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-----Original Message-----
From: Tom Bauer <tomb@ccpl.carr.org>
To: Military Vehicles List <mil-veh@uller.skylee.com>
Date: 24 January 2000 16:07
Subject: Re: [MV] Fw: Fuel Pump - vapour lock.
>since 'vapor lock' seems to affect many of you on hot days, it is
>important to understand what it is. gasoline has a fuel vapor pressure,
>and when it is heated above a certain temp in relation to the pressure
>it will vaporize.
>
Absolutely right. Reducing the (atmospheric) pressure on the fuel has the
vapour point decrease also. Automotive gasoline is now highly volatile (easy to
vaporise), more so than higher octane aviation fuel. Even aero engines approved
for unleaded pump gas here are not permitted to fly with the fuel above 20ºC or
over 5,000ft; warm fuel + reduced pressure = vapour lock.
>some cars with weak fuel pump pressure will actually
>stop running while traveling. now the problem is that most mechanical
>fuel pumps run very low pressure AND the fuel line is poorly situated in
>a hot engine compartment.
>
Typical fuel pump pressure is about 2 - 5 psi anything more will overcome the
float needle valve.
>when you shut down the engine, the engine heat
>heats the fuel line and vaporizes the fuel.
>
Sometimes whilst running this may occur.
>when you restart the fuel
>vapor flows to the carb slowly through the float valve,
>
Disagree, vapour would flow exceedingly fast past the float valve since it is
thousands of times thinner than gasolene.
>but any new fuel
>introduced to the fuel line just vaporizes so you are in a 'loop'.
>
The vaporisation occurs at the pump inlet not outlet. With the reduced pressure
at the suck side of the pump and extra heat, vapour forms with each suck stroke
and the pump shifts nothing. It is exacerbated by the pump valves being
adequate for the viscosity of fuel but likely inadequate for vapour or gasses.
eg, we know you can liquefy a gas by pressurisation and make water boil without
adding heat by depressurising it. The vapour point is increased with pressure,
a pressure cooker for example.
The Ford CMPs suffered so badly in hot climates with the location of mechanical
pump, being conduction heated by the engine and together with copper pipework
that an extra electrical pump (autopulse) was fitted outside the body and
gravity fed to force liquid fuel to the engine pump thus raising the _inlet_
pressure.
> there are several good ways to reduce/eliminate this problem. one is to raise
>the fuel pressure by taking the pump apart and using a stronger spring
>to push the diaphragm.
>
That might be a red herring, the stronger spring just shifting vapour faster
that it forms and thus getting a "bite" on liquid fuel.
>another is to put a metered return line to the
>tank so that the circulating fuel cools the line. still another is to
>cover the lines with insulation or route them where there will be less
>heat. ( you certainly don't want them running over the exhaust for
>instance.) also be sure your engine is not running any hotter than it
>need be.
>
In some arrangements the pump is totally remote from the hot engine and engine
bay even to the extent of being in the tank itself and cooled by the mass of
fuel therein.
Richard
(Southampton UK)
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