On 7/21/07, Yvon Cournoyer <geryvon@videotron.ca> wrote:
----- Original Message -----
From: "Rotary Engine" <
rotaryeng@earthlink.net>
To: "AARotary Engine" <rotaryeng@earthlink.net>
Sent: Friday, July 20, 2007 2:37 AM
Subject: FUEL SYSTEM PRESSURE
(snip)
> Not really, the pressure regulator will keep the fuel pressure
> constant
> right up until the pump can no longer supply enough fuel for the
> engine, and
> by then it is too late. Once the fuel pressure starts to drop, the
> return
> flow had already hit zero and your power will already be limited at
> best,
> and the pump is in its final hour. Still, with a second pump it
> usually
> would not be a big deal.
Allo David, may I ask that you elaborate on the meaning of this
paragraph. What do you mean by ''and by then it is too late''? Too late
for what?
Also, ''and the pump is in its final hour''. Do you mean that the pump
is being damaged? Why should that be?
I need to understand and my knowledge of English is insufficient, as
well as my understanding of EFI pressure systems.
Thank you. Yvon Cournoyer
Yvon, don't tell me you are thinking about switching to EFI? :-)
Most EFI systems used for rotary aircraft installations work like this:
The pump gets turned on, and just pumps away as hard as it can at a more or less constant flow. Pressure is created in the system by the regulator that is placed after the fuel rail. It works like a thumb over the hose to create back pressure, and lets the excess fuel flow back to the tank. So as long as the pump is able to pump just slightly more than the engine is using, fuel pressure remains constant. As soon as the pump cannot keep up with what the engine is burning,
i.e. the engine is using more than the pump is pumping, then the fuel pressure very quickly drops and the engine begins to falter. (ie, once the pressure starts to drop the engine starts to falter - skip, misfire, run lean, loose power, surge, or just plain quit).
Normally, the pump puts out several times what the engine needs. As the pump starts to fail, flow is reduce but pressure remains unchanged for a while (depending on how fast the pump is failing - in my experience of 1 pump failure, it failed over the course of a few hours). By the time the flow is reduced enough that the pressure starts to drop and the engine starts to falter, the pump is only putting out a fraction of its rated flow.
i.e. it has almost failed completely.
Better to catch it early if possible. I found the return flow meter useful for that purpose.