flying dog Posted January 3, 2012 Posted January 3, 2012 Ok, another question. Though I should probably ask Jabiru....... but here goes: Ok, start up for the Jab' is basically: Master on, fuel pump for a few seconds, then off. Mags on, and start. Take off, is fuel pump on and take off. In flight turn fuel pump off - to help with fuel flow / litres/hour calcs. Granted. But this is where I am stuck: Reading about fuel systems in planes - which may or may not include Jab's - there is the main fuel line to the carb'/injectors, going via an "optional" electric fuel pump. There is also a RETURN line for "excess fuel" to return to the tank. As a worded sequence would go it would read: Fuel tank - fuel pump - fuel flow gauge - carb' -(split to) engine and return line. (or something like that.) Isn't there a fuel PRESSURE gauge in there somewhere? Anyway..... I would have thought it would be "smarter" to have the fuel flow AFTER the return line? Then, when you get in you turn on the master, and turn on the fuel pump. You would see the fuel pressre go up, so you have established the fuel line is ok. Zero fuel flow - or you have a leak. Turn off fuel pump. Start engine. Monitor fuel flow. Turning on the pump would/SHOULD not alter the fuel flow, ONLY the pressure. Again: Am I missing something here? I'm only asking with the hope of learning.
Alan Posted January 8, 2012 Posted January 8, 2012 In a Jab 230 it is: Wing tanks are both connected to the central (header?) tank. The fuel then goes through the electric pump, the mechanical fuel pump to the carb. There is no return to tank fuel. The pressure guage sender is between the mechanical fuel pump and the carb. If you have a fuel flow meter it should be in the line before the electrical pump. Start up is. Electrical pump on for a few seconds then off - Primes fuel system and verifies function of electric pump. Start motor, taxi and do checks on mechanical pump only - verifies function of mechanical pump. Switch on electric pump for take off, landing and anywhere else you want to be doubly certain of fuel feed. Alan
flying dog Posted January 8, 2012 Author Posted January 8, 2012 Thanks. So, riddle me this: (wink wink) The reves on the engine are determined by the ammount of fuel. The ammount of fuel governs the ammount of fuel/hour the engine uses. After I start the engine I get a Litres/hour figure shown. Swithing on the electric fuel pump the number increases BUT the revs don't. There has to be a return line: Otherwise the "extra" fuel flow is not being burnt/combusted/used. Or am I missing something?
Alan Posted January 11, 2012 Posted January 11, 2012 Is the fuel sensor between the pumps or anywhere after the electric pump? The electric pump will put pulses in the fuel pressure that affects the fuel flow sensor rotor. Alan
flying dog Posted January 11, 2012 Author Posted January 11, 2012 Just spoke to Jab (won't say the person's name.... - parly because I've already forgotten it) He offered the possibility that with the electric fuel pump on, the mixture is increased and so the engine would be running with a higher fuel/air mixture. And.... As the butterfly isn't any more opened, the revs won't increase. Makes sense I guess, but he wasn't 100% sure. Anyone with thoughts on that?
Guest davidh10 Posted January 12, 2012 Posted January 12, 2012 If it is a conventional carburettor engine, the fuel is pumped to the carby bowl(s) via a "float valve" in each bowl. Thus the fuel is maintained at a constant level in the bowls and the jets suck it from there. The fuel isn't under pressure into the jets, but rather sucked by the vacuum in the venturi. The only way the fuel level should change in the carby bowls is if fuel is restricted below demand or the float valve is leaking and allowing increased fuel pressure to increase the flow into the bowl. The explanation given by the person you spoke with, seems unlikely, to me. The idea that there is some return line or a leak would seem a better explanation, but having a fuel flow metering sensor before the return relief valve makes no sense except as a design / build mistake.
flying dog Posted January 12, 2012 Author Posted January 12, 2012 David, I agree, but "they" say there is no "return line" from the carb'. I should have mentioned the problem with the explination but I wasn't wanting to put him off-side. He was on the spot and was only sumising - he actually admitted he didn't know and I am kind of respectful of him for admiting that. Not many people can admit they don't know something, especially if it is to do with something they should know about. Anyway, I'm still stuck on what is going on with the extra fuel.
Guest davidh10 Posted January 12, 2012 Posted January 12, 2012 A return line would usually be before the carb and fuel filter, and operate on a pressure relief basis, if it exists. Alan's hypothesis about fuel pulses sounds reasonable if there's no return line or leak. Sorry, as I'm sure you know, I don't know anything about Jabs.
flying dog Posted January 12, 2012 Author Posted January 12, 2012 No worries. I am just asking. Live and learn!
will kirkbride Posted January 12, 2012 Posted January 12, 2012 No worries.I am just asking. Live and learn! Hi there. Just my 2 cents. If you have a carb system, like the Jab, there is no return line. You only need a return line in a fuel injected system because it operates differently and at a higher pressure. The typical carb type set up only uses about 3 to 5 pounds of fuel pressure to the carb or carbs. The fuel is only used as the engine runs through its rev range. At low revs it doesn't need as much as at higher revs/power settings it uses more fuel. Try to think of the carb float bowl like the toilet cistern at home (it is the same principle). When the cistern is full, no water flows into the bowl, even though the water pressure in your plumbing is always there. Release the water and the float drops, the valves opens until the cistern is full again. In your carb/ fuel pump/ set up, once the float bowl in the carb is full, the fuel flow stops until the float drops and allows more in. This happens constantly as the engine runs. The fuel pump is set at a low pressure and once this pressure is reached, the fuel pump is idling (the fuel bypasses internally in the pump until the pressure drops again. When it does, the fuel starts flowing to fill the float bowl again). So the amount of fuel passing through your flow meter should not change as long as there are no leaks etc. The boost pump in this set up is also low pressure, so that it doesn't overcome the float valve pressure setting or the engine driven fuel pump setting. In an injected engine, the fuel is delivered at around 38 to 43 pounds,usually to a fuel rail. The injectors are attatched to this rail and the amount of fuel allowed into the engine is determined by engine load through either a vacumn arrangement on the older systems or by electonics as in the new type of system. The fuel pressure is controlled by a pressure regulator at the end of the fuel rail and the excess fuel is sent back to the fuel tank. This is why the injected type engines MUST have a fuel return line . 1
dazza 38 Posted January 12, 2012 Posted January 12, 2012 Hi there. Just my 2 cents. If you have a carb system, like the Jab, there is no return line. You only need a return line in a fuel injected system because it operates differently and at a higher pressure.The typical carb type set up only uses about 3 to 5 pounds of fuel pressure to the carb or carbs. The fuel is only used as the engine runs through its rev range. At low revs it doesn't need as much as at higher revs/power settings it uses more fuel. Try to think of the carb float bowl like the toilet cistern at home (it is the same principle). When the cistern is full, no water flows into the bowl, even though the water pressure in your plumbing is always there. Release the water and the float drops, the valves opens until the cistern is full again. In your carb/ fuel pump/ set up, once the float bowl in the carb is full, the fuel flow stops until the float drops and allows more in. This happens constantly as the engine runs. The fuel pump is set at a low pressure and once this pressure is reached, the fuel pump is idling (the fuel bypasses internally in the pump until the pressure drops again. When it does, the fuel starts flowing to fill the float bowl again). So the amount of fuel passing through your flow meter should not change as long as there are no leaks etc. The boost pump in this set up is also low pressure, so that it doesn't overcome the float valve pressure setting or the engine driven fuel pump setting. In an injected engine, the fuel is delivered at around 38 to 43 pounds,usually to a fuel rail. The injectors are attatched to this rail and the amount of fuel allowed into the engine is determined by engine load through either a vacumn arrangement on the older systems or by electonics as in the new type of system. The fuel pressure is controlled by a pressure regulator at the end of the fuel rail and the excess fuel is sent back to the fuel tank. This is why the injected type engines MUST have a fuel return line . Hi Will, not 2 cents worth mate.At least $200 bucks worth mate. Good to here from you again,Cheers
bilby54 Posted January 13, 2012 Posted January 13, 2012 I had this same problem with the Aero Club SP480 Jabiru. When the electric pump was running, the fuel flow increased by around 3 litres per hour (I think) but there was no difference to the engine output. The aircraft was run out of fuel and forced landed in a paddock due to the pilot leaving the pump on for the duration of the flight - yes, and not monitoring his fuel load but this was covered in another post some time back. I could never satisfactorily find where the fuel was going with the boost pump on and even bench testing the complete fuel system did not solve the mystery. The underside of the Jab had a very distinct stain and I can only assume that excess fuel was coming from somewhere around the mechanical pump. Jabiru were unable to help with the problem then either.
Alan Posted January 14, 2012 Posted January 14, 2012 My J230 has the symtoms described, but I have the fuel flow sensor installed in the wrong place! (I could not feed the sensors cables down the central duct - try as I might with all sorts of cable threaders). My fuel pump is in the fuel line between the electric pump and the mechanical pump. This is warned against in the fuel computer installation instructions. As a result, when the electric fuel pump is on my fuel flow appears to increase 2-3 litres per hour at idle and 5-6 litres per hour at 2900 rpm. I never noticed any fuel leakage or engine condition change when the electric pump is switched back on. For example after a long flight when the engine is stable when I switch back on the electric pump in preparation landing, the motor note, revs, CHT, and EGT, remain the same no matter what the pump configuration. So I think it is the pulses after the electric fuel pump unevenly driving the sensor rotor speed; the signal processing must not average out this affect leaving the fuel flow/consumption reading erroneously high (safer than an under reading error). I have never left my electric pump on in the cruise long enough to check whether extra fuel is actually being consumed, but I believe it is not. If running both pumps together caused additional fuel usage then either there is a leak in the pipe work or the Carb needle valve is becoming compramised. If anyone can tell me the trick to threading cables down the central spine of my J230c (it has been done on another J230c with no problem), I would be grateful. My J230c came out without the central tank warning light and I would like to fit one. While not trusting the Jab fuel guages I have never had a problem. Pre flight tank dipping, calibating the fuel flow sensor makes me feel reasonably secure, even when I can end up with one tank indicating almost empty and the other 3/4 full. Regards Alan
bilby54 Posted January 14, 2012 Posted January 14, 2012 The Jab referred to above ran out of fuel after 3 hours and 10 minutes which is around 20 litres per hour with the electric pump on. Fuel burn without the pump on was approx 14 lts per hour See the picture for the fuel stain on the underside of the fuselage
flying dog Posted January 15, 2012 Author Posted January 15, 2012 Bilby, Well I have asked, asked and done research. What I have found - and belive to be true: The fuel pump is in the wrong place and it is interfering with the sensor. The sensor SHOULD be as far away from the pump as possible, OR: BEFORE the pump. Leaving the electric fuel pump on does NOT change the fuel consumed by the engine. There is a slot/slit/groove in the pump's piston which allows the fuel to back flow once pressure is reached.
jetjr Posted January 15, 2012 Posted January 15, 2012 There is no return line from the engine in a standard Jabiru setup The pumps - both of them - run against closed head when fuel not being used or not full flow being used. The flow is the same irrespective of what the sensor says, electric pump on or not. The engine driven pumps are highly reliable however IF it does ever fail theres a good chance of an engine flame out under high fuel demand, in order to be sure an additional electric pump is used for critical parts of the flight. It is also possible that the engine mounted pump could fail WITHOUT symptom in flight and when full power demanded engine could starve, ie in an emergency go around My experience with a number of fuel flow meters in all sorts of applications shows them to be highly unreliable, placement and even orientation, let alone vibration and heat all upset them. Pulsation from diaphram pumps being a big problem. Many have very fast rotation impellers which are easily blocked by dirt or air. Id suggest NOT using them for much more than cruise and secondary information only. Certainly not for planning etc. All aircraft are "supposed" to have reliable fuel gauges of some sort and watching flow sensor instead of visual gauges is a easy mistake. Using the electric pump at all stages of flight can reduce the uneven feed from Jab wing tanks, and can help with crazy readings on gauges too. It really shouldnt harm anything leaving on and there are plenty who do. Re fuel starvation and photo, . Id also be very surprised if the pilot couldnt smell fuel with it being that bad. I have had a carb bowl gasket leak and there was a very strong smell especially in turns. 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. Allan - re threading wiring, Jabiru just ran some new wiring through for me in an J200, they must have a method.
Guest Andys@coffs Posted January 16, 2012 Posted January 16, 2012 My setup exactly as per Alan's, my results exactly as per Alan's. with E Pump on gauge shows extra burn but doesnt really (tested at refuel time). I believe stain on bottom of fuse is just overflow oil, most who dont know will fill to 1/2 way on the dipstick and that extra is ditched, usually in the first 5 mins of high speed running, usually into the overflow bottle, or if thats full all over the bottom of the plane. Its been my experience that AVGAS stains to a colour that closely approximates Copper oxide not used oil colour I further believe that it is very easy to run the engine higher than 2850rpms which moves off the cruise part of the needle to the flat out part ehich drives the fuel flow upwards by around 10Lts per hr for not much appreciable increase in power or rpm. This would account for fuel starvation, not 3lts per hour which should be covered by 30min reserve etc. Its a real plane and people have to do fuel management and take it seriously. In my aircraft I believe Jabiru when they say drive it hard and I do so, but I have to ensure Im operating at around the 25 to 27lts per hour range and not 33 to 37lts. Andy
bilby54 Posted January 16, 2012 Posted January 16, 2012 Bilby,Leaving the electric fuel pump on does NOT change the fuel consumed by the engine. Hmm! Lots of contradictory stuff in this post. FD - You are correct in saying that having the electric pump on does not change the fuel used 'by the engine'. Read #12 where I said there was no change to the engine power as that is almost impossible due to the carby float as was well described by Will K. I referred to the fuel flow increasing when the boost pump was on. Interestingly enough, this aircraft was not fitted with a fuel flow indicator at the time of the accident but was fitted after it was repaired. The stain on the underside started very close to the vent line from the mechanical pump and the aircraft got through 65 litres of fuel in exactly 3 hours and 10 mins of engine time. 15 litres per hour is just over 4ml per second so pulsing from the pump is not an issue but high pressure in the delivery line is and is why it is called a 'boost' pump so it is very feasible that the mechanical pump is relieving.When the aircraft was flown with the electric pump in the off position, it would fly safely for almost 4.5 hours. When the engine was checked, it still was almost full of oil. The Flight Manual states that all flights are to be planned at 15 litres per hour so where did the fuel go on that trip?? Why are questions asked in this forum if no one likes to hear the answers? Wouldn't everyone like to know why this happens because what if one day you have a mechanical pump fail so turn on the electric pump and then run out of fuel short of your alternate.
flying dog Posted January 16, 2012 Author Posted January 16, 2012 Bilby, Yes, I am saying that having the electric fuel pump turned on (always) should not/will not change the fuel used by the engine. RE post 12: Can't say. Insufficient details.
Guest rocketdriver Posted January 16, 2012 Posted January 16, 2012 just my 2 cents worth ..... any increase in fuel pressure will richen the mixture somewhat. This is because the float has to push harder to close the valve, which can only happen if the fuel level in the float chamber increases. This effect is clearly outlined in books on tuning carburettors and is why many competition engines (and all fuel injected engines) use a fuel pressure regulator. So, assuming that there is no fuel pressure regulator in the system, if the electric pump causes an increase in fuel pressure at the carby fuel inlet, then expect the mixture to richen up a bit ...... especially if the mechanical pump is a bit down. cheers RD
flying dog Posted January 16, 2012 Author Posted January 16, 2012 Rocketdriver, I can't argue with that, but the "increase in pressure" is another thing. Can anyone say - definitivley - how much more pressure the electric pump put in/on the fuel? A car's electric fuel pump - as an analogy: Most blokes can pi$$ faster/harder than the pump. The needle/seat in the carb should negate the extra pressure - unless we are talking about going from (example) 2 PSI to 20 PSI. It would be foolish to add a fuel pump to the system which (by default) puts too much pressure for the carby to handle. Fuel injection systems as you mentioned are another kettle of fish and do have the extra things, so really theyy aren't applicable to this. Are they? Now to something you posted: This is because the float has to push harder to close the valve, which can only happen if the fuel level in the float chamber increases. Sorry? The float level doesn't change irrispective of the pressure. When the fuel is at "level x" the needle pokes into the seat and bloks the fuel flow. (Again, this is with normal pressures. If you push the pressure into another set of figures, the needle/seat won't work.) So I can't understand what you are wanting to say here. P.S. (via edit). And you mentioned "COMPETITION ENGINES". I agree that they have special tricks they do to optimise their systems. But again: How does this relate to Jab' engines? And how much "richer" will all this make? less than 10%, less than 20%, 30, 40...? A small increase of mixture would be "ok" and this would need quantative testing to see what really happens to fuel consumption. Also EGT's would need to be measured. (And/or CHT)
Guest davidh10 Posted January 17, 2012 Posted January 17, 2012 Rocketdriver,.. Now to something you posted: This is because the float has to push harder to close the valve, which can only happen if the fuel level in the float chamber increases. Sorry? The float level doesn't change irrispective of the pressure. When the fuel is at "level x" the needle pokes into the seat and bloks the fuel flow. (Again, this is with normal pressures. If you push the pressure into another set of figures, the needle/seat won't work.) So I can't understand what you are wanting to say here. ... The float valve is closed by pressure caused by the rising level of the fuel in the float bowl. Once closed, the fuel flow stops and thus the level does not increase, however if we were to add some fuel by another means and raise the level in the float bowl, then the float cannot rise any further, so it floats lower (displaces more fuel volume) and thus applies more pressure to the closed valve. This is the old physics balance arm with weights on both ends, but the upward movement of one end of the arm is limited by closing the valve. If the fuel pressure is increased then it pushes harder on the valve and if hard enough will depress the float and let more fuel in until the level rises to cause the float to displace enough fuel to equalise the sealing and opening pressures of the valve. If it was a rotary valve, like a gas tap, this wouldn't apply. How much increased fuel pressure is required to increase the float chamber level by how much is a function of the mechanical advantage of the float / valve combination. When you put pumps (or any force generator) in series, the end pressure will be the sum of all. It is reasonable to expect that the electric pump would at least supply the same pressure as the mechanical one, so it could suffice alone if the mechanical one failed. Under that scenario, the fuel pressure would have to be at least double with both pumps operating, unless there is some pressure regulator / relief, which I think has been established to be absent. If indeed the substantial extra fuel is due to a richer mixture for the above reason, then one could also postulate that the carburettor was not designed to operate under the pressure range variance that is delivered by having the electric pump in circuit. Perhaps there should be an alarm or flashing light to draw attention to the fact that the electric pump is active, if it is really causing a change in fuel consumption that significantly decreases flying range.
jetjr Posted January 17, 2012 Posted January 17, 2012 If theres a fault in a pump or a hose ie its broken and leaking ........ of course it will see higher fuel flow rates - not much you can do to except high level inspection and maintenance The hose from the mechanical fuel pump is not a pressure bleed and should never bleed off fuel unless somethings wrong Newer LSA versions have a Fibreglass cup and drain under the pump to catch and divert fuel in case of a failure All things running correctly the electric pump should have NIL impact of fuel burn or leakage A fuel flow sensor is unlikely - with any certainty - pick up this sort of slow leak. Id reckon youd be chasing phantom leaks fairly often if you paid that much attention and faith in them. David is correct the pressure of pumps in series is the sum of their outputs but flow is not. In jabirus we are talking just a few PSI so highly unlikely to overpower needle and seat and the carb is designed to handle it. Flow and pressure checks for leaks and hose security are part of 100 (and 50???) hr services. Hose have to be replaced every ?? many years too IF there was extra fuel going into carb, unless the engine could burn it up ie high rpm then it wouldnt run very well
facthunter Posted January 17, 2012 Posted January 17, 2012 I wasn't going to buy into this, but where a pump is driven by a camshaft etc. the constant output pressure is provided by a spring. If you put a lighter one in the delivery pressure will be less. The lever will in normal operation, move the diaphragm against the spring compressing it. If an extra pump is activated in the line beween the tank and the pump there will only be a pressure increase if the second pump is delivering more pressure than the engine driven one would have. That pressure will compress the diaphragm spring and the engine driven pump will not stroke . the fuel just passes through the two one way valves. Two centrifugal pumps will add the two pressures if they are in line, but that is no what we have here. Nev
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