Engine Governing/Fuel Consumption

[skippydiesel]

(Not much activity on the Forum , thought I would put this up for debate)

 

I seek your learned clarification on the above topic;

I have always understood that a carburettor engine speed (rpm), is governed/controlled by the amount of air:fuel allowed into the engine.

The PRIMARY mechanism that determine the amount of air: fuel is  the  "butterfly" valve,  which the pilot controls using the throttle.

The butterfly is opened/closed at will, influences the air pressure/vacuum in the carburettor body, which then opens/closes a needle valve allowing more/less fuel to mix with the air.

Then there is the SECONDARY mechanism, the propeller. Depending on blade pitch the propeller will also act as a governor. Fine pitch will allow higher RPM, Coarse pitch will restrict RPM.

So - IF I have my 912ULS at Wide Open Throttle (WOT) and I have a Constant Speed Propeller (CS) I know I can use the pitch of the prop to  control engine RPM  but can I also control fuel consumption.

 

Put another way;

With  fixed pitch prop, I want to select 5200 RPM for a reasonably economy/ good power, cruise setting.

With a CS, engine at WOT, can I select a pitch, that will reduce engine RPM to 5200 FOR THE SAME FUEL FLOW  as above (no change in WOT)?😈

[facthunter]

Coarsening the Prop is like selecting a higher gear. It loads the engine.  Nev

[facthunter]

Hello. Anybody home?

[Thruster88]

3 hours ago, skippydiesel said:

(Not much activity on the Forum , thought I would put this up for debate)

 

 

 

Put another way;

With  fixed pitch prop, I want to select 5200 RPM for a reasonably economy/ good power, cruise setting.

With a CS, engine at WOT, can I select a pitch, that will reduce engine RPM to 5200 FOR THE SAME FUEL FLOW  as above (no change in WOT)?😈

You certainly can reduce the rpm to 5200 WOT using the prop control, however this will be a much higher power setting than the fixed propeller doing 5200. The difference will be the amount of manifold pressure which is controlled with throttle.

 

This power chart for a lycoming shows multiple combinations of manifold pressure and rpm to get the same power and hence fuel flow. 3 fuel/power settings are shown in the chart. All piston engines behave the same way.

[skippydiesel]

Okay -

 

So I have a Rotax 912uls with Bing carburettors (below)  - the amount of fuel being delivered depends on manifold pressure, which can be controlled either with the throttle/butterfly or the propeller pitch. It seems to me that, in theory, the manifold pressure/vacuum, at a given rpm, will be the same whichever governor is in use.

 

My theory does not seem to hold good for load (needing power/fuel to move) must come into the system somewhere.?😈

 

 

 

 

[facthunter]

With no prop at all you'll get destructive revs with about 1/8th throttle. Like running a car in neutral. . A CV carb is different to all others and there's always some pressure drop across it.  At higher altitudes the dashpot doesn't fully open.  At lower revs your engine doesn't Pump as much air. so won't make as much Horsepower..  I also understand Rotax's don't like over propping. . Nev

[Thruster88]

Skippy, your thinking is wrong, the propeller control only adjusts engine rpm, not manifold pressure. The throttle only adjusts manifold pressure, not rpm in a constant speed propeller system. There are exceptions to this outside normal climb, cruise, descent power, throttle will reduce rpm during landing etc.

 

It would be nice if Rotax published a comprehensive chart like the one above for the 912, showing 75%, 65%, 55% power and how that can be achieved for a given altitude if a constant speed propeller is used.

Edited February 13 by Thruster88

[Thruster88]

2 hours ago, facthunter said:

  At higher altitudes the dashpot doesn't fully open. 

I have always wondered about this. We know with Lycons above 7500 feet we usually have the throttle wide open (FT) as the chart I posted above shows.

 

The Rotax 912ULS performance chart at altitude shows the 100hp engine only making about 38 hp at 7500 feet when it should be capable of approximately 75hp at that altitude. What is going on in that carburetor??? Is the chart wrong? 

[facthunter]

The dashpot is only elevated by the pressure difference across it.  As atmospheric pressure drops the difference does also. This is how the taper of the needle adjusts the carburettor for altitude mixture. (stops it being too rich).  The fuel is always sucked out of the needle jet. It's NOT an injector.   Nev

[pluessy]

3 hours ago, Thruster88 said:

I have always wondered about this. We know with Lycons above 7500 feet we usually have the throttle wide open (FT) as the chart I posted above shows.

 

The Rotax 912ULS performance chart at altitude shows the 100hp engine only making about 38 hp at 7500 feet when it should be capable of approximately 75hp at that altitude. What is going on in that carburetor??? Is the chart wrong? 

Density altitude is in METERS, not feet. 7,500' is about 2,300m and the power at WOT and 5,500rpm is around 56kW (or 75hp which is your 75% of sea level max power). Same difference, different units.

Edited February 13 by pluessy
spelling

[Thruster88]

Metres for altitude, yes of course, we wouldn't want the Chinese or Russians to feel left out.

[skippydiesel]

15 hours ago, Thruster88 said:

Skippy, your thinking is wrong, the propeller control only adjusts engine rpm, not manifold pressure. The throttle only adjusts manifold pressure, not rpm in a constant speed propeller system. There are exceptions to this outside normal climb, cruise, descent power, throttle will reduce rpm during landing etc.

 

It would be nice if Rotax published a comprehensive chart like the one above for the 912, showing 75%, 65%, 55% power and how that can be achieved for a given altitude if a constant speed propeller is used.

Are you sure? - This is a Rotax 912 - there is no direct mechanical linkage to the fuel metering system (as in a LyCon).

 

As I understand it, fuel metering is achieved through influencing the manifold pressure, which is a direct result of rpm.

 

The RPM can be manipulated by the;

• throttle which is connected to the "butterfly valve ",No 3 in above diagram
• propeller pitch

😈

Edited February 13 by skippydiesel

[facthunter]

The performance chart above has all the information you need to know about the effect of varying RPMs.   Nev

[skippydiesel]

Brains Trust stumped?😈

[Thruster88]

10 minutes ago, skippydiesel said:

Brains Trust stumped?😈

No however without a performance chart like the one I posted above for the carburetor 180hp lycoming it is not possible to give advice how you should operate your aircraft with its constant speed? Or in-flight adjustable? Propeller at any given altitude. If you have an accurate fuel flow that could guide you while avoiding any low RPM/high manifold pressure limits that Rotax has published. 

[Area-51]

You can use Pipistrel Vario Prop performance charts from either Virus or Sinus POH; plenty of relevant information in there.

 

Long story short, you acquire your set engine rpm then coursen the pitch to your set MAP, and then make fine adjustments for desired cruise speed. In general 26 MAP at 4800rpm will be most economical cruise for a 912 up to critical height for that MAP.

 

When you adjust back to fine pitch you adjust your rpm down first, then pitch fine to MAP, to avoid over speeding the engine... Going to a course pitch its the opposite, pitch course, rpm increase to MAP

 

Cruise 25-28 MAP

Downwind 23 MAP

Final 18 MAP

 

With all those flight modes and MAP settings you can set 5000rpm as your target for all of them; the CSU governor will hold 5000rpm

Edited February 14 by Area-51

[Area-51]

Pipistrel Sinus POH.pdf

[Area-51]

Sorry threw up wrong file... see attached SW121 POH

 

SW121 POH.pdf

[Thruster88]

I looked up the pipistrel virus 121, nice performance chart in section 5.9. Ignore the KTAS, the percentage of power and fuel flow at 2,4,6,8,10,12 thousand feet is what you need. Make your own palm sized card with that data for reference while flying at altitude. 

 

Interesting how the 55% power fuel flow increases fairly dramatically as altitude increases. Carb not totally compensating, rich mixture? And engine speed (increased friction losses) to get required power at altitude.  

[Area-51]

Pipistrel Sinus - Vario Prop POH

 

PIPISTREL SINUS POH.pdf

[Area-51]

3 minutes ago, Thruster88 said:

I looked up the pipistrel virus 121, nice performance chart in section 5.9. Ignore the KTAS, the percentage of power and fuel flow at 2,4,6,8,10,12 thousand feet is what you need. Make your own palm sized card with that data for reference while flying at altitude. 

 

Interesting how the 55% power fuel flow increases fairly dramatically as altitude increases. Carb not totally compensating, rich mixture? And engine speed (increased friction losses) to get required power at altitude.  

Bing64 runs richer as pressure altitude increases; at 4500' and up its running at about 12.3 Stoich

[Thruster88]

Would be interesting to see how the 912iS performs at altitude. What aircraft has that engine with a constant speed prop?

[Area-51]

2 minutes ago, Thruster88 said:

Would be interesting to see how the 912iS performs at altitude. What aircraft has that engine with a constant speed prop?

The iS maintains around 14 Stoich; this is where they achieve the 6% fuel burn reduction across the board over the ULS, its also monitoring and adjusting on a per cylinder basis.

Edited February 14 by Area-51

[skippydiesel]

13 hours ago, Thruster88 said:

No however without a performance chart like the one I posted above for the carburetor 180hp lycoming it is not possible to give advice how you should operate your aircraft with its constant speed? Or in-flight adjustable? Propeller at any given altitude. If you have an accurate fuel flow that could guide you while avoiding any low RPM/high manifold pressure limits that Rotax has published. 

Not asking for advice on how to operate prop/engine at altitude - asking for opinions on the effect/influence of prop, as governor, on fuel flow in a Rotax/Bing carburettor😈

[pluessy]

the fuel flow is controlled by that little lever near your hand. The prop is only trying to resist your command.

The Bing is no different to any other carburetor other than having a variable venturi instead of a fixed one. The throttle plate (butterfly) controls the air volume and the piston moves to vary to venturi to add the correct amount of fuel.

A fixed carburetor has a number of passages and jets (air bleed, emulsion tube, enrichment etc) to do what the piston does in the Bing.