vapour lock

[Guest PeterMJ]

some information on rotax engines (probably goes for other engines I gather) mentions that vapour lock may occur with use of unleaded mogas, less likely with avgas. A manual advises to keep an eye out for signs of vapour lock. Can someone educate me on exactly what vapour lock is, and what the signs are?

 

 

[facthunter]

Vapour lock.

 

Mogas is a mixture of many substances all of which will vapourise under the effect of temperature. The substances (formulation) vary from summer to winter with the winter mixes being more liable to form "vapour" in the fuel lines, or anywhere in the fuel system for that matter, where elevated temperatures are reached. It doesn't matter that most of the fuel hasn't vapourised, only some of it needs to be vapourised, and then your engine will starve for fuel, and quit. ( or a float type carburettor will foam in the bowl, the float sinks in the less dense fuel, and it will flood). Both these possibilities are a bad idea.

 

The method of prevention involves 2 processes;

 

Insulate the heat from the fuel system eg Heat shield baffles and running fuel lines inside heat insulating sheathes. These are also fireproof and should be employed firewall-forward.

 

Use a fuel pump to provide extra pressure to the fuel. This inhibits the formation of vapour. It will still form but at a higher temperature. Sometimes surplus fuel will be flowing through the system and returned to a fuel tank, (by design). this can reduce the temp in the particular fuel line. Nev..

 

 

[icebob]

Hi,

 

Vapor lock(fuel converting to gas) was far more common in older mogas fueled systems incorporating a low-pressure mechanical fuel pump driven by the engine, located in the engine compartment and feeding a carburetor. Such pumps were typically located higher than the fuel tank, were directly heated by the engine and fed fuel directly to the float bowl inside the carburetor. Fuel was drawn under negative pressure from the feed line, increasing the risk of a vapor lock developing between the tank and pump. A vapor lock being drawn into the fuel pump could disrupt the fuel pressure long enough for the float chamber in the carburetor to partially or completely drain, causing fuel starvation in the engine. Even temporary disruption of fuel supply into the float chamber is not ideal; most carburetors are designed to run at a fixed level of fuel in the float bowl and reducing the level will reduce the air:fuel mixture delivered.

 

Carburetor units may not effectively deal with fuel vapor being delivered to the float chamber. Most designs incorporate a pressure balance duct linking the top of the float bowl with either the intake to the carburetor or the outside air. Even if the pump can handle vapor locks effectively, fuel vapor entering the float bowl has to be vented. If this is done via the intake system, the mixture is, in-effect, enriched, creating a mixture control and pollution issue. If it is done by venting to the outside, the result is direct hydrocarbon pollution and an effective loss of fuel efficiency and possibly a fuel odor problem. For this reason, some fuel delivery systems allow fuel vapor to be returned to the fuel tank to be condensed back to the liquid phase. This is usually implemented by removing fuel vapor from the fuel line near the engine rather than from the float bowl. Such a system may also divert excess fuel pressure from the pump back to the tank.

 

Most modern engines are equipped with fuel injection, and have a high pressure electric fuel pump in the fuel tank. Moving the fuel pump to the interior of the tank helps prevent vapor lock, since the entire fuel delivery system is under high pressure and the fuel pump runs cooler than if it is located in the engine compartment. This is the primary reason that vapor lock is rare in modern fuel systems. For the same reason, some carburetter engines are retrofitted with an electric fuel pump near the fuel tank(I did this).

 

Other solutions to vapor lock are rerouting of the fuel lines away from heat generating components, installation of a fuel cooler or cool can, shielding of heat generating components near fuel lines, and insulation of fuel lines. An older alternative was to solder penny washers along the fuel line to act as mini heat dispersers.

 

A vapor lock is more likely to develop when the vehicle is in traffic because the under-hood temperature tends to rise. A vapor lock can also develop when the engine is stopped while hot and the vehicle is parked for a short period. The fuel in the line near the engine does not move and can thus heats up sufficiently to form a vapor lock. The problem is more likely in hot weather or high altitude in either case.

 

Do not assume that a gravity feed fuel system is immune to vapor lock simply because there is no fuel pump to "upset.” Much of the foregoing applies equally to a gravity feed system; if vapor forms in the fuel line, its lower density reduces the pressure developed by the weight of the fuel. This pressure is what normally moves fuel from the tank to the carburetor, so fuel supply will be disrupted until the vapor is removed, either by the remaining fuel pressure forcing it into the float bowl and out the vent or by allowing the vapor to cool and re condense.

 

Vapor lock has been the cause of many a forced landing in aircraft. That is why aviation fuel (AVGAS) is manufactured to far lower vapor pressure than mogas. In addition aircraft are far more susceptible because of their ability to change altitude rapidly. Liquids boil at lower temperatures when in lower pressure environments.

 

Bob.

 

 

[Guest ROM]

Many years ago we switched our glider towing 235 HP Pawnee over to Mogas.

 

Worked Ok in winter but the first hot days we got, the Pawnee would take off OK and about over the fence would suddenly cough, splutter, hick-up, miss a few beats and then resume running as per normal.

 

Made the tuggies eyes go real wide and got his completely undivided attention as I can personally testify!

 

After that we went back to Avgas over summer and Mogas during winter and then decided it was not worth the trouble so settled on Avgas thereafter.

 

No vaporisation problems ever after that even towing on 42 degree days.

 

3 or 4 years ago we did some research on Mogas for a two stroke self launching glider engine with oil added.

 

When we found out just how little actual petroleum was in the Mogas and how much plain s--t, otherwise known as volatile substances were in Mogas we most definitely won't use Mogas in any of our aircraft engines unless really stuck.

 

Rotax recommendations for Mogas in Australia surprise me no end.

 

European Mogas is refined to very tight tolerances by law and European vechiles go very well on their Mogas but these same vechiles in Australia performances drop off a lot when run on the supposed equivalent rated Australian mogas.

 

Up until a few years ago there were NO specifications set down for Mogas or Diesel fuels in Australia.

 

[ I don't know the current status of the fuel specs. ]

 

It was up the the oil companies to set the specifications.

 

Due to the low quality of Australian fuels and the very high percentage of non petroleum volatiles or cousins to petroleum volatiles I am surprised that there is not more problems with the Rotax engines.

 

25 years ago I got mixed up fairly heavily with the technical marketing managers [ the tech guys between the refiners and the sellers. ] of the then 27 oil companies in Australia over parrafin wax freezing out of diesel fuel in cold frosty mornings and destroying the fuel pumps of tractors, trucks and various heavy diesel fuel engines.

 

Believe me it was quite an education for a lil ol country boy [ there were three of us from the national farming organisations and although the junior, I had the lead as I had the info. ] to take on this high status mob in Shell House.

 

Over the couple of months beforehand I learnt some really surprising aspects of the petroleum industry and of fuel qualities such as the change overs between summer and winter and Alpine grades plus lots of other tech info on refining and storage.

 

Winter grades are more volatile. Summer grades less so so don't use winter grade fuel in mid summer if you want to avoid vaporisation problems.

 

I was pinned down at the fancy meal that was laid on and was demanded to tell them where the hell I had got all my info from as some of it was only known internally in the oil companies and then only to a limited group in the companies.

 

Vell ve had vays and means and one hell of a lot of time on the phone all over Australia beforehand got that info from some strange sources at times.

 

Yes, the wax content was reduced drastically as a consequence and our diesel fuel freezing problems went away until recently when the next generation of fuel managers with no memory of those problems have moved into power.

 

 

[Yenn]

While vapour lock is caused by fuel converting to gas. The real problem is the fact that the pumps cannot handle the gas, they need liquid. The vapour will most likely form in a high point in the piping and the only cure is to cool it or pressurise the pipeline with another pump downstream.

 

 

[Seal]

While vapour lock is caused by fuel converting to gas. The real problem is the fact that the pumps cannot handle the gas, they need liquid. The vapour will most likely form in a high point in the piping and the only cure is to cool it or pressurise the pipeline with another pump downstream.

Well, sort of. The real problem is that the carburettor requires liquid fuel. The fuel pump is designed to provide that liquid at the required pressure but it can't work either if it is not supplied with a liquid feed. The vapour will form anywhere the pressure in the line is below the vapour pressure. Often this is the highest point, but if there is a high point in cold air and a low point in hot conditions then I'd go for the hot bit as the most likely location.

 

Take the Pawnee ROM was talking about earlier. The highest point in the system was the fuel level in the tank. It didn't vapourise there. The fuel line came over the top of the muffler, into the fuel pump attached to the hot block and thence into the carby which was the lowest point in the system. While it was running, the flow of cool fuel out of the tank kept everything ticking along nicely. When you'd landed, taxied in front of the glider and waited while he was hooked up and all the rest, the fuel somewhere in the pipes and pump reached the magic combination of head and temperature. You'd then open the tap and the thing would roar off on the fuel in the carby bowl until it was about airbourne, then it would stop for just long enough for you to say "good golly goshkins", then it would spring into action again and all would be well. It pumped the hot fuel through, or it was pushed through by the pressure head in the tank and the flowing fuel never got hot enough to reach vapour pressure.

 

The margin between vapourisation and not was quite small. The Pawnee would only vapourise below about one third fuel so just an inch or two of head made the difference. I flew the same aeroplane so I know this first hand.

 

Were we silly or dangerous? I don't think so. We knew the systems and had a pretty good handle on what was going on. If we had had been operating from low wing tanks then it would have been a different story, hiccup, cough, die and all that went with that.

 

As Yenn said, if you have a vapourisation problem, you need the either control the temperature in the lines or increase the pressure. To increase the pressure you need a pump upstream to take the cool fuel out of the tank and stuff it up the pipe.

 

 

 

If you are having a vapourisation problem with avgas then you almost certainly have a design fault as the vapour pressure of avgas is controlled. If you have a problem with mogas then it could be design or fuel as the vapour pressure of mogas could be anywhere. If you need to run mogas then keep on insulating fuel lines and putting pumps closer to your tanks until you get the reliability you desire. Just remember that there is no way of knowing if the next batch of mogas you get is more volatile than anything you have seen before. Fly accordingly.

 

 

 

And just remember, we do this for FUN.