916 fuel pressure problem

Rotax Wizard wrote:

Hi All

The newer pumps that Rotax recommends were to cover the very high volume that the high output engines have for fuel supply.  The smaller pumps are just not enough flow at full power.  The pressures however are the same, the regulator is the same.  Perhaps there is an issue with your return line or restriction with the check valves. (?) 

Cheers

 I ran a test with the fuel circuit split directly after the regulator and directing it into a container.  No drop in pressures - so the return line was eliminated.  

Are you referring to the check valves at the pumps or the one bypassing the filter?  I can't see how any of those could increase pressure?

Jeff Blakeslee wrote:

Yes I was talking about part 34, and it was a long shot anyway and probably would yield the same result.  My first reaction to your problem was the same as yours, to make sure there was no restriction in the return lime so the regulator can do its job. At least you have now verified that the electronics are accurate and the return line is clear.

I would think the fine filter offers some resistance to flow, so make sure you really did install the (temporary) bypass check valve in reverse so that it is forcing the fuel through the fine filter.  Also, are you using the Rotax fine filter or another type?

And then of course you could try a new regulator, but I think it’s likely that will turn out to be a waste of money. I’ve got a couple old regulators left over from 5-year rubber changes, maybe you could find a shop locally that would have a used one around you could use as a test.

I know a few years back Rotax issued a service bulletin that raised the max allowable pressure by a few PSI, up to 46, which is what it is listed as now in the manuals. This was based on reports from aircraft manufacturers that they could not get the fuel system to meet the original spec. So it sounds like being on the high side is somewhat common.  I don’t have any experience with these new style (larger) pumps, can Sling offer any advice? Maybe they are seeing similar results with other builds.

I’m also curious if the fuel pressure would fall a bit with the engine running. You would not think there would be enough additional fuel flow to make a difference, but sometimes these things are counterintuitive. On my 912iS Bristell, the fuel pressure does not change with the engine running or not. 

I will check with the Dealer to see if they have a spare regulator on Monday. Unfortunately, there's no dealers in Ireland.

I first noticed the problem when the engine was running (~2500 rpm) and the pressure was about 1psi higher than the equivalent statis test.  I put this down to the pumps getting ~14v of Alternator power instead of 12.5v from just the battery.  Not sure if this is valid though.

RW,

Regarding the larger pumps, you state the pressure is the same but the flow is higher.  I agree. However, both the smaller and large pumps are capable of much higher system pressure than the engine requires.  The pressure is controlled by the regulator metering the return flow. My question is this: Since the larger pumps provide more flow, and flow and pressure are inversely related, could this flow be more than the pressure regulator can handle to keep the pressure within spec? 

Hi Jeff

The Bosch pumps have been tested and used for several years in development and about 1 year in final configuration.  It is the only approved pump that Rotax recommends for the type 916 engine.  The very high flow requirement at WOT, 160HP, was not possible with the old version ones.  (in my opinion the old ones are marginal even for the type 915) 

The block and field testing has shown no issues in any of the dyno run or field tested versions with the fuel systems designed to the outlined specs in the installation manual. 

I can confirm that the mechanical regulators may not all be exactly the same, they are after all mechanical in nature.  We know from the many years of use with the existing smaller pumps that fuel flow is affected by small changes inside the regulator.  So much so that it is a listed as a 5 year part for change based on time rather than hours.  

ian cassidy wrote:

Rotax Wizard wrote:

Hi All

The newer pumps that Rotax recommends were to cover the very high volume that the high output engines have for fuel supply.  The smaller pumps are just not enough flow at full power.  The pressures however are the same, the regulator is the same.  Perhaps there is an issue with your return line or restriction with the check valves. (?) 

Cheers

 I ran a test with the fuel circuit split directly after the regulator and directing it into a container.  No drop in pressures - so the return line was eliminated.  

Are you referring to the check valves at the pumps or the one bypassing the filter?  I can't see how any of those could increase pressure?

RW,

Any thoughts on this?

Many Thanks 

Ian