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humptybump

Well Known Member
I have my fuel flow transducer K-factor set for my EMS so it is close but reports a little high. On a recent set of trips I noticed I started out with pretty accurate reporting and then it started reported much higher (more than 15% off).

I have concluded the fuel flow reports differently if I have the tanks full of 89 zero-e vs 100LL.

I am assuming this is to be expected.
  • Do others experience this same issue?
  • Where can I find the technical data for why this occurs?
  • Is there is a formula to predict it?
 
Probably the Flow Meter

Glen,

I would look to the transducer for the variance, not the fuel. Here is why:

The fuel flow meter is almost certainly a positive displacement meter, which means that a change in reading of 15% from one fuel to another would actually require a density change that is proportional. When you consider AvGas has a density of around 6.0 lbs/gal, then you would expect to see the density increase to 6.9 lbs/gal with a 15% increase in reported fuel flow.

The density of Jet A is 6.7 lbs/gal (12% greater than AvGas) and the density of diesel fuel is around 7.1 lbs/gal (18% greater than AvGas).

This kind of change in fuel composition would almost definitely result in very pronounced changes in engine performance.

Based on this reasoning, I would put my money on the transducer as the root problem with your variation in readings.

All the Best,

Clayton
 
You need to know the calorific value or BTU's per pound and the density of the fuel.

Then work out how many BTU's per litre/gallon you are getting.

As an example the new G100UL fuel that will be available soon has a lesser BTU/lb score but it is much denser, so the end result is you get about 2% more BTU/L or Gallon.

15% sounds too extreme to me. I could see a possible 5-7% or even 10-11% if you had fuels at the extremes but not 15%. And that would be the extremes.

To give you an idea in the history of Gasoline the BTU/lb has only varied by +/-2% for all sorts of blends, so that could only account for 4% and density about 7%.

So is it possible perhaps but not likely.

Here is a list of things to answer and I might have a guess?and a WAG at that.

1. Supplier and grade of AVGAS
2. Supplier of the 89 mogas
3. Were you flying similar speeds and similar OAT/Density Heights
4. Which fuel read what flow rates
5. where is the fuel flow sensor mounted in the fuel system

Any additional info that you might think is useful.
 
All good questions (and further proof I didn't explain my question well).

The actual fuel burn between the extremes of fuels is very close.

I burn about 7.2 when I fly cross country and I pick 2450 rpm and get about 140 kts. The O320 is carbureted and I lean until the engine runs rough and then enriched a bit.

I get the 89 MoGas at one local station. When I travel I usually get 100LL but occasionally find 93 zero-e. When I fill up the tanks, the calculated fuel flow is very close, regardless of what fuel I burned.

What I meant to ask is "why does my fuel flow 'report' higher fuel burn with 100LL vs 89 MoGas even through the actual fuel burn is very close once I fill up?"
 
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Is the red cube or other transducer?

Is it located downstream of the mechanical pump or other?

If the transducer is upstream of the mech pump, you could, possibly, maybe, be getting slight "boiling" of the fuel that results in the higher indication. If it is not that, then I don't know. It certainly is not a true density issue as confirmed by your fuel refill experience.

BTW, even the red cube is a pelton wheel and not a true positive displacement instrument.

Kudos for paying attention and getting to the cause, this is where real learning will occur.
 
I think Bill is onto the same thing, the location of the fuel flow sensor might matter. Refer my post above.

The RVP of mogas is higher than avgas, and quite a bit higher in winter, which it is for you guys right now.

I have this debate with people all the time about running mogas and RVP and many claim they never have a problem etc etc??.truth is they do not realise they do and while it may not be significant most of the time one day it will be.

So where is the sensor?
 
humptybump said:
What I meant to ask is "why does my fuel flow 'report' higher fuel burn with 100LL vs 89 MoGas even through the actual fuel burn is very close once I fill up?"
Click to expand...

Are you saying that despite the higher fuel flow indication on 100LL, when you gas up the actual fuel required still matches the prediction based on the fuel remaining calculated by the fuel flow system?

If the fuel required to fill the tanks matches the values shown by the fuel flow system, that suggests the fuel flows must be accurate. The question becomes "why is my cruise fuel flow higher when burning 100LL as compared to that when burning Mogas?"

Do you get the same speed when burning 100LL as when burning Mogas?
 
Hi Kevin etal. An example may help ...

Sample test case: fly for 3 hours with the same leaning technique, same RPMs, and same IAS & TAS (within the margin of error)
If I am flying with my local supplied 89, and I fly for 3 hours, the flow will report about 7.5 and fuel totalizer will report something close to 22 gallons.

If I have fueled up with 100LL and fly for 3 hours, the fuel totalizer will report something close to 24.5.

However, in both cases, when I go to fuel up, I will be full at about 21.5 gallons.​

So, in both cases, the fuel flow and fuel totalizer agree. In both cases, the actual fuel used is the same. However, in the case of 100LL, it is reporting notably higher than actual, where as with 89 mogas, it is reporting very close to actual (because I have calibrated the k-factor using the 89 mogas)..

The transducer is the FloScan 201 and installed between the mechanical pump and the carburetor.

I went back and looked at my data and the above has been true for two different "fuel computers", using the same transducer. I originally was operating with a FloScan 450 and the 201 transducer. When I switched to the MGL Mini EFIS, I kept the transducer. This tells me the transducer is spinning faster with 100LL that it does with the 89 MoGas.
 
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According to Flowscan this is how the device works.

Series 200 Flow Sensor Specifications

Description:
Series 200 Turbine Flow Transducers measure flows of hydrocarbon fuels such as gasoline, kerosene, and #2 diesel fuel and other light transmitting, non-corrosive liquids of similar viscosity. Typical fuel flow applications include aircraft fuel monitoring systems; gasoline, diesel, and gas turbine engine test stands; and industrial furnaces.

The transducers give repeatable signals on gasoline across a 100 to 1 flow range down to 0.3 GPH. The higher viscosity of diesel fuel reduces signal repeatability at flow rates below 2 GPH. Pressure drops are very low compared to other turbine flow transducers. The transducer bearing system is rated for continuous operation at the upper end of the flow range.

The transducers produce a current pulse signal from an opto-electronic pickup with a preamplifier.

Principal of Operation:
Liquid enters the flow chamber tangentially, follows a helical flow path, and exits vertically, thereby venting any entrained vapor bubbles. The rotational velocity of the liquid is directly proportional to flow rate. A neutrally buoyant rotor spins with the liquid between V-jewel bearings. Rotor movement is sensed when notches in the rotor interrupt an infrared light beam between an LED and phototransistor.

The vapor venting design requires that the transducer be positioned with the electrical connectors pointing up. Turbulence caused by valves or sharp elbows mounted close to the transducer inlet can affect transducer K-Factor and should be minimized.


I do not believe the type of fuel going through the device matters, the flow rate should be the same.
 
Flowscan and FC-10

Just one data point, mine is plumed between the mechanical pump and the fuel servo, it doesn?t care weather I burn pure 92 or 100LL, no difference.
 
Just a shot, more vapour bubbles are likely with hot fuel in the post pump to carb and under reading?

Can you mount another one inside the cabin / tunnel or somewhere cool and try it again?

Despite what people think, RVP of mogas is a fair bit higher than avgas. The one good thing you have going for you is that the sender is after the pump.

Puzzling but I am sure there is an answer. :cool:
 
Glen, contact the Floscan folks and ask. Chances are they know why you're seeing what you see.

Remember, those of us with the cube mounted between the electric pump and the engine driven pump routinely see an increased fuel flow indication when the electric pump is running. Builders with the cube between the fuel servo and the spider do not see a changed indication under the same circumstances.

The only real difference is fluid pressure change in the first case and no change in the second case...the servo regulates pressure.

I seem to recall something about a small passage and bubbles? A pressure change would change bubble formation.
 
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