Message text written by Fred Fillinger
>I don't understand swapping a mechanical pump for an
electrical system, unless vapor-lock prevention schemes can't be made
to work,<
The existing system with a mechanical pump makes for a lot of connections
and lengths of fuel hose in the engine compartment. The pump position and
bleedback requirement also necessitates having the fuel pressure and flow
sensors in the engine compartment. Even with hose insulation the fuel temp
exceeds 40 degC after shutdown. So many connections in the engine
compartment also increases the risk of a fuel leak and therefore engine
fire. If twin electrical pumps are used the pipework forward of the
firewall can be minimised to just two individual pipes direct to each carb
---From the firewall. To include fuel temperature a single connection by the
hotter carb could be used if required. All other sensors could then be aft
of the firewall.
>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.<
Electronic regulators can and do occasionally fail to a full output mode (I
had it on a motorbike) and unlike car alternators, pmg generators have no
field connection making them more difficult to control. Whilst the Rotax
generator output is relatively small (about 20A max) so is the rest of the
system. Batteries are often only 17AHr and the system load can also be
quite small. Mine is often below 10A and only exceeds 14A for short bursts
when transmitting. The systems might be able to keep the generator from
cooking the battery if they can keep the output voltage below 16V but why
risk it. A crowbar unit is relatively cheap, weighs next to nothing and is
easily fitted. The recommendation comes from Bob Nuckolls who is a
recognised expert in this field.
>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.<
Regulators and alternators usually fail suddenly whereas batteries usually
give some warning (unless cooked by overvoltage). For this reason I would
prefer to use the battery as an emergency source. That aside the most
important thing is to avoid the two electrical pumps sharing the same
busbar or main fuse.
>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.<
Whilst I am sure this will work OK it involves system knowledge and pilot
input. On the day I would sooner have a straightforward automated system
which looks after itself but allows pilot intervention. A crowbar unit is
in effect a slightly more sophisticated Zener and also has only two wires.
It is the way it is connected we prefer to differ. Wiring so that it does
what you are going to do anyway seems a sensible idea. If an backup battery
is to be used there is even less reason to use the direct generator output.
Nigel Charles
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