanical
> pump with all the pipework across the engine.
> ...
It is worthwhile to note that without 2 alternators/batteries, this
system does not meet the independent sources of power for fuel
delivery that would be required here for production A/C. But FAA has
recently approved for IFR certification a setup where backup power to
electric gyros is simply a pack of alkaline D-cells with an expiration
date. Although I don't understand swapping a mechanical pump for an
electrical system, unless vapor-lock prevention schemes can't be made
to work, such a backup power source would be simple to add and solves
the same problem for the 914.
> one of the pumps should not receive its power from the main panel fuse
> and to guard against total electrical failure a overvoltage 'crowbar'
> unit should be installed if this is not already the case.
I don't think a crowbar system is worth much on a Rotax setup, as the
failure mode of the solid-state regulator is highly likely to be
little or no output, unlike mechanical regulators (and a 70-amp
alternator). The puny Rotax alternator, if connected to a good
battery, just cannot generate much overvoltage to damage anything that
quickly if at all. The systems it supplies will simply sap most of
what it can put out unregulated.
On the 914, Rotax says wire the main pump directly to the alternator
regulator, best through a pullable breaker. In most fault scenarios,
this allows pump operation if the alternator still works, but the
battery relay must be killed, or the alternator output to the bus
killed, or its CPD pops.
The only problem with this would indeed be an overvoltage, but with a
crowbar it could be disastrous, if you wire it to break alternator
output. If the pump is wired to the regulator as recommended, then you
may send very high voltage to the pump when overvoltage trips the
crowbar, and how much it can take is secondary to the fact that it is
carrying fuel while the pump heats up with full unregulated alternator
output. Pump destruction and fire could take mere seconds. A better
way is a normally closed relay wired to break alternator AC to the
regulator, which keeps the pump on battery.
On my 914, there is no crowbar, but simply a 50W, 17V Zener diode on
the regulator output, with panel voltmeter and aural/visual warning.
It will reasonably clamp any spikes accompanying regulator failure
that can hurt some avionics, and within the cook time of the Zener (if
it cooks at all) in an unlikely overvoltage condition anyway, there
should be plenty of time to turn the aux pump on (switchable but
default connected to aux alt/battery), kill the main pump, and then
pull the main alternator. It will also send 17V v. 14V to the main
pump, which it can certainly withstand longer than needed to switch
stuff around.
Regards,
Fred F.
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