How do aircraft fuel injection systems work?

[Jim_CAK]

Check out "The Finer Points", too. Really good channel that doesn't get much attention

Yes - I watch his channel all the time. He seems like a great instructor.

 

[Checkout_my_Six]

so....what happens when the fuel distributor thingy sticks open?

 

[Dan Thomas]

so....what happens when the fuel distributor thingy sticks open?

The fuel in the line between the servo and distributor can boil from engine heat after shutdown and badly flood the cylinders. It's bad enough that the lines between the distributor and injector boil off and let fuel in there even if the valve closes.

If it sticks closed the engine won't start or will be hard to start. Back around 2006 Precision Airmotive built a batch of distributors with the valve machined too closely, and it would stick closed when cold. We had a 2006 172SP that gave us grief until I replaced that distributor after talking with the OEM. Here's their SB:
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=2ahUKEwjJ0e6qv8HgAhWCTt8KHZa9CNIQFjABegQICBAC&url=https://www.lycoming.com/sites/default/files/Reprint%20of%20Precision%20Service%20Information%20Letter%20No.%20SIL%20RS-84.pdf&usg=AOvVaw2f2Po268oUJhENbrjO9Lh6

 

[PaulMillner]

Hi, I know a thing or two about this.

Thanks for the explanation, Ted!

On your Lycoming the basic description is as follows. You have a mechanical fuel pump that will generally run around 35 psi or so to a servo (exact settings vary). This servo has a venturi (similar to a carb).

Nope, no venturi. There are instead four impact tubes that sense the mass flow (!) of air into the throttle body. No venturi there.

The delta-P that you get from this venturi moves a piece of rubber inside which then kicks that ~35 PSI down to a lower number. The delta P is caused by airflow going through the venturi, so this works well for measuring actual airflow.

The impact pressure of the four tubes guides the fuel flow. There is no venturi.

Either before or after that metering (I forget which) is the pressure reduction caused by the mixture knob, which is essentially a variable orifice to reduce pressure.

Well, it's neither before or after... there is one fuel valve, and it's pushed one way by the air impact pressure which represents mass flow of air, and the other way by the flow of gasoline... to deliver the required air/fuel ratio. So no before/after, the two flows act against each other, as modulated by the mixture valve. The throttle opens/closes the butterfly.

After the servo, the fuel goes up to a fuel distributor (or spider) which then splits that fuel flow (pressure) to each of the nozzles (one per cylinder). Those nozzles have calibrated orifices to produce equal flow. This is part of why it's important to keep them clean. The nozzles have screens to allow some small amount of air in with the fuel to help atomize it. This is a continuous injection system, so it's a constant dribble/spray/whatever. However the pressure at the nozzles will vary depending on the engine and the power setting. More pressure = more fuel. I forget the exact range here.

If you have a turbocharged engine the nozzles have boost pressure fed on the outside of the engine as well to prevent the boost from trying to blow the fuel out. It's important to keep these lines clear if you have them or it can cause all kinds of weird fuel flow imbalances.

Certain Lycomings have a fuel return for venting purposes to aid hot starting. I think the restart Cessnas were the main ones with that.

Not aware of that. ECi's retrofit systems offer that feature, however.

Continental is another ball of wax. It's a simpler concept that ends up being more complex in execution.

 

[Dan Thomas]

Thanks for the explanation, Ted!



Nope, no venturi. There are instead four impact tubes that sense the mass flow (!) of air into the throttle body. No venturi there.



The impact pressure of the four tubes guides the fuel flow. There is no venturi.



Well, it's neither before or after... there is one fuel valve, and it's pushed one way by the air impact pressure which represents mass flow of air, and the other way by the flow of gasoline... to deliver the required air/fuel ratio. So no before/after, the two flows act against each other, as modulated by the mixture valve. The throttle opens/closes the butterfly.

Look at this:

The Venturi (yes, it's there) generates a low pressure that controls one side of one diaphragm, and the impact tubes control the other. The differential between them determines airflow and density. Another diaphragm takes signals from unmetered and metered fuel pressure. All four inputs control the fuel flow to match the airflow.

The throttle opens and closes the butterfly and also controls the idle fuel flow valve via an adjustable link.