"Vapor Lock"

[NzCaptainAndrew]

Does anyone have any good reading material regarding vapor lock? Any experiences and knowledge is appreciated. Thanks.

[phiggsbroadband]

Hi Nz, only on a Lawn Mower...

It was a hot summer day, loads of long grass to cut, and the engine kept stopping after over 15 minutes running. It did this several times, but would restart after a 10 minute break.

When I finally twigged what was happening, I could see bubbles from the petrol pipeline bubbling into the fuel in the tank. This pipe was sitting very close to a hot part of the engine.

Re-routed the pipeline, and held it in place with a cable-tie, and all Ok ever since.

Pete

[boguing]

I had a car that suffered badly in warmer weather.

Pa's suggestion of wrapping the fuel lines with several layers of cooking foil in the engine bay worked a treat.

[Mariner9]

A surprising discovery from seeing what happened inside was that a major potential cause of vapour lock was a fuel pump as the suction side could pull the fuel below its vapour pressure allowing vapour to form in the line

Shouldn't have been surprising. Piper surely were well aware that they need to ensure a net positive suction head (NPSH) across all reasonably foreseen operating criteria (ie atmospheric pressure, altitude, temperature, fuel flow rate etc). A simple pressure gauge on the suction side of the pump and then a relatively straightforward calculation could tell them that. I suspect therefore that they knew or suspected they had a problem and were trying to identify the source.

Transparent piping may have helped locate source of any specific trouble spots in the system visually, but a far better way would have been to install a series of pressure sensors at various points in the system and look for any significant pressure differentials.

Increased pipe bore and reduction of bends/joints etc are indeed straightforward ways to improve NPSH, so I don't think your memory is failing quite yet DV :ok: They both work to reduce friction/turbulence thus reducing resistance to flow. If fuel heating (thus increasing fuel vapour pressure and reducing NPSH) is an issue, insulation will help so above advice regarding foil etc would also help, subject of course to approval from appropriate authority in the case of aircraft!

[Mike Cross]

A friend of mine died from what was probably vapor lock. Hot day, had been using Mogas mixed in with Avgas. There were a number of contributory factors. You'll be aware of the old one about not being able to brew a proper cuppa up a mountain. The water boils at a lower temperature due to the reduced pressure. It also plays havoc with timing your boiled egg. Conversely the pressure cap on your car radiator increases the pressure and so raises the temperature at which the coolant boils. Releasing the pressure on a hot engine is spectacular and dangerous as the entire cooling system boils at once.

Well vapor lock is just the same, fuel boiling. If you have a low wing and an engine driven pump then the pressure at the pump inlet will be below atmospheric and if it's low enough for the temperature and the fuel then vapor will form. If it does the pump will stop pumping because it's not designed to pump a compressible gas.

AAIB report here Air Accidents Investigation: Fairchild 500172 (http://www.aaib.gov.uk/publications/bulletins/april_1999/fairchild_500172.cfm)

[Maoraigh1]

A sunny spring afternoon, a Jodel DR1050 with an O200, and mogas. Fuel tank top temperature was below 20C. Aircraft had been flown, and parked into a warm (for Inverness) wind for a short time.
The gascolator is behind the engine. Take-off is on the front tank, which is exposed to the sun. I used carb heat at the checks.
When I opened the throttle to take off, the revs rose to about 1600, then dropped, and the engine stopped. It restarted on the starter, and, after clearing the runway, ATC gave permission for me to shut down, exit and check the fuel. When I put the fuel tester to the gascolator, a froth spurted out. I restarted, returned to the hangar, and discussed the problem with our engineer. In the cool evening, I took off with no problem.

[patowalker]

Not unusual when burning Mogas: warm engine, shut down, park in sun, return a while later, attempt to start, no joy, off with the cowling, wait a few minutes for heat to dissipate, vrooom.

Item 7. bottom of page 3 http://www.lightaircraftassociation.co.uk/engineering/Mogas/OPERATING%20INFORMATION%20Iss%205.pdf

[ShyTorque]

Here's a useful link about the use of MOGAS:

http://www.caa.co.uk/docs/33/20120117SSL04.pdf

[Jabawocky]

Andrew

Avgas will also vapour lock, and given the right circumstances, say your typical Bonanza, parked out on the ramp on a 35-40 degree day, even better with dark paint or boots. Full tanks. Nice and warm.

Take off, and climb, so the angle from the wing pick up top the engine is even further up hill, and passing through 7-8000 feet you get stumbles..... Bingo! Every time. And you think it can't happen with AVGAS :ouch:

And yes, this is not abnormal!

Now on a 206 for example it is less of a problem due to the gravity feed, but it is still possible with the TC machine you have. Just a bit harder to achieve.

Cheers :ok:

[xrayalpha]

Vapour lock: it is a strange one.

Almost all microlights run almost exclusively on mogas, and vapour lock does not seem to be a big issue.

Is it a question of more modern design, where people are now more aware of bore of pipe, route of pipe, length of run, kinks, etc.

Or is it that many microlights can be successfully landed in small areas and then people do not report it?

Am also thinking of carb ice. Some microlights, like the C42, are designed with no carb heat and some have very different types of permanent carb heat, of sorts that have nevr gained certified approval.

Modern cars, unlike me mum's old VW Polo which could never get up Shap in one go because of carb ice, don't get carb ice now they are injected.

I presume injection won't solve vapour lock since it is a presurised pipe, so why don't we hear much about vapour lock in cars. Has it been designed out of them?

So, to sum up. Is there a lesson we can learn from microlight/cars. Or is this a problem just faced by older designs?

Or is the fuel in the Alps and the Rockies so, so different as to work in cars at altitude with no difficulties?

[ShyTorque]

XA, most modern (fuel injected) cars use a high pressure fuel pump located in the fuel tank. This pushes fuel up the system, in excess, with a bleed return back to the tank, normally through a 3 bar regulator. This is far less likely to allow fuel vapour lock to form than does pulling it via a mechanical suction pump, as most carburettor equipped cars used to do.

I have a car that uses an old design of engine notorious for fuel vapour lock. I replaced the mechanical suction pump with an electric pusher type, located right next to the tank and it has never suffered that problem.

[mad_jock]

XA, most modern (fuel injected) cars use a high pressure fuel pump located in the fuel tank. This pushes fuel up the system, in excess, with a bleed return back to the tank, normally through a 3 bar regulator. This is far less likely to allow fuel vapour lock to form than does pulling it via a mechanical suction pump, as most carburettor equipped cars used to do.

Which is why you must turn the ignition off if you have a crash. If the fuel line is rupture you can get loads of fuel outside the relative safety of the fuel tank rather quickly

[phiggsbroadband]

Hi, just to mention that some old petrol cars had a bi-metalic valve in the air inlet pipework, that allowed air to be taken from near the exhaust pipe.
The reason given in the owners manual being that it increased the vapourization of the fuel in cold weather.

Nowadays Intercoolers are used to reduce the air temperature between a turbo, and the engine, as the turbo heats the air by compression.

[mad_jock]

Not on the older models and sometimes it doesn't work.

I have had to leg it across a pool of fuel to turn the ignition off when a lady had managed to spin out of a corner on black ice and mounted a dry stone dyke sliding along the top of it before coming to rest.

She was just screaming and trying to haul her kid from the back. The sump had been shattered and the engine had siezed and the fuel was still pumping.

[mad_jock]

Better safe than sorry and crispy and turn the ignition off if you have a crash.

[ShyTorque]

Which is why you must turn the ignition off if you have a crash. If the fuel line is rupture you can get loads of fuel outside the relative safety of the fuel tank rather quickly

Of course, I have a battery isolator switch as well, as the car is used for competitions. Another safety device sometimes used is to wire in an engine oil pressure sensing switch. This will not allow the live feed to connect to the fuel pump unless the oil pressure is up. If you fit one of those you also need a fuel prime (bypass) switch to get the fuel system filled to start the car, but the pump will stop running as soon as the engine stops turning.