K-Factor Fuel Flow

[JimJab]

Can anyone tell me what the k-Factor number for the fuel flow sender means.

Is the K-Factor a number of pulse equal to a known amount of fuel going through the meter?
I see that different flow meters have different K-Factors so this makes me think the K-factor is a standardized meaning or value.

I put an oscilloscope on a plastic flow meter to have a look to see how clean the pulse signal was with the Jabiru 3300 mechanical fuel pump working and this pic was taken with the engine at around 1700 rpm and the pulse frequency was 19 Hz.

I think the scope signal looks good, but some one with more experience may like to comment.


Anyone who can help explain what the relationship is between the fuel flow amount and the K-Factor, I would really appreciate it.

Cheers
JimJab

Plastic Flow Meter Pules on a Jab 3300 Engine running

Scope Fuel Flow with EngineR3.jpg (50.54 KiB) Viewed 6859 times

[JimJab]

Hi All,

I asked the question about the K-factor in my above post and I had not googled K-Factor, so I did this morning and it bounced back with many pages, but it was the K-Factor in aviation that caught my eye, I found this article and it answered many of my questions.

I have put this screen shot up of the article I found, but you will need to enlarge (zoom) your browser to read the article or click on the link if it's still working at the time you happen to reading this post.

https://jlcinternational.com/liquid-tur ... explained/

K-Factor ExplainedV1.png (137.59 KiB) Viewed 6852 times

This is a screen shot of a page from either the Fuel Sensor manual or installation manual for the MGL plastic Flow Sensor.

K-Factor for Plastic Flow sender

K- Factor Fuel for Flow setting.png (225.95 KiB) Viewed 6852 times

Cheers
JimJab


Here is another scope picture of the Fuel Flow Pulse when we just had the electric pump going and collecting the fuel to check LPM.

Scope Pulse with Electric Fuel pump, fuel into Bucket.

Scope Fuel Flow No Engine Electric Fuel Pump OnlyR1.jpg (133.23 KiB) Viewed 6852 times

[rainier]

Not really that happy with the slow rising edge of the signal you are showing.

K-factor in our case is simply the number of pulses per liter of fluid passing through the sender.
There are three pulses for every full rotation of the internal impeller.

Getting really accurate flow numbers is a bit of an art - fluid can behave in odd ways in particular if you add a pulsed pressure pump and possibly flexible fuel lines (which act as balloons - expanding and contracting with pressure fluctuations). Viscosity of the liquid plays a role as well (but once calibrated that should remain constant).
A customer recently wrote how he solved unreliable flow readings on a Rotax 912 - he simply inserted a fuel primer bulb bought at a boat shop between the fuel pump and the sender (mounted downstream from the pump just before the splitter to the tank return). The bulb acts as perfect flow damper. That is a good idea.

Another "gotcha" - be aware of installing any flow sender on the suction side of a fuel pump (does not matter what kind or make of flow sender). Orifices in the sender coupled with low pressure can lead to instant fuel vapor bubbles forming. That usually results in gross over reading of the flow (the meters measure volume). This does not happen if you install on the pressure side of the pump.

[JimJab]

Not really that happy with the slow rising edge of the signal you are showing.

Rainier thanks for the reply, what do you think about this pulse pictured below.

This fuel flow sensor pulse was taken with the engine being ground run and after I found the problem of the fuel tank level sensors being incorrectly earthed back to the instrument panel negative bus.

Once I changed these wires to earth back to the RDAC negative bus point, my EFIS fuel tank gauges actually worked as they should and my oil pressure came back to a more realistic oil pressure,(from 75-80 psi to 45-55 psi). This has been a slow trip for me with my secondhand aircraft, but I now feel I am getting on top of the EFIS issues having found this problem.

So I am now getting this type of pulse signal from my fuel flow sensor, I am not expert with a scope, but I am thinking this pulse signal is more like what you would like to see coming from your fuel flow sensor?

Also the measured pulse frequency was 19.1Hz, so I will do some math's to see what type of fuel flow this should work out to be. But, before I get to far into this I better check what jet size is actually installed in my fuel flow sensor and to make sure it's correct.

Fuel Sensor Pulse Generated with Engine Running

Scope Fuel Flow with Engine RunningR3.jpg (50.54 KiB) Viewed 6839 times

Getting really accurate flow numbers is a bit of an art - fluid can behave in odd ways in particular if you add a pulsed pressure pump and possibly flexible fuel lines (which act as balloons - expanding and contracting with pressure fluctuations).

An art, yep I think I understand what you are saying Rainier, every installation could be slightly different with it's pulse waves being generated, so they could be cancelling out or causing problems depending on fuel pipe lengths and wave frequency being caused by the fuel pumps etc.

Would it be possible to see any of these added pulses or fluctuation on the scope, I am thinking this type of thing would show up as random changes in the pulse period, -width or -duty times, or if really bad, just an ugly random pulse rhythm?


Cheers
JimJab

[rainier]

Yes the rise times in that pic are much better.

The main problem with flexible fuel lines and pulsed pressure pumps is much more simple:

The pump forces fuel through the sender into flexible tubes downstream - typically not all of that goes to its destination so on the pumps downstroke some fuel simply flows back through the sender in the opposite direction. The sender cannot "subtact" fuel flow. It just causes instability that is typically highly dependent on RPM.

[JimJab]

Thanks Rainier,

That's great to hear we are heading in the right direction with this pulse wave with the engine running.

So the next step is to see how changing the fuel sensor position may improve the K-factor number. I have added the pic just in case someone might see an issue. Yes, the fuel sensor is sitting lower at the output than the input, so that needs to change and I will look at putting an inline fuel bulb between the electric fuel pump, as it is about about 10 cm further down the fuel line from the edge of the photo.

It's about 2.5 meters from the fuel flow sensor to the engines mechanical fuel pump. So I was originally thinking that distance would far enough away not to have an effect on the Fuel flow meter pulses, but it sounds like it may not be.

Any thoughts would appreciated, I am shooting for the best result I can get?

Cheers
JimJab

Fuel Flow SensorR3.jpg (189.87 KiB) Viewed 6827 times

[rainier]

That is a good orientation but I would raise the sender outlet a bit to make sure no vapor can get trapped.

This orientation is great as it results in the sapphire spindle resting in one of the sapphire cups on its tip - resulting in an extremely small contact area and near zero friction. In this mode it can go down to really low flow values accurately.

Generally we have little issues with these flow senders in Jabiru aircraft - with the exception of early models using wing tanks made here in South Africa - in those days some form of white waxy chemical was leached out of the new tanks for a while which jammed up the sapphire bearings solid. It's easy to clean out and this tended to go away after two or three fills of the tank.
I have not heard of this however since those days - about 15 years ago or a little longer even.

Our biggest issue is the 912 engine with the Italian fuel pump - I have a 912 myself with that pump - I fitted the original Pierburg pump which works well with the sender but the Italian job just does not. I have got a report from somebody that fitted a simple fuel primer bulb between the Italian pump and the return splitter that sorted it out.

[JimJab]

Thanks Rainier for your post about the position of the Fuel Flow Sensor in our Jab 430.

I will change longitudinal angle to sit higher at the out flow, I may print a up a flow sensor block to hold the sensor in the right horizontal and longitudinal position. After another look at my photo, maybe it would be easier to print up a simple raiser block to lift the black clip up so the outflow hose sits high coming from the Fuel Flow Sensor.

Thanks for the confirmation Rainier, it does really help to know when the install you have is on the right track.

Cheers
JimJab

[jwaldrip]

That was interesting about the flow transducer not being on the suction side of the pump. I just read the installation instructions for My Mizer fuel flow and it said to put it on the suction side. But I don't think it is. I bought the plane used. I'm upgrading and wanting to keep the transducer and use it with the RDACXF and the Challenger Efis. It was working before I took the panel apart, so I know it works. Just have to make sure it will work with the EFIS.

[rainier]

flow senders installed on the inlet side of a pump usually not a good idea (OK if you have wing tanks as then the fuel here is under some pressure if the flow sender is installed lowisch in relation).
You can end up with instant carburation problems in the flow sender leading to vapor bubbles in the fuel. Usually not good to have that going on.

Much better to have these senders installed on the pressurized side - no chance of any vapor forming unless your sender is really restrictive with respect to the flow rate.