Fuel burn

[gbigs]

Remember, there is a difference between carb'd 912ULS and the fuel injected quasi-FADEC controlled 912iS.  Fuel and air are managed by software, not a needle in a venturi tube on a carb.

[Anticept]

After a certain point of throttle, the carbeurator piston fully opens, and the mixture is only limited by the main jet orifice size. There's no circuit switching or anything. The video that FastEddieB linked that I had found a while ago is accurate except for where the vacuum port is (located on the piston, not near the throttle valve).

 

CharlieTango: I mentioned last year that what might help you power wise is switching from needle position 3 to needle position 2 (per eric tucker). However, as it was noted, you're restricted to high altitude operation at that point because it would be far too lean at low altitude.

[Ed Cesnalis]

After a certain point of throttle, the carbeurator piston fully opens, and the mixture is only limited by the main jet orifice size. There's no circuit switching or anything.

 

CharlieTango: I mentioned last year that what might help you power wise is switching from needle position 3 to needle position 2. However, as it was noted, you're restricted to high altitude operation at that point because it would be far too lean at low altitude.

 

That is the termonolgy used by Bing.  After idle and before needle is fully open is the needle jet circuit and above 92% when needle is fully open is main jet circuit.

[Anticept]

That's their perogative. I find that their choice of langage makes it sound like there's some sort of mechanical linkage that does it, or there's a separate pathway.

[FastEddieB]

Really, the needle is just raised out of the way, allowing the main jet to flow at maximum rate, right? Same circuit, so the BING language is a bit misleading.

 

Hence, how would any leaning be occurring?

[FlyingMonkey]

 

Not so.  When you open the throttle, you allow more air in. 

 

At idle, the engine is virtually strangled – there’s a very small hole for air to get in when the throttle is closed - that’s why inlet manifold vacuum is at its highest when the engine is idling. 

 

Opening the throttle makes the hole bigger, so more air rushes in, (manifold pressure increases) and more fuel is sucked up into the venturi.  The cylinders receive more air & fuel, producing a bigger explosion and more power.

 

 

I was thinking that valves open, and air at atmospheric pressure rushes in.  I knew, but forgot, about the intake and heads efficiency.  So when carbs are not fully open, as you say the engine is "strangled".  The fuel/air mix is the same, but at less density since not much mixture gets into the cylinder before the valves close.  So at WOT you get the maximum flow rate that the system is capable of, and thus maximize the amount of combustion mix.  Simple!

 

Thanks for saying it in a way that made it clear and jogged my memory for this stuff!

[FlyingMonkey]

Thanks guys, I finally got it.  IrishAl locked it in and the rest of you confirmed it,

 

Next question:  Given all these variables, why do the Rotax power tables list fuel burns at a given RPM?  Are they assuming WOT at sea level, or something else?

[FastEddieB]

In the fuel injected/variable mixture world, one acronym in WOTLOP - Wide Open Throttle Lean Of Peak.

 

In that world, a partially closed throttle is likened to a dirty air filter, in that they both restrict air flow.

 

So, if a pilot wants to set 65% power, let's say, the most efficient way is to be WOT but lean to the fuel flow that results in 65% power. That's more efficient than just pulling back the throttle, sticking a plate in the way of the incoming air.

 

Not relevant to much, but just for your edification.

[FastEddieB]

BTW, I've been futzing with these BINGS since my 1986 BMW R80RT. That's nearly 30 years and I STILL don't quite have them figured out.

 

Different bike in the early 1990's:

 

 

If that makes you feel any better!

[Anticept]

All airplanes that I know of have fuel consumption tables. They are approximate and are very useful for diagnostics and trip planning. It's not going to be perfect, but reliable enough to make it close.

 

We're supposed to monitor fuel gauges anyways. If you spring a leak, it doesn't matter how much fuel that engine is consuming if the fuel is spilling on the belly!

[FlyingMonkey]

All airplanes that I know of have fuel consumption tables. They are approximate and are very useful for diagnostics and trip planning. It's not going to be perfect, but reliable enough to make it close.

 

We're supposed to monitor fuel gauges anyways. If you spring a leak, it doesn't matter how much fuel that engine is consuming if the fuel is spilling on the belly!

Tru dat!

[Anticept]

BTW, I've been futzing with these BINGS since my 1986 BMW R80RT. That's nearly 30 years and I STILL don't quite have them figured out.

 

Different bike in the early 1990's:

 

 

If that makes you feel any better!

 

I need to fly to Georgia. You look like one coo sum'bitch and would be a hoot to drink with.

[Doug G.]

I'm sure you would be Ed-ified! Our maybe Eddie-ified.????

[IrishAl]

Next question:  Given all these variables, why do the Rotax power tables list fuel burns at a given RPM?  Are they assuming WOT at sea level, or something else?

 

The reason they can quote fuel burn at a given RPM is because 'all these variables' are not present with a fixed pitch prop flying straight and level.

 

Imagine a car driving down a long hill at 70mph & 2000rpm - you'd be very light on the throttle because the engine load is light, and you'd be doing 80mpg.  Up the same hill at 70 & 2000rpm you'd be hard on the throttle and doing 20 mpg because load is heavy.

 

But when an engine is driving a fixed pitch prop, the load doesn't vary - there's a fixed ratio between prop and crank and the prop 'bites' the same amount of air once it gets up to speed and is flying S & L.

[Roger Lee]

and this is all done on a bench under their controlled parameters. In the field you have many variables so these charts are just close estimates.

[Ed Cesnalis]

Thanks guys, I finally got it.  IrishAl locked it in and the rest of you confirmed it,

 

Next question:  Given all these variables, why do the Rotax power tables list fuel burns at a given RPM?  Are they assuming WOT at sea level, or something else?

 

The power/torque tables are done WOT at standard conditions,  I think.  They adjust the load to change RPM 

[FlyingMonkey]

The power/torque tables are done WOT at standard conditions,  I think.  They adjust the load to change RPM 

 

Thanks, that makes sense.  I'm just wondering what the applicability is to real-world conditions for most of us.  It seems that there is quite a bit of fuel use variability in the members here in similar cruise conditions, judging by the posts. 

[FastEddieB]

I have found the cruise performance charts of my Sky Arrow to be essentially worthless:

 

 

Also does not indicate much if any leaning occurs, with fuel flow actually going up with altitude!

 

 

This is contrary to any piston aircraft performance I've ever seen. Also the RPM figures are off now that I took some pitch out of my prop.

 

 You might be interested in what a Cirrus SR22 looks like up high:

 

 

Really rather amazing efficiency - I like the 171k at 12.6 gph for example! Or 160 kt at about the same fuel burn as my Tiger going around 30k slower!

[Ed Cesnalis]

Thanks, that makes sense.  I'm just wondering what the applicability is to real-world conditions for most of us.  It seems that there is quite a bit of fuel use variability in the members here in similar cruise conditions, judging by the posts. 

 

As soon as I saw the graph I knew I wanted a flatter pitch and the ability to cruise at 10,000' @ 5,500 RPM.  Best performance is a few thousand feet lower but around here that is often under ground.

 

The chart is useful for determining where best power and best torque are I don't think they are useful for determining fuel burn because they are done at WOT and except for 5,500 - 5,800 you wouldn't want to use a WOT. 

[Roger Lee]

The charts were done with an in flight adjustable prop. They were done with a max rpm setting of 5800. They were one under temperature control. They were done on a test bench under controlled throttle inputs and no aircraft attached and no owner variables thrown in.

Flow charts for compressors and engines are best case scenarios and possible best output, but not usually what you will ever see in the field an individual which will introduce all kinds of variables.

Rotax would prefer to use in flight adjustable props all the time, but that isn't practical or even legal in some countries. 

 

I had to go through all this with Honda and Thomas compressors when I MFG'd dive compressors and I know how Rotax does their testing too. The charts are just good guide lines.

[Ed Cesnalis]

Might be helpful to post the charts, the only ones I have seen were Rotax not Flight Design.

[Doug G.]

Performance manual FD USA.