Vapour Lock?

[Alan]

Jab 230. Engine Gen 3 with all latest mods (18 hours since overhaul). Running on BP95 Auto Gas (Avgas is now too hard to get) . CHTs pretty even and don't exceed 170°C.  No previous problems experienced. 

 

Until a few days ago - Air temp 35°C, all checks normal, put out taxing call and held short of runway while student in 150 landed who then came in off the runway onto the taxiway and turned back towards the runway in front of me.  Then I waited and waited while monitoring CHTs. Eventually the instructor got out, gave me a wave then leaned in to the door for further chat.  OK so a student doing a solo.

 

What this meant was I was idling for about 15 to 20 mins total, but the CHTs all looked OK.  The student cleared and I back tracked and took off.  Just after rotation (app. 150-200ft) my engine started running really roughly, revs dropping and loosing height. I was preparing for a cane field arrival when sufficient power came back to hold altitude, not too turn back, but I had more options.  After gaining circuit height I prepared for a normal landing, but with a now normal engine behaviour, I gained further height but remained local within gliding distance of the airport. 

 

Plugs, leads, filters checked. Fuel checked for water and any solid residues after evaporation and then donated to the lawnmower.  The engine has behaved normally for about 5 flying hours since.  

 

Lesson learned, I was not far from flattening the sugar cane. I believe the rough running was a vapour lock caused by the long waiting time and high ambient.  I had the electric fuel pump selected and could hear it running. 

However, I was thinking it would be pretty simple to set up a temperature monitoring of the firewall forward fuel lines, possibly on the inlet to the mechanical pump or inlet of the carb, but what temperatures would be regarded as critical?  I'll take some of the fuel into the lab and look at its boiling point but that would not test actual vapour pressure nor allow for the 4psi of the fuel system. 

 

Any thoughts out there?  

[Kenlsa]

Running when only on idle revs for a long period can make the plugs very sooty. That’s one of the potential problems with mogas. Our entire club Jab fleet, and many Jabs here run on mogas all the time, but it is start, taxi and go. Don’t have any trouble.
General practice is not to spend anymore time than needed at idle, and that’s on any plane and fuel combination.

 

you MAY have had a temp soak on the fuel line due to no air (120kts) cooling the crank area….remember that the muffler is right there….this is facilitated by those 2 x 25mm holes at the front of the air shrouds.

 

Bottom line…….if you are going to sit there…..switch off

 

Ken

[facthunter]

I think I would have shut down the engine in those conditions if you were sure you would still have an operating radio. You may have been slightly down wind at the time also and there's the issue of holding on the brakes for the time and who's Paying. Idling is not enough rpm to distribute oil properly in the engine, either.. Keeping the electric pump on is a consideration also. It may rely on fuel flow to cool itself..  Nev

[IBob]

Does the Jab have a return line bleeding fuel back from the engine compartment to one of the tanks?

[Alan]

I had no idea how long the student would themselves be idling at the threshold while being instructed, they knew I was there when they first left the runway (airmanship?);  had i known how long I would have shut down .  Idle was low enough not to require brakes.  I was paying - my plane.  I must admit I was not aware of additional sooting of plugs on mogas; not heard of this before.  Plugs were all perfect when I checked afterwards, but that does not mean much after further running.

 

No the jab is standard and has no fuel return line.

[Old Koreelah]

1 hour ago, facthunter said:

Idling is not enough rpm to distribute oil properly in the engine…

Some recommend warming up a Jab at 1000rpm.
Nev do you know at what minimum revs a Jab oil pump works properly?

 

[facthunter]

They work OK and provide oil to galleries and bearings but don't fling oil to pistons and camshafts adequately. Idle speed is not for extended periods on anything. 10 Psi will provide oil to bearings at lower speeds and loads. At higher speeds and loads the bearings need more oil for heat removal.   Insulation lagged fuel pipes help but don't stop all heat  from getting to the fuel. You best course of action is to face into wind and run a few more revs and I'd research the fuel pump cooling. These principles apply to most piston engines fitted to aircraft. Modern cars bring on a fan when in traffic. Nev

[kgwilson]

Vapour lock will only occur when the engine is sucking the fuel. In a Jab the fuel is gravity fed from the wings to a header and via an electric boost pump to the engine. Even with the electric pump off there is plenty of gravity fuel flow. I agree with Kensla it may have been plug fouling with very rich mixture at idle. Idle at 1000rpm may not have been enough either and would have got your head temps up as well.

 

I had a similar thing but with Avgas in a C152 back in the 80s. At the time I had an instructor & after sitting for what seemed forever he took control & gave it full power on the ground for at least a minute on the threshold. There was a lot of rough running to start with but it eventually cleared & we took off without further problems.

 

My 3300A engine oil pressure is 250kpa (36psi) at idle before takeoff when the engine is cool which is more than enough. Once the engine is warm & I am at cruise RPM it setlles at 350kpa which is spot on according to the Jab specs. That is with Aeroshell W100Plus.

Edited January 8, 2022 by kgwilson

[facthunter]

I wouldn't want any engine of mine run for over a minute at full bore on the ground in those circumstances.  Leaning at much lower revs works with less risk. Then confirm with a normal mag drop check.  If it doesn't make the normal figures or run smoothly, return for some PLUG service. Nev

Edited January 8, 2022 by facthunter

[IBob]

2 hours ago, kgwilson said:

Vapour lock will only occur when the engine is sucking the fuel. In a Jab the fuel is gravity fed from the wings to a header and via an electric boost pump to the engine. Even with the electric pump off there is plenty of gravity fuel flow.

 

KGWilson the Savannah fuel is gravity fed from the wings to a header, and via an electric booster pump to the engine.
At the manifold where the fuel line splits to the 2 Rotax carbs, there are 2 other spigots with very small orifices: one goes to the fuel pressure gauge, the other is a return line, normally hooked to a high point in one of the wing tanks.

On several occasions I have had the following scenario:

Land on private strip, spot the owner, park up for a yarn.
15 or 20mins later, prepare to leave, activating the electrical fuel pump prior to starting.

**Listen as the fuel pump runs very rapidly for 5 or 10 secs, then slows down as the fuel pressure rises.**

Start engine, take time taxiing and prepping  to allow cooling of engine compartment.

Fly away.

**What is happening at this point is that the the fuel in the pipework in the engine compartment has vaporised, the electric pump runs rapidly as this vapour is pushed through the return line orifice (or possibly to the carbs) then slows once the pipework is flooded.

 

This rapid electric pump run only happens under the above circumstances, and I take it as a warning not to hurry my departure until I am confident the fuel system is delivering correctly.

[walrus]

We call it vapour lock. I would call it a slug of fuel that was so hot when it reached  either the electric pump, mechanical pump or carb float bowls that it boiled. Even on the injected (812 iS) engine, when you switch on one of the pumps, you can hear it stutter at high revs for a few seconds before it clears the fuel rail of vapour and settles down to the usual rpm as the fuel pressure comes up.

[IBob]

Okay, vapour lock.

To consider that more closely (in the context of a non fuel injected engine) :
Some of the pipework is full of vapour instead of liquid. And if this includes the main fuel pump, then the pump can get only limited traction, so we are now relying on gravity feed to push the vapour out via the carburettor float needle jets. Furthermore, if part of the pipe work is still hot, the arrival of liquid fuel may result in more vapour.

 

In the case of the Savannah, the electric fuel pump is low on the LH firewall, receiving fuel from a line under belly of the aircraft. Some  builders now prefer to site it behind or under the seats. Either way, it is well placed to receive and pump unheated fuel.

Meanwhile, at (or just before) the carburettor end of the setup, the return fuel line mentioned above allows vapour to bleed out of the system and back to the tanks.

This seems to me to be a pretty good system, in terms of reducing the likely impact of vapour lock situations.

 

[kgwilson]

Fuel delivery lines should be insulated and fire sleeved. Mine certainly are. The carburettor is underneath the engine at the rear and away from hot exhaust pipes. With the carbs on a Rotax on top of the engine this would seem even more critical so after sitting around after a flight especially on a hot day I can well believe the fuel has had time to become overheated and boil. You can call this vapour lock if you like but it not so in the sense of fuel being sucked by the engine pump from tanks that do not gravity feed to the engine. Having a reliable electric boost pump to push fuel to the engine is critical in this situation because if it fails you are very likely having a forced landing.

 

There is a good reason why engine manufacturers specify running the electric boost pump for several seconds before every start. Generally starting should always be with the electric boost pump off as this will prove the engine driven pump is operating. Cessnas do not have any boost pumps just a selector switch for off, left, right or both. In my Cessna days the switch was always left on both except if parked on a slope when it was switched off.

 

Return lines to the tank is an added safety measure but not necessary if you have the correct PSI value pump for your engine. A Facet pump will continue to operate even after the float bowl is full and the valve shuts off without damage. If you have to pump fuel back up to the tanks on a high wing you will probably need a higher value PSI pump than if you don't have return lines. 

[facthunter]

The return line allows fuel to return to a tank and be replaced by cooler fuel from outside the cowl. If there's cavitation at the pump it won't work at all if it's centrifugal. A displacement pump will move vapour but nowhere near as well as a solid liquid. Testing the engine driven Pump is well worth doing. every time you engine start. Lagged and sheathed lines help but if little fuel is flowing they still get some heat and enough to raise the temp too high. Vapour is diminished by applying pressure to it and/ or using avgas. Running electric pumps continuously when fuel flow is low is something I would avoid. If  your carbs are above the tanks a lot depends on all this  working reliably . It's essential to understand your fuel system even beyond what's written in the POH.. Even the best  engine needs fuel. Nev

[IBob]

KGWilson, all good. And yes, the siting of the Rotax fuel pumps is less than ideal.

 

I think there is sometimes a misunderstanding regarding the Zenith/Savannah return fuel line: I come across references to it as 'returning excess fuel' or 'limiting excess pressure' but the orifice is so small (0.4 or 0.5mm, see pic) it will do neither of those things.
I believe the purpose of it is to reduce the likelihood of vapour lock, as described.

 

As to where or not it is 'necessary', for the cost of a couple more fittings and a run of pipe I would think it an excellent innovation.

 

(PS my prestart routine includes:
*Turn on aux fuel pump, observe pressure rise to > 2 PSI.
*Turn off aux fuel pump, observe pressure fall to 0PSI to ensure fuel return orifice is not blocked.)

[skippydiesel]

First; I do not believe that the term "vapour lock" is a good descriptor of what is happening. There is no "lock" or blockage as such. The "block" if there is one, is the carburettors full float bowl, causing the valve controlling fuel delivery to the bowl to remain closed. - not allowing liquid fuel or vapour to move in the fuel line. This often results in an initial engine start and brief run, followed by silence as the float bowl fuel is exhausted. The next problem is fuel vapour is not readily moved/pumped by standard fuel pumps, designed to process liquid. They will usually do it but at low efficiency. Gravity fed systems will clear the in line vapour bubble but often only after extended engine rotation, to allow the vapour to be "processed out" by the engine pump.  Careful/short use of the choke may assist in clearing vapour but may also result in a flooded engine. Throttle should always be at or near idle setting.

 

"Fuel delivery lines should be insulated and fire sleeved." Insulated only if/where likely to be subjected to significantly higher than ambient temperatures - I see lots of expensive orange fire sleeve, where they will have little benefit other than to the supplier. Also fire sleeves are neither required or particularly effective (in fire situation) in the overwhelming majority of RAA class installations - cheaper automotive insulators or even home made, will do just as well for a fraction of the cost.

 

Vapour lock/fuel boiling is almost always an on ground phenomena. Has little if any relationship with fuel tank location (high/low) . Rotax 9 carburettor installations are particularly problematical, as even with the best fuel pipe routing & insulation, there is going to have a significant section over the top of the engine, the area most prone to heat soak while standing after a flight. Hot weather (or in air craft with a known problem)airmanship/management should always have the possibility of sudden loss of power on take off/climb.

 

As Nev said, the effects of using your boost pump  pre engine start (in this situation) is to raise the boiling point of the fuel (pressure) and to force fuel vapour, out of the main delivery lines, into the return fuel line. In my experience hearing your boost pump "rattle" away ,for a longer period then normal, is indicative of at least a small vapour bubble. Rotax return lines are usually located where the main delivery line "splits" to the two carburettors - this always leaves a length of hose where a vapour bubble may reside. This must be cleared befor reliable engine running can be expected.

 

"engine manufacturers specify running the electric boost pump for several seconds before every start" This  is just good airmanship and applies to all aircraft where a boost pump is part of the fuel delivery system Further its not for "several seconds" per say, its until a satisfactory fuel delivery pressure has been achieved. For aircraft fitted with a fuel pressure gauge this is easy to monitor. With no pressure gauge I would suggest running the pump until the "rattle" drops to just the occasional/slow beat.

 

Good fuel boost pressure management should include; Before start fuel pressure, Start Engine, fuel pressure , followed by Boost pump on and again monitor changes to fuel pressure readings - all indicative of fuel flow "health" - Boost pump should remain on for TO and climb out to safe altitude and only then be shut down.

[kgwilson]

The Jabiru engine manual specifies pump on for 10 seconds then off before start. Facet electronic cube fuel pumps (without a positive shut off valve) have no electrical contacts and have an electro magnetic free floating piston which operates by the polarity being switched constantly. When the carb float bowl is full and the inlet valve closes the pump will continue to operate to its maximum pressure (mine is 1.5-4 PSI) but the piston will not move as far as it can't push the fuel through the line. When there is air or vapour in the pump it will clatter quite loudly as there is no fuel for it to move or dampen its travel and it will hit the ends of the cylinder, hence the noise. When the pump is switched off the piston centres and fuel flows freely around it.

 

When I transfer fuel from my wing tanks into the centre fuselage tank I have a 4-6 PSI Facet auxilliary pump & know it takes about 20 minutes to move the 35 litres in the wing tank to the main tank. If I forget to turn it off it reminds me when the tank empties and the clatter can be easily heard over the engine noise. I generally wait till the fuel gauge shows about 1/2 full then transfer for 10 minutes from each wing tank. The gauge then shows about 85% full & I transfer the rest when it gets back to 1/2 again.

 

The fuel in the main tank is gravity fed to the engine but the wing tanks are lower than the engine which is why that fuel is always transferred to the main fuselage tank.

[turboplanner]

Vapour lock was a common event in the days of side valve car engines fed by side-draft or Up-draft carbies which put fuel lines in close proximity of the exhaust pipe.

Petrol turns to gas at 95 degrees, so a 35 degree ambient day boosting engine compartment tepmerature to 50 degrees is probably not going to do it.

It was followed by a long stop by the side of the road because the pump didn't turn over fast enough to push the new cold fuel past the hot-spot before it turned to gas.

Winding a rag around the fuel pipe and wetting it was usually the fix that got the engine running.

 

With a mechanical pump operating and an electric pump boosting, you would think there would be enough flow to cool the pipe below boiling point.

The description of electric pumps cavitating trying to pump vapour would need the supply pipe to the pump running close to an exhaust pipe, abd could alternatively be the downstream pipe from the electric pump running next to an exhaust pipe, or the supply line downstream of the pump running next to an exhaust like the old sidevalve. Or the electric pump may have been sitting for a while and the valves are sticking. Anyone who has lived with English cars from Morris Minors to Jaguars is usually very good at rebuilding a Lucas pump on the side of the road.

 

As much as it's good to discuss details of vapour lock, I think there's still a possbility that 20 minutes of idle just fouled the plugs.

 

 

 

[skippydiesel]

1 hour ago, kgwilson said:

The Jabiru engine manual specifies pump on for 10 seconds then off before start...........................................................

 

Does the 10 seconds accommodate a vapour  "bubble" in the fuel delivery system ? Common sense would suggest not - the same sense would suggest modifying your engine management to the conditions present rather than some rigid formula..

[kgwilson]

I am just stating what the manual says. After a 100 hourly I replace the in line filter, remove the bowl & check for debris check the float level etc. It only takes a few seconds to fill the filter & float bowl. There is likely fuel still in the line between the filter which is in the cabin just downstream of the electric pump and carb so the pump will start filling the carb with fuel left in the line & then the air from the filter will be pushed through before the whole line is recharged. This takes less than 10 seconds & perform a start and engine test run. Never falters.

[Alan]

Thanks for all the comments.

 

The electric pump was used to bring the fuel up to pressure as the Jabiru 230 POH at start up Then switched off for the start, taxi and run up to verify the mechanical pump.  The electric pump is switched back on for the take off run and switched off once safe height is achieved. The electric pump in  a Jab 230 is in the back next to the header tank.  All the fuel lines forward of the bulkhead have fireproof sleeves as factory fitted.

This is the first time I have had a Jabiru motor grumble in 14 years.  No problems in the 5 hours since.

 

I am more or less convinced this was my fault, with me tolerating a long idle having to wait for the student aircraft caused the fuel to vaporise.  - what fooled me was a normal run up check and the initial take off run at WOT was fine.

 

 

[Thruster88]

A sticking valve would cause rough running and power loss.

 

The way the jabiru fuel system is designed makes vapour lock very unlikely.

 

Piper pa28 with a mogas STC have two electric pumps mounted in the engine compartment pulling fuel UP from the tanks, that system works. It would be subjected to all the heat and long delays at busy airports. 

Edited January 9, 2022 by Thruster88

[IBob]

Thanks for putting it out there for us to pick over, Alan.

It's had me thinking through what I might do in similar pre-takeoff circumstances.

[turboplanner]

In GA students are taught why the electric fuel pump is there and when to turn it on or off.

 

In RA it seems some students are not taught this, and from time to time home builders etc. invent their own reasons and habits, some double dipping by running both manual and electric at the same time, and some leaving it turned off “for an emergency”.

 

Electric fuel pumps are a double-edged sword.

 

The Warrior POH says that the electric pump should normally be off so any malfunction of the engine driven pump is apparent.

 

So it can take over if there is a partial or full engine pump failure.

 

The reason it is normally off is that it can cause a disaster if there is an engine bay fire and fuel lines melt or fracture, and also if the carburetor float valve is leaking

 

In both cases this is because it will continue to pump fuel up to it’s pressure limit which coincides with the maximum carburetor float valve capacity.

 

Electric pump checks in a Warrior are:

 

Pre-start

Fuel pump on for five seconds > Pressure.  (a) it’s pumping, (b) it stops when it fills the carb bowl, (c) it doesn’t run on, indicating there’s a faulty float valve. All the above usually happen within 5 secs.

 

Engine started

Fuel pump off   (allows you to identify that the engine pump is delivering)

 

Pre Take Off

Fuel pump on> pressure OK  (Now standing by in case of an engine pump failure up to 1000’)

 

1000’

Fuel pump off  (safeguards against pump filling engine bay if fuel line leak or float valve leak start.

Also extends electric pump life which is relatively short).

 

En-Route

Turned on for fuel tank changes then off again for same reasons as 1000’

 

Pre-Landing (or downwind) Checks

Fuel pump on

 

After landing (clearing runway)

Fuel pump off (to remove the chance of pumping fuel line/float valve leaks into engine bay)

More from this link to Warrior POH

https://www.sfcaero.com.au/wp-content/uploads/2019/06/PA-28-161-Warrior-III.pdf

 

[RFguy]

Alan, vapour lock on a heat soaked Jabiru engine  is not uncommon, the fuel pump gets absolutely cooked by the adjacent cylinder barrel.

Many over put in a blow pipe (like 3/8" ID)  from the rear of  cooling plenum  to blast the fuel pump. Some put a shiny aluminium heat shield  between the cylinder and the fuel pump.

Edited January 9, 2022 by RFguy