Why does Rotax use a fuel pump?

[CTSW Bob]

May be a stupid question, but why do our engines use fuel pumps?  Our CT’s are high wing airplanes capable of flowing 150% or better of needed fuel by gravity alone.  Both my 172 and 182 had much bigger engines and required more fuel without a fuel pump.  So, why do the carbs need a pump?  
 

Just curious.

 

[Tom Baker]

Cessna was a big enough company that Lycoming and Continental were willing to customize engines to their need. Piper did the same thing. That is why you could not just simply swap a Lycoming o-320 from a Cessna 172 to a Piper Warrior. Continental and Lycoming wound up with so many different and intricate model differences for some of those engines. Rotax chose to basically have one design for their engine that fits all airplanes. The only difference being simple add on accessories. 

[CTSW Bob]

Thanks for the reply Tom.  So, in essence, one could remove the fuel pump and the engine carbs should still have plenty of fuel and run fine with our high wing configuration?  
 

[Roger Lee]

yes and no.

It would be foolish to do that. How do I know I did a research project on just that. fuel pressure would be so minimal that certain maneuvers would make it even worse. Leave the fuel pump in. There is a reason the minimum fuel pressure limit is set at 2.2 psi.

[FlyingMonkey]

I could imagine some situations where you might not get enough fuel flow from gravity alone to run the engine correctly.  Like tanks 1/4 full and in a steep climb where the engine is closer to the height of the wing tanks.

[AGLyme]

Re the injected version... need a pump to push the gas over and up from the header tank in the baggage compartment.  

[FlyingMonkey]

Just now, AGLyme said:

Re the injected version... need a pump to push the gas over and up from the header tank in the baggage compartment.  

Plus injectors use higher fuel pressure to atomize the fuel.

[AGLyme]

Didn’t know that... 

[FlyingMonkey]

To be fair the highest pressure is at the injector nozzle itself where the aperture is tiny  50-100psi+ is normal for many injectors.  I could not find the spec for the Rotax injectors, just the part number:  874 780.  The overall fuel system pressure doesn't necessarily have to be that high,and I don't know what is normal fuel pressure in the 912iS.  But I assume with the need to maintain high injector pressure at the cylinder to maintain mixture, the 912iS is probably more sensitive to low fuel pressure than the 912UL/ULS.

[AGLyme]

Makes sense thx for explanation 

[CTSW Bob]

I would never remove the fuel pump, but I was curious why it was there on my carbureted engine when it appears it may never be needed in the first place.  Just seems like another failure point.   If the pickup was in the right location, in the back of the tank aka Cessna 172/182, it should always have fuel except in a dive.  Then again, if you run it that low would a fuel pump make any difference if the fuel can’t get out of the tank and into the fuel line?  Does a carb need fuel pressure to fill a float bowl more than gravity feed?  Our fuel systems, if my memory is correct, are designed to flow 150% of needed fuel by gravity alone.  This is part of the annual inspection.  You measure the flow into a bucket from the sump area by taking loose a line.  As long as the floats are full on a carb, the carb takes what it needs.  Fuel pressure has nothing to do with that once the fuel is in the float bowl. 
 

I may be all wrong. I do understand fuel injection is a whole different beast that needs pressure.

Thank you for your responses.

[FlyingMonkey]

Bob, the fuel pump is a feature of the 912 engine, and in many installations (esp. low wing) it is a requirement for good fuel flow.  It could be a case where it was easier for FD to just plumb to the fuel pump than to omit it and make changes to the engine and standard Rotax fuel system. 

Rotax is pretty particular about things, it could also be that they don't allow manufacturers to omit the fuel pump without affecting the Rotax warranty.

That's all speculation, and in the end it doesn't really matter.  The pump is designed to "fail open" so if there is a problem fuel will continue to flow using gravity feed.  Possibly not enough to run full RPM, but it should be more than sufficient to keep you in the air.  I for one am happy that at least it's an engine driven pump and not an electric boost pump that requires another checklist item.

[Roger Lee]

No one has hit on the reason for the pump. 

[GrassStripFlyBoy]

1 hour ago, Roger Lee said:

No one has hit on the reason for the pump. 

Vapor Lock

[Roger Lee]

11 hours ago, GrassStripFlyBoy said:

Vapor Lock

Partly.

Think about what activity your doing and forces generated.

[Madhatter]

I can only assume Rotax is providing a fuel pump to cover all installation configurations in all types of airframes,  kind of like one size fits all. That would make sense financially and provide mass production simplicity.

[Tom Baker]

1 hour ago, Roger Lee said:

Partly.

Think about what activity your doing and forces generated.

For a CT I don't think the forces generated are a factor in needing a fuel pump. For most float type carburetors they are not an issue. One thing a fuel pump does for all applications is provide steady fuel pressure to the carburetors, at least it is supposed to. 

[JDL]

On 12/19/2020 at 8:35 PM, Roger Lee said:

No one has hit on the reason for the pump. 

Is there a reason for the fuel pump, that you know of, for a plane like the CT series, with tanks in high wings ? 
As others have pointed out, C-150, 172 etc, with carbs, have no fuel pumps.  The carb is under the engine for O-200 & O-320 etc on Cessnas, so a bit lower, but if gravity is enough with the higher carb position on a Rotax 912, then why do we need a fuel pump ?
There is a very good description of the fuel pump at 
 https://electricmotorglider.com/wp-content/uploads/2018/09/912-Fuel-Pump-Web.pdf
and it can be seen there that the mechanical fuel pump has two check valves, so that fuel can only flow one way.  The check valves are critical for the proper operation of the pump. It's difficult to guess how the pump would fail, if/when it does fail, but some failure modes would mean that the diaphram does not move to pump fuel, however in that case, the spring-loaded check valves at the pump inlet and outlet would still be operating, so the pump would not really 'fail open', the check valves would still provide resistance to prevent free flow through the pump.  An electric boost pump can easily overcome that resistance, to force open both check valves, but gravity flow might have a harder time doing that.  In other words, if the pump fails in this way, and there is no boost pump, then the available flow rate to the carbs would likely be a lot less flowing through the failed pump, than if the pump was bypassed altogether.   When you did the research project, did the gravity flow pass through a disabled pump, or bypass the pump ?
 

 

 

 

 

[Anticept]

Rotax engines are standardized. They dont make "high wing" and "low wing" engines. They're all the same.

[Mike Koerner]

What about when we are climbing at a high angle of attack - max power at low speed? Is the wing root fuel port still above the carb bowl fuel level? High enough above so that the fuel head is sufficent to overcome the line losses at the maximum fuel flow rate?