Basic Fuel Injection Questions

[Toobuilder]

Depends on if you are interested in what the "ship" side of the fuel system is providing (GUMP check), or if you want to know the servo is scheduling fuel. An ailing/failing boost pump or main fuel pump will likely not show up if you are reading pressure at the divider. Not until it quits, anyway.

Generally speaking, "fuel pressure" is the pressure available TO the engine (servo/carb), not what happens after.

[Berchmans]

Ok, so sometimes I'm a twit. After looking in the box the throttle body came in I found the gasket in a brown paper envelope taped in one end...in my defense the color is the same as the cardboard...well close anyway. Now on to fitting the cowling.

[Don at Airflow]

Originally Posted by penguin

With due respect I disagree. The pressure indication at the servo provides much more information that at the pump. For example that fuel has reached the spider when priming, that fuel is flowing to the spider when the engine is running, and a zero indication means there is no fuel flowing to the injectors. Yes, the indicated pressure will vary with throttle opening. Reading pressure straight from the pump just says the pump is making pressure, doesn't say anything about the servo unit. I have both pressures measured, a sender in the spider should be rated to withstand engine vibration (or use a hose and a remote sender).

Pete

With all due respect for Mr. Penguin?s comment on monitoring the ?spider? (technically the flow divider) pressure. The pressure is a relationship to nozzle backpressure, so it really only measures what the nozzle backpressure is. If you know the size of the injector nozzles then you can establish the relationship between nozzle backpressure and fuel flow. The nozzle size must be held accurately +/- 0.25% on flow) for this to give meaningful data. If the nozzle restrictor sizes are changed for nozzle tuning purposes then this relationship is out the window and is pretty much meaningless. Also the nozzle backpressure curve is based on a fixed orifice size so follows the formula for incompressible fluids, which is a square curve. The result is a having fairly inaccurate readings at low flows as most pressure gauges are +/- 2% on flow.

As far as a troubleshooting device, measuring nozzle backpressure is only really good for determining if there is a clogged or partially clogged nozzle. If the gauge reads zero, why is the gauge reading zero? This in it self doesn?t tell you much. Maybe the flow divider is stuck closed, maybe the boost pump or engine driven pump is not working, maybe the mixture control is rigged wrong, or maybe there is no fuel. In a flight situation since the ?servo? (technically the fuel control. The RS model of Bendix fuel injection systems were actually servoed regulators, the RSA design as well as the FM Airflow system is a direct acting regulator. No servo valve in these designs.) will compensate for inlet fuel pressure, if there is a drop in inlet fuel pressure the nozzle pressure will stay constant until the inlet fuel pressure drops to a point where the fuel control regulator can no longer compensate and the engine will quit. I think monitoring inlet fuel pressure is far more important in this respect than monitoring nozzle backpressure. I may be wrong here, but probably 60% of the RV?s here running fuel injection monitor fuel flows electronically with a digital flow meter, which is far more accurate than the old pressure gauge measuring nozzle backpressure.


Don

[penguin]

Don,

Thanks for the education!

What I was trying to say (clearly not very well) is that if only one pressure is measured, the pressure in the flow divider gives potentially more information than the pump outlet pressure. I'm not suggesting it should be used to give a fuel flow indication. I have a flow meter.

I find the flow divider pressure is useful when priming (Precision system) to know when fuel is flowing to the injectors.

Pete

[Toobuilder]

Pete, I think Don just explained why monitoring nozzle pressure is not "better"... The fact that the fuel control/servo compensates for a fairly wide range of inlet pressure means that you could have a partially failed fuel pump, yet would never see it looking only at the nozzle pressure. Someone else can jump in here with the exact numbers, but I think the fuel control will operate with pressures from 15 PSI to over 45. So your "normal" 40 PSI system could degrade down to 15 and you would be none the wiser. I don't know about you, but an ailing fuel pump is more critical information to me than knowing my primer system is working.