Thursday, April 3, 2008

AV - Vapor Lock

Re: Vapour Lock (Question for Mr. Hoover)

--- In volksplane@yahoogroups.com, Stefan Vorkoetter wrote: > Can you tell me (and the group I guess) what vapour lock is?

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Gasoline, or whatever passes for gasoline nowadays, has a low boiling, typically between 80* and 90* on the Fahrenheit scale, the range depending on the formulation of the fuel, in that 'gasoline' containing a higher percentage of alcohol boils at a LOWER temperature. And to make matters worse, the boiling point decreases as the air pressure is reduced.

Heat your gasoline to its boiling point, it starts to do just that. Not like your tea kettle, but the liquid gasoline is still rapidly turning into gasoline vapor.

Did you get that bit about pressure? Reduce the pressure on your hot gasoline and it will start to boil at temperatures well below 80 to 90F (I forget the Celcius). Higher you fly, the lower the boiling point.

The pressure factor is really important if you have a fuel pump anywhere in your system and is of CRITICAL important is you're using the typical two-chamber automotive type pump, such as a stock VW mechanical fuel pump. That's because when the pump's diaphram is pulled down it effectively REDUCES THE PRESSURE on the fuel that appears in the inlet-pipe of the pump. And if the liquid fuel is close enough to the boiling point, the drop in pressure fills the lower chamer with gasoline VAPOR instead of liquid fuel. (This is the classic definition of Vapor Lock, by the way. Which never happened on the early Fords because they used a gravity-fed system.)

Wanna know the main difference between gases and liquids? (They're both 'fluids,' by the way.) You can COMPRESS a gas whereas liquids are virtually incompressible. And that's exactly what your fuel pump does -- it compresses the bubble of vapor in the lower chamber. The bad news is that compressing the bubble of vapor does NOT allow any gasoline to be pumped.

The really bad news here is that an automotive fuel pump (and most others) are not positive displacement devices. The outlet pressure of the typical fuel pump is generally less than 1 psi. If the lower chamber were filled with LIQUID, 1 psi would be more than enough to push the check-valve off its seat and force liquid gasoline into the upper chamber, where it would bet fed to the carb according to the position of the carb's float-valve. But when the lower chamber is filled with vapor, the pressure of the return spring under the diaphram simply compresses the bubble. Some of the vapor may get into the upper chamber but the bottom line is that within three or four cycles the fuel pump stops delivering fuel to the carb, the engine burns off the fuel in the bowl (if its a real carb) then dies.

That's the typical 'vapor lock' scenario. But there are others.

If the hot gas is in a pipe at atmospheric pressure, you're going to get SOME amount of vapor formation. In a properly designed fuel delivery system, your fuel pipe should ALWAYS have some degree of slope toward the carb, even when the airplane is climbing at its steepest angle. The reason for that is to allow any vapor to travel UP the pipe and be vented from the tank and for any debris that got past your finger-strainer to travel DOWN the pipe and end up in the gascolator. (Your fuel line is always sized to PERMIT the down-hill passage of any debris large enough to get through the strainer in the tank.) Sounds like plain old fashioned Common Sense but you'd be surprised what a rare commidity that has become :-)

One of the other vapor-lock scenarios is when the body of the carburetor becomes so hot that the flow of gasoline through the carb is not enough to cool it off. The gasoline boils and since the bowl is vented to the atmosphere, escapes. But in some cases, especially with automotive carburetors, the bowl is vented into the throat of the carb resulting in a mixture so rich that it can kill the engine. The Tillotson and, I think, most of the Zeniths, are vented directly from the bowl to the atmosphere so this type of 'vapor lock' but the Solex (all models) and most other automotive carbs dump the boiled-off vapor into the manifold.

As you can see, the root-cause of vapor lock is pretty simple, which makes its prevention a no-brainer. Unfortuantely, in the case of the VP, in an effort to keep the costs down the fuel system violates a lot of rules, such as mounting a Solex carb on top of the engine, forcing you to use a fuel pump. Which is fine for a car, where you can pull over and park, but it's not real smart on an airplane. (Yeah I know, Rotax does it too. Of course, when you pay twelve grand for a 1300cc engine you expect it to come with fully insulated fuel lines.)

-R.S.Hoover

PS -- a lot of modern vehicles use positive-displacement pumps with pressures as high as 135psi. (Under that kind of pressure you don't have to worry about vapor lock.) A pressure-regulator controls the feed to the injectors and any excess is returned to the fuel tank, which is usually fitted with a vapor recovery device and electronically controlled venting system, hilariously complex, ridiculously expensive and not repairable at all; replacement only. Which is one reason my daily driver is forty years old :-) (Runs fine, thanks. And its emissions are barely a tenth of the legal minimum.)

(the above was posted to the Volksplane Group in 2005)
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