At 09:04 AM 11/19/99 -0000, Paul Hunt wrote:
>....
>I've never really understood 'vapour lock' in connection with our fuel
systems. If the fuel level in the float bowls drop the valve opens. With
the pump at the back of the car pushing, even if there was vapour in the
pipes wouldn't it get pushed straight through to be followed by liquid
fuel? Or is the theory that the pipe gets so hot that the liquid fuel also
vapourises at that point? But that takes energy that would tend to reduce
the temperature of the pipe, unless there was so much heat being generated
in the engine compartment that there was more than enough to keep the
vapourisation going. That's an awful lot of vapour. My factory V8 has
never suffered from vapour lock even on a 90+ day stuck in traffic, with
both fans on full time and the temp needle well into the red.
I can answer that one. You are right, that if vapor occured in the fuel
line the pump would just push it through into the float chamber where it
would be vented to the atmosphere (or into the carbon canister in those
"newer" cars). However, heating of the fuel line can preheat the fuel,
which can contribute to vapor lock, which happens not in the fuel line but
farther down stream.
Vapor lock occurs in the body of the carburetor, downstream from the float
chamber. Heat soak from the exhaust manifold creeps up into the carb(s) to
make the carb body hot enough to boil the fuel inside. This is more often
a problem when the exhaust manifold is on the same side of the engine as
the intake mainfold, as it is in the MG B-series engines. When the carb
body gets hot enough to boil the fuel it can do one of two things, both
with the same effect, sputtering or stalling engine.
Vapor pressure created by the boiling fuel can prevent the liquid fuel in
the float chamber from getting into the body of the carb. This would
generally occur after a hot run when you turn the engine off and let it sit
for several minutes. As the liquid fuel is fed by gravity only from the
float chamber to the body of the carb, there is very little pressure
pushing on the fuel at this point, so it doesn't take much back pressure to
stop the flow. In this case the engine may not start at all.
In a slightly less severe case of vapor lock (much more common), there is
hot fuel vapor in the body of the carb, but fuel can still flow from the
float chamber into the body of the carb, pushing the vapor ahead of it
towards the venturi opening. As more liquid fuel enters the body of the
carb it continues to boil from the residual heat in the carb body, such
that the resultant fuel mix is part liquiid and part vapor. In this case,
as the fuel enters the venturi to mix with the air there is insufficient
fuel to make up the correct fuel/air mixture, so the engine runs extremely
lean, coughing and sputtering with very little power. The object here is
to keep the fuel cool enough to prevent boiling in the carb body.
Under normal driving conditions there is enough fuel flowing continuously
through the system that the fuel itself acts as a heat sink and keeps the
carb body temperature below the boiling point. I occcasionally have
conditions with my MGA where the engine is working hard enough on a hot day
to peg the temperature gauge, but the engine keeps soldiering on none the
less with the high rate of fuel flow keeping the carbs cool (or at least
cool enough). For a reference picture go here:
http://www.ntsource.com/barneymg/pic89/trmont.htm
I do occasionally encounter vapor lock when I stop for fuel and shut it off
after a long run on the road. By the time it's gassed up and I try to
start it again the hot carbs are boiling some fuel. This condition is of
course aggrevated by the hot exhaust manifold being directly underneath the
carbs, and the engine compartment being closed acts like an nice oven to
keep the heat contained around the carbs.
Much more often I get vapor lock in between runs while autocrossing. With
the engine fully warmed up it gets maybe a 60 second run at nearly full
throttle and at high RPMs, which is by nature the condition where it uses
the maximum amount of fuel in the shortest time possible and generates the
maximum amount of heat in the engine and in the exhaust manifold. If it is
then shut off or left to stand stationary and idling, the heat from the
exhaust manifold can do it's nasty work on the carbs in short order. When
it's time for the next run it may start but run very badly.
For SCCA Stock class I am not allowed to modify the fuel system, so extra
heat shielding or forced cooling air or fuel cooling cans are not options.
The expedient solution here is to pull the manual choke full on and pan the
throttle some to get a lot of fuel flowing through the carbs. The flow of
fresh fuel then in turn cools the carbs enough to allow it to run normally
again (eventually). In the more severe cases this could take a few minutes
of fiddling and choking and tromping on the throttle before I can take the
next autocross run. For road driving it usually just takes some manual
choking and a couple of minutes of driving down the road for it to clear
up. Then again, this entire proceedure would be very difficult on the
later cars with automatic choke, so for those owners, you have my
condolences, but I dunno what could help there.
Barney Gaylord
1958 MGA with an attitude
http://www.ntsource.com/barneymg
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