JET PUMP? Fuel System

[gnx99]

In a fuel system, what is a JET PUMP? How does it differ from a boost pump?

Thanks!

 

[Wiskey Driver]

A jet pump works like a venturi (spelling) it requires motive flow in order to operate. Motive flow is fuel moving fuel so boost pump goes on and fuel begins to move low pressure thru front of jet pump hi pressure out the back. REMEMBER that in order for it to work fuel must first be moving.

 

[Timebuilder]

Jet Pump, or motive flow pump

The engine driven fuel pump is a mechanical pump that runs off of the accessory drive that runs off the tower shaft. That's a shaft that turns because it is geared to the spool shaft of the engine (Garrett 731).

This mechanical pump can obviously only make pressure when the engine turns. Some of the pressurized fuel gets diverted to the jet pumps by a "bleed", or small portion. The jet pumps sit on the bottom of the fuel tank or wet wing, or on the bottom of the tip tanks (Lear). The jet pumps have a screen on the bottom to pick up fuel. They have one fitting where the pressurized fuel from the engine driven pump goes in, and another where the fuel goes out. The pressurized fuel goes through a venturi, where a small "jet" of fuel accelerates and its pressures decreases, and moves on into the output fitting. The lower pressure fuel "jet" is exposed to the fuel of the tank inside the strainer, which is at a slightly higher pressure, and the tank fuel is drawn along into the output fitting as the pressure attempts to equalize. The jet pump has no moving parts, except for the fuel being forced into it by the engine driven pump.

The boost pump is usually an electrically driven pump. On a Lear, the electric pumps can be called either "standby" pumps or "transfer" pumps, depending on where they are located.

Questions?

 

[Cornelius]

Jet Pumps use the High pressure fuel from excess fuel at the engine driven pump and converts it to low pressure high volume fuel. Usually fuel from the jet pumps are the sole means how the engine driven pump gets its head pressure. Head pressure is very important for the main engine driven pump so it won't cavitate and overheat. Jet pumps are also used some times to transfer fuel from one tank to another usually working its way inboard.