3
$\begingroup$

For example let’s take the PT6A-27.

In every video related to engine start I see, the fuel boost pump is turned on, and then the fuel pressure goes up.

Example:

My question is: where is that fuel going if the condition lever is still at the idle cutoff position? Don’t you flood the engine?

$\endgroup$
2

3 Answers 3

4
$\begingroup$

I can’t say for sure about the jet engine mentioned. If I think of it like a piston engine, the fuel pump pressurizes the fuel lines. That in no way means that the fuel is entering the combustion part of the engine (cylinders in my case). It just means that the fuel lines leading to the fuel distribution point (carburetor or EFI) are being fed fuel at a certain pressure. A primer is a different story. It is feeding fuel directly into the combustion chamber by using pressure.

As mentioned in another post, more pressure and fuel is supplied by the fuel pump than is needed by the engine. In the case of a Cessna 172SP, the excess is routed to a reserve fuel reservoir. Here is a schematic of the fuel system from the POH.

enter image description here

$\endgroup$
1
  • $\begingroup$ … and turbines don't have primers. Fuel is only introduced once the starter spun it up to certain RPM and the igniters are on. $\endgroup$
    – Jan Hudec
    Jun 7, 2020 at 20:58
3
$\begingroup$

On most (probably all) PT6 installations, the fuel pumps send much more fuel than the engine needs. That extra fuel is then returned to the fuel tanks. This is true both during operation and before engine start.

$\endgroup$
2
  • $\begingroup$ So your position is that fuel just starts dumping into the combustion chamber as soon as the boost pump is turned on? What does the fuel condition lever do then? $\endgroup$ Jun 7, 2020 at 18:32
  • 2
    $\begingroup$ @MichaelHall, not the combustion chamber, just circulating more fuel through the pipes than the injectors will use when actually open. But they are controlled by the condition lever, so they are still closed at that point. $\endgroup$
    – Jan Hudec
    Jun 7, 2020 at 21:06
3
$\begingroup$

The pump just creates the pressure. The fuel doesn’t have anywhere to go until you move the condition lever to idle. Think of it like your faucet: There is water pressure there, but it doesn’t flow anywhere until you open the valve.

$\endgroup$
3
  • $\begingroup$ fuel pumps don't like to run without fuel moving through the lines. The excess fuel is returned to a collector tank. The condition that fuel has nowhere to go should never happen. $\endgroup$ Jun 9, 2020 at 13:25
  • $\begingroup$ @Peter Kampf, yes good point. Still, the pump simply creates pressure, and doesn't flood the engine until the valve is opened. $\endgroup$ Jun 9, 2020 at 15:24
  • $\begingroup$ @PeterKämpf Depends on the pump design. I have seen positive displacement pumps where the fuel either moves or something breaks, these usually have return lines. There are also spring solenoid designs where an electromagnet pulls back a piston which then returns under spring pressure, so if the fuel doesn't move then it just sits there and no harm done. I've never personally seen a centrifugal pump design for gasoline, but it wouldn't be impossible. Not sure which of these designs might ever be used in aviation, but the piston types are pretty common in other areas. $\endgroup$
    – Perkins
    Jan 26, 2022 at 23:46

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .