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Disappointment in Rotax 9 series
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Thom Riddle

Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)

Posted: Mon Feb 11, 2008 5:41 am Post subject: Re: Disappointment in Rotax 9 series

Giles,



You are correct in that there are many combinations of mp/rpm that will produce equivalent power, which is as you stated, limited by atmospheric conditions in a normally aspirated engine. That is why Rotax provides tables for the operators that show the most efficient combination of mp/rpm to achieve standard power outputs of 55%, 65% and 75% of max.



What is interesting to me is that the combinations Rotax recommends are markedly different for the 912 and 912S (80 vs 100 hp) engines. See attachments for these tables. Your statement that higher MP vs rpm combinations are more effecient is evident most of all in the 80 hp engine table.



Without looking at the Katana manual, I don't recall if the particular mp/rpm combinations shown in their table are the same as recommended by Rotax. If not, then perhaps there is room for improvement in fuel flow by conforming to the Rotax recommended settings.

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912S power.jpg

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_________________
Thom Riddle
Buffalo, NY (9G0)

Don't worry about old age... it doesn't last very long.
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Mark Burton

Joined: 10 Jan 2006
Posts: 74

Posted: Mon Feb 11, 2008 3:23 pm Post subject: Re: Disappointment in Rotax 9 series

Howdy folks,



I don't have anything to add re altitude compensation (or lack of it) but you may be interested in the following:



I have in my (912 powered) aircraft an instrument that calculates %power from the engine RPM and differential manifold pressure. By differential, I mean it measures the difference between manifold pressure and the ambient pressure at the engine side of the firewall rather than absolute pressure which is the norm. You can think of DMAP as being an indication of throttle position. 



Now as %power is closely related to fuel flow, you can easily calculate an estimated fuel flow from RPM+DMAP. And, of course, by integrating the estimated fuel flow you end up with an estimate of fuel used.



So it gives you a similar readout capability to using a fuel flow meter but without any fuel flow sensors to leak or clog up (or purchase!)



It's turned out to be amazingly accurate for the low-level flying that I do. I would say that the error was no more than 5%, probably better.



Cheers,



Mark

Thom Riddle

Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)

Posted: Mon Feb 11, 2008 3:29 pm Post subject: Re: Disappointment in Rotax 9 series

Mark,



That sounds very interesting. Is this system a commercially available set up or did you put it together? I'd like to learn more about how you did it and how you estimate the power and fuel flow from the RPM and DMAP.



The more info, the better. I think others might also be interested.

_________________
Thom Riddle
Buffalo, NY (9G0)

Don't worry about old age... it doesn't last very long.
- Anonymous

Mark Burton

Joined: 10 Jan 2006
Posts: 74

Posted: Mon Feb 11, 2008 4:42 pm Post subject: Re: Disappointment in Rotax 9 series

Hi Thom,



The instrument I describe is actually one of my CSC units that I sell. The %power calculation has been in the commercial unit for some years now but the estimated fuel flow stuff is only in my own aircraft because I want to give it a really good evaluation before I consider making it available to others.



As for the %power calculation, it's straightforward. I based my code on the curves given in the Rotax 912 documentation. I interpolate in 2 dimensions (RPM and MP) to produce an estimate of %power. It's not rocket science.



The problem with using absolute MP is that for a constant throttle setting, the measured MP goes down as you climb so the calculated %power goes down as well. But by using differential MP the calculated %power doesn't change much with altitude so you end up with a %power figure that relates to throttle position. i.e. for a given throttle position, the displayed %power doesn't change much with varying altitude.



Fuel flow is simply calculated based on the assumption that  it scales linearly with %power. If you look at Rotax's curves, they match pretty well so it's not an unreasonable assumption.



As I said, it gives remarkably accurate results and would make a good backup system to cross-check other fuel gauges against.



Cheers,



Mark

rampil

Joined: 04 May 2007
Posts: 870

Posted: Mon Feb 11, 2008 5:30 pm Post subject: Re: Disappointment in Rotax 9 series

Mark,



Percent power relative to engine rated max decreases with DA.

What ever fraction of max power (75% for example) at SL some 

arbitrary throttle setting will decrease with altitude if the throttle 

stays put.  I don't believe you are saying otherwise.



Just to avoid hand waving, Engine power for an Otto cycle recip run

rich of peak is a function of MAP, and RPM.  As a first linear

approximation, Power declines 3.3% per inch Hg MAP decline at fixed

RPM.  Power also declines linearly with decrease in RPM. For example,

at fixed MAP, if the RPM declines from 5800 to 5500 (a drop to 94.8

percent of max RPM), the engine power declines to 95%.  Therefore,

absolute pressure in the manifold is central, as is RPM relative to max

RPM speed.



What your device does, if I understand it is to display percent power

available as a percent of the max at any particular altitude.  My 

question is Why?  What's it good for?

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rv6ejguy

Joined: 18 Nov 2007
Posts: 2
Location: Calgary, Canada

Posted: Tue Feb 12, 2008 10:21 am Post subject: Re: Disappointment in Rotax 9 series

EGT is not, strictly speaking, a measure of mixture. Many factors affect EGTs so this is not a valid way to determine AFRs. You can fit a wideband O2 meter to determine AFR changes with altitude.



Programmable fuel injection is now available for for 912UL and 912ULS engines if you dislike the carbs: http://www.sdsefi.com/rotax2.htm

_________________
Subaru EJ22 turbo RV6A Flying
Subaru EG33 twin turbo RV10 building

Gilles.Thesee(at)ac-greno
Guest

Posted: Tue Feb 12, 2008 10:51 am Post subject: Disappointment in Rotax 9 series

 	  Quote: 	
		   EGT is not, strictly speaking, a measure of mixture. Many factors affect EGTs so this is not a valid way to determine AFRs. You can fit a wideband O2 meter to determine AFR changes with altitude.

   


	


I'll second that.



 	  Quote: 	
		   Programmable fuel injection is now available for for 912UL and 912ULS engines if you dislike the carbs: http://www.sdsefi.com/rotax2.htm



   

Please be aware that electronic fuel injection means higher part count, 
	


and increased complexity because of the need for electrical redundancy.

It is just the same with the 'electrically dependant' 914.



The injection kit you are referring to is most interesting.

Elsewhere on their site is a most interesting story about a subaru RV 

project with fuel injection. Beautiful engineering, but unfortunately 

the plane experienced an engine failure due to the lack of backup 

electric supply.



Fuel injection in an airplane is inseperable from electrical redundancy.



Best regards,

-- 

Gilles

http://contrails.free.fr

ami(at)MCFADYEAN.FREESERV
Guest

Posted: Tue Feb 12, 2008 1:34 pm Post subject: Disappointment in Rotax 9 series

I think I would dislike this system even more than carbs.



The manifold design on the 912 provides uneven charge to the front and back 

cylinders (except, I am told, at full throttle).

This EFI system, using the stock inlet manifolds will do exactly the same in 

terms of the air delivered to the front and back cylinders.

But now, the mixture will be different front to back as well. Because the 

injectors only meter according to the air demand of the whole engine, not 

the air demand of individual cylinders. It gets worse still; the mixture for 

all cylinders will need to be set up to the leanest cylinder, meaning the 

others run rich.



The advantage of having individual and tuned inlet runners to each cylinder 

has been lost with this specific arrangement.



Duncan McF.



Been there, done that (with other engines), not worth the hassle for the 

benefit obtained (unless lower emissions is high on your list of 

priorities).

---

rv6ejguy

Joined: 18 Nov 2007
Posts: 2
Location: Calgary, Canada

Posted: Tue Feb 12, 2008 2:46 pm Post subject: Re: Disappointment in Rotax 9 series

 	  ami(at)MCFADYEAN.FREESERV wrote: 	
		  I think I would dislike this system even more than carbs.



The manifold design on the 912 provides uneven charge to the front and back 

cylinders (except, I am told, at full throttle).

This EFI system, using the stock inlet manifolds will do exactly the same in 

terms of the air delivered to the front and back cylinders.

But now, the mixture will be different front to back as well. Because the 

injectors only meter according to the air demand of the whole engine, not 

the air demand of individual cylinders. It gets worse still; the mixture for 

all cylinders will need to be set up to the leanest cylinder, meaning the 

others run rich.



The advantage of having individual and tuned inlet runners to each cylinder 

has been lost with this specific arrangement.



Duncan McF.



Been there, done that (with other engines), not worth the hassle for the 

benefit obtained (unless lower emissions is high on your list of 

priorities).

--- 	




By making an assumption like this, you'd be incorrect. Testing showed the EGT spread was narrowed at 5750 rpm from 110F with carbs to 10F with the EFI. At 5500 rpm, spread was 60F with EFI and a whopping 130F with carbs. Intake length tuning is largely unaffected typically with the siamesed runner design used on this engine when only dry air is flowing through the manifolds. The EFI demonstrated much better fuel distribution than the carbs at typically run rpms as well as a power increase of about 4-6 hp depending upon ambient conditions.

_________________
Subaru EJ22 turbo RV6A Flying
Subaru EG33 twin turbo RV10 building

daveaustin2(at)primus.ca
Guest

Posted: Tue Feb 12, 2008 4:05 pm Post subject: Disappointment in Rotax 9 series

Hey folks..  Go flying!  My 912 has sung with oil and coolant change each 

year for 15 years!

Dave Austin  601HDS - 912, Spitfire Mk VIII

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