Fuel flow vs pressure. Pumps etc.

[jetjr]

Nev' s right

 

I was thinking other types of pumps

 

Outcome is the same though, flow and pressure should be the same unless its leaking out somewhere.

 

 

[facthunter]

Most more conventional pumps do add, but we have a lot of ones that don't in our aircraft. Over-pressurising the fuel system is not a good idea as the float chamber is more likely to flood. A 5 foot head would be about all that you need. That would be about 2 1/2 psi. (less in fuel than water.). Nev

 

 

[bilby54]

Any more theories??

 

The needle does not shut off in normal operation as it is supplying fuel to the engine.

 

 

[flying dog]

GUYS!

 

Hello!?

 

This is getting a wee bit "off thread".

 

I am asking about Jabirus and the increased fuel flow when the electric fuel pump is turned on.

 

Not other aircraft. Not centrifical pumps. Not fuel injection systems.

 

I appreciate there is a bit of lattitude, but this is just getting WAY OFF the original question.

 

 

[turboplanner]

I also heard there was also a rare problem of where needle and seat couldnt close against pump pressure and was overfilling carb bowl, but only with electric pump running.

This is a common issue with custom or home built cars, boats etc.

 

The float valve in a carburettor opens and closes to meter the amount required by the carburettor.

 

As the valve opens, fuel flows into the bowl.

 

The fuel pump is always designed to exceed the capacity of the main jet(s) so that the float valve never starves the carby (becoming the defacto main jet)

 

This means it oversupplies, so a float is used to shut off the fuel supply at a preset bowl level.

 

As the carburettor sucks fuel out of the bowl the cycle begins again.

 

The fuel pump pressure output is designed to be less than the pressure which the float valve is capable of holding.

 

The fuel pump plays no part in boosting power or increasing fuel consumption (WHEN CORRECTLY SET UP)

 

Turning it on before takeoff and before landing (where takeoff power may be required) elminates the chance of vaporisation on a hot day, and makes up for any unexpected poor performance in the mechanical pump (WHEN CORRECTLY PLUMBED)

 

Sometimes people don't design the system correctly, or substitute a non standard fuel pump. If the pump pressure exceeds the float valve resistance capacity then the float bowl fills, but the float is pushed down by the excessive pressure, opening the valve.

 

There is an overflow hole in the carburettor and fuel flows out through this hole as long as the (incorrect) electric pump stays on.

 

This would account for the smell of petrol (in which case there's a chance of fire), and the stain under the aircraft.

 

This can also happen when a standard electric fuel pump has been fitted if the float valve needle or seat are worn, or jammed by a foreign object. (which is why we turn it off after takeoff)

 

 

[facthunter]

The topics was," fuel flow vs pressure pumps etc", wasn't it? This is a subject where you must get it right. The best way, is to understand the principles that you are dealing with . If someone says" I fixed mine this way", your problem may be quite different, and from another cause.

 

In any case, the thread belongs to the forum when it starts to flow. I haven't seen anything that is off topic.There are plenty of further possibilities if you want to consider them. Nev

 

 

[bilby54]

Hi, sorry for my earlier frustrations but I, like everyone else, am only trying to find out why this happened.

 

I have had practical experience that it does occur and all of the components in the aircraft were the ones supplied by the manufacturer. I remember asking the pilot at the time if he could smell petrol while flying and there was no smell inside when I picked it up from the paddock. I cannot ask him anymore questions as he is no longer with us.

 

It really doesn't matter how the fuel system should work, there maybe an underlying problem that either has not been addressed or admitted by the manufacturer or has not been reported to them.

 

 

[facthunter]

Unfortunately there will always be things that are not sorted out in our branch of aviaion. There are so many variables in the way things are installed and modified.

 

Check .... Needle and seat float tab contacting at the correct angle, no foreign matter.

 

fuel pressure. Check with a gauge.

 

The other thing is check the carburetter is not getting too hot. This used to cause flooding in Mitsubishi Colts. The fuel would effervesce with bubbles forming in the petrol IN THE BOWL, as the more volatile elements would vapourise (boil) and this would reduce the density of the fuel in the bowl, and the float sinks and would not close off the fuel flow and the carb would vent into the throat of the carb because that is where the balance tube goes. The engines would blow copious amounts of black smoke and often stall completely. Note this is not vapour lock where the fuel vapourises in the lines. The carb you have would vent overboard instead. Nev

 

 

[turboplanner]

It really doesn't matter how the fuel system should work, there maybe an underlying problem that either has not been addressed or admitted by the manufacturer or has not been reported to them.

It does matter how the fuel system should work because that's how you can identify the underlying problem, even with manufacturing design faults (one shown by Facthunter above in #33)

 

High pressure doesn't make the electric pump a "boost" pump, it's the extra volume, which helps fill the bowl if the mechanical pump is worn, fails, vaporisation or air locks in the line have formed etc.

 

Having the fuel pump turned on will not change the fuel used by the engine, but other issues will pump fuel out of the AC during flight, including float valves being forced open by excessive pressure, elecrtric pump fuel lines leaking, electric pump filters etc leaking. i.e. anything faulty in the electric pump circuit.

 

Fuel pressure regulators are used to reduce the pump pressure below the float valve closing pressure, usually because someone has fitted a bigger volume pump as you would sometimes do in a race engine. Often there isn't much science in those conversions, they are correcting a bowl overflow problem.

 

The float valve needle DOES shut off in normal operation. It is not supplying fuel to the engine, the Bernoulli effect in the carburettor throttle body is creating suction at a rate varied by the butterfly valve, and through most of the throttle cycles the demand is well below the capacity of the float valve. You could theoretically build an engine where the float valve flow equalled the main jet flow, and THEN the valve would need to stay open, but there's no safety margin with that - if there was a float valve restriction the engine would lean and you'd melt pistons, so float valve capacity is always well above main jet capacity. That's why the operation in the float chamber is a series of opening valve-filling chamber-closing valve-opening valve etc.

 

 

[bilby54]

I gotta tell you TP, I have a lot of respect for your inputs but you lost me on this last one.

 

The average bog standard aircraft has a mechanical fuel pump and quite often an electric (boost, auxillary, optional or call it what you will) pump. We are all good pilots and read the flight manual so take off and land with the electric pump on - no issues so far. In cruise, if the engine shuts down for whatever reason we turn on the electric pump and the engine continues to produce power. We could feasibly conclude that the mechanical pump has failed and that is the extent of identifying the underlying problem so that is why I openly stated that it does not matter how the fuel system works because essentially they all work in very similar ways.

 

I know about floats sinking in boiling fluids within the float chamber and appreciate that it has been aired in this post.

 

As for the needle valve opening being "a series of opening valve-filling chamber-closing valve-opening valve etc" I think is not entirely correct. At very low power it does happen that way but at cruise power it becomes a process control situation and the needle valve throttles to supply the demand from the float chamber level. Fuel is continually entering the chamber to replace that being sucked up the main jet. As I mentioned earlier, 15 litres fuel flow per hour - Jabiru Flight Manual - is 4 millilitres per second and as the float falls, fuel starts to flow and will maintain the valve opening to provide a constant level in the float chamber. This can be seen from any breed of fuel flow indicator fitted in the aircraft and seems to have started this whole session off.

 

I also stated that the fuel flow indicator can indicate an increase in fuel flow but not necessarily into the engine - I think that you and other mentioned exactly the same thing. The carby on Jabiru engines is located below the engine so any excess fuel will tend not flow down the throat but would more likely over flow the float bowl as someone else mentioned. I could accept this as being a reasonable cause of the accident that I had to clean up.

 

 

[turboplanner]

I gotta tell you TP, I have a lot of respect for your inputs but you lost me on this last one.The average bog standard aircraft has a mechanical fuel pump and quite often an electric (boost, auxillary, optional or call it what you will) pump. We are all good pilots and read the flight manual so take off and land with the electric pump on - no issues so far. In cruise, if the engine shuts down for whatever reason we turn on the electric pump and the engine continues to produce power. We could feasibly conclude that the mechanical pump has failed and that is the extent of identifying the underlying problem so that is why I openly stated that it does not matter how the fuel system works because essentially they all work in very similar ways.

 

I know about floats sinking in boiling fluids within the float chamber and appreciate that it has been aired in this post.

 

As for the needle valve opening being "a series of opening valve-filling chamber-closing valve-opening valve etc" I think is not entirely correct. At very low power it does happen that way but at cruise power it becomes a process control situation and the needle valve throttles to supply the demand from the float chamber level. Fuel is continually entering the chamber to replace that being sucked up the main jet. As I mentioned earlier, 15 litres fuel flow per hour - Jabiru Flight Manual - is 4 millilitres per second and as the float falls, fuel starts to flow and will maintain the valve opening to provide a constant level in the float chamber. This can be seen from any breed of fuel flow indicator fitted in the aircraft and seems to have started this whole session off.

 

I also stated that the fuel flow indicator can indicate an increase in fuel flow but not necessarily into the engine - I think that you and other mentioned exactly the same thing. The carby on Jabiru engines is located below the engine so any excess fuel will tend not flow down the throat but would more likely over flow the float bowl as someone else mentioned. I could accept this as being a reasonable cause of the accident that I had to clean up.

Para 2

 

I was skimming information from several posts.

 

You're quite right as a means of identifying an engine failure. Sorry, I thought you were referring to what we were talking about on float valves etc.

 

Para 4

 

You could design the float valve system to do what you describe - stay open and flow the same rate - so with a wide open throttle the float valve orifice would be the same as the main jet. However if there was bouncing, tilting, a grass seed, any small restriction the float valve would restrict the flow and lean the engine with potential piston damage, so in a good system the float valve always has a big margin over the main jet(s), so it overruns the demand and has to close, if only momentarily.

 

The fuel flow meter is measuring the net fuel flowing down the pipeline over a given time. I wouldn't expect it to show the float valve action.

 

Para 5

 

I think that's the most likely scenario. usually the bowl has its own overflow hole, sometimes with a nozzle and tube attached, and yes the fuel flow meter would indicate an increased flow.

 

 

[turboplanner]

Memory is slowly coming back.

 

To give you an idea how much spare capacity float jets can have, when I converted Mikunis to run on methanol I bored out the main jet to approx double the diameter for petrol, but never had to increase float jet size.

 

I made up a test bowl by tapping an elbow into the bottom and bringing a clear plastic pipe up the side to see how constant the float level was.

 

At idle it would be pretty much at float level.

 

At wide open throttle acceleration it would half drain the bowl and then with the float valve fully open quickly fill back up to float level with the engine sitting on 10,000 rpm for two or three seconds.

 

 

[Yenn]

Just a question. How does the electric pump work. I have one in my plane but have never tried to see how it works. The old pommie A.C. pumps were similar to the ordinary engine pump, using a spring to control pressure, but I am not sure how our aircraft pumps work.

 

 

[turboplanner]

Touch your scrotum with a cattle prod - much the same principle.

 

 

[Guest Andys@coffs]

Touch your scrotum with a cattle prod - much the same principle.

Ok.....see you learn something every day on this site, I didnt know that an electric pump had as major subcomponents a cattle prod and a scrotum. There must be a fair bit of soundproofing as well, they are noisy buggers when on, but nothing approximating a police car sirren that I would have expected. How many litres per scrotum? Scrotums do seem to have a reasonable life on them generally living as long as there owner, but im not sure I 've seen one rated for continuous cattle proding.....perhaps something more likely seen at the blue oyster bar.......

 

 

[turboplanner]

Yenn there are quite a few different types these days. Get the make/model from your aircraft and google it. I'm amazed at what comes up on google searches including excellent schematics. Some people really get interested in the mechanics of their equipment.

 

 

[flying dog]

Yenn,

 

There are different types/styles of pumps.

 

There are the centrifical ones, and then there are the ones like the one discribed as the "engine pump" where a cam lifts a diaphram so suck in fuel, and then the cam "retracts".

 

There is a spring which is then used to return the diaphram to the normal position and so expelling the fuel at a given pressure.

 

And then there are other types as well.

 

I'm not into that so I shall stop there.

 

 

[jetjr]

Im fairly sure a wing tanked Jabiru with BOTH pumps off will still run OK, gravity from wing tanks is enough to keep ordinary fuel flows up.

 

If an engine driven pump were to fail in flight, you may not see symptoms until full power applied and even then a few seconds later after it empties the bowl, hence the use of additional pump for TO and landing.

 

Jabs use an electric solid state Facet or similar style pump as the boost pump. They use switching of a coil to move plunger/diaphram back and forth, they are also self regulating, bypassing excess pressure back to suction side I think.

 

I also have a fuel pressure sender before the carb and it doesnt change with boost pump on or off - flow does however (according to the flow meter) which I know is not the actual case.