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On 10/10/2013 12:17 AM, Alan Carter wrote:
> Why hasn't the 914 a mechanical pump, as well as an electrical
> pump. What's the reason not have a mechanical pump in the system,
> the 912 have both.
There are two reasons:
1) The 914 needs a higher fuel pressure.
2) The 914 runs hotter, and the inlet air has an elevated temperature
because of the compression by the turbo. A mechanical fuel pump
connected to the hot engine, plus its long hose routing through the
cowling, allows considerable heat transfer to the fuel. Then you have
the combination of hot inlet air and hot fuel. This increases the
chances of vapor lock (and detonation?) considerably. The Rotax
engineers designed the fuel system of the 914 setup such that you can
keep the fuel pumps outside the cowling, with no mechanical connection
to the hot engine, and with very little fuel hose exposed inside the
cowling. Together with the high return flow this ensures that the fuel
reaching the carbs won't get too hot.
The concept works, I don't remember having seen reports of a double
electrical fuel pump failure. Don't fix something that isn't broken!
If you want additional redundancy, I would recommend putting an extra
alternator on the engine. There are very small light weight
alternators that fit on the vaccuum pad of the engine and are able to
run a fuel pump plus some additional items. You can only use it of
course if you have no vacuum pump on the engine.
Frans
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