Why Fuel Pressure?

[dodsond]

This is a generic topic, not specific to Dynon products.

I would like to start a discussion on the benefits of having a fuel pressure indicator when a fuel flow indicator is already installed.

Many airplanes have fuel pressure indicators or fuel flow indicators, but not that many have both.  Some use fuel pressure to deduce fuel flow and display the reading in gph, not psi.

I am adding a (non-Dynon) EMS to my Twin Comanche and the system did not come with fuel pressure senders but those can be added for an additional price.  The original airplane used pressure converted to flow.  It was modified with a Shadin system that actually measures flow and the old pressure system was removed.  The new Auracle system uses flow meters for fuel flow like the Shadin.

So, what would be the benefit, if any, of adding the optional fuel pressure system?  I don't see much benefit of knowing pressure if I know fuel flow.  Please tell me if I am missing something.

FWIW, the sales guy from the other company is actually discouraging adding the optional fuel pressure sensors since the TC for the PA30 doesn't require it (true) and fuel flow is included.

-Doug

 

[dynonsupport]

Doug,

Assuming a injected engine:
The fuel pressure most people install with our products is before the injection system, but after all the fuel pumps, gascolator, fuel filters, etc. It's the pressure being delivered to the fuel pressure regulator, not after it. If you want to use fuel pressure to estimate fuel flow, you put a fuel pressure sender on the fuel spider, where the pressure is regulated to create a specific flow. In this case, a fuel flow sender is much more accurate, and the fuel pressure sender is redundant.

The advantage of fuel pressure before your fuel injection system is that it gives you early warning to failures. Before you take off, you can see that when you go full throttle you will have enough pressure to generate full power. If you are in cruise and your fuel pressure falls, you may not have a reduction in power now, but you won't be able to create full power later. Neither of these can be detected by fuel flow since they are limits to more power, not the power you have now, and flow is only based on now.

Finally, if your fuel pressure drops as your flow goes up, you have a blockage in the system. This is hard to find with only flow, since you only see that you can't get flow above a certain GPH.

Another advantage is that you can put an alarm on fuel pressure. You should never have less than X PSI pressure or more than Y, but you can't do this on fuel flow since both 1GPH and 20 GPH are logical. So it can be a nice early warning as things go wrong.

So, it is a backup and a nice to have, but so are a lot of instruments in a plane. It's only about $50 for the sensor and a few wires, so almost everyone installs one in an experimental.

As we all know, experimentals are way ahead of certified planes, so it doesn't surprise me nobody wants one in a certified spam can

--Ian Jordan
   Dynon Avionics

 

[dodsond]

FWIW, I have an D-120 in my fuel injected Glasair which I built myself. I have had some fluctuations in (indicated) fuel pressure but flow was not affected and the engine ran just fine. It is startling to get the low fuel pressure warning, but I have done some testing and don't believe there is an actual pressure problem.

I started the topic with the PA30 (also injected) because of a combination of what I believe are false alarms and the surprising response from a sales rep that recommended I don't buy his product.

I thought I thought this through but wanted to hear from the field so I posted this story on several forums. to see what I might learn. Thanks for your input.

Depending upon what I learn, I was considering removing the pressure system from the Glasair due to the false alarms. I don't know if the gage fitting (restrictor) is causing an issue or its a failed or otherwise inadequate sender. My boost pump always fixes the issue, but even without the pump, no fuel flow anomalies or engine symptioms during the intermittent drops to very low numbers.

-Doug

 

[Battson]

Short answer mate - the fuel injection servo needs BOTH flow and pressure to operate correctly.
The two parameters can vary totally independantly, an acceptable measurement of one parameter DOES NOT infer the other must also be running in the green.
If one drops off below limits, you're engine is in grave distress.