| [GMCnet] Ethanol vapor pressure [message #99222] |
Fri, 10 September 2010 08:40  |
Gary Casey
Messages: 448
Registered: September 2009
Karma: 0
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Senior Member |
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I posted information that the fuel companies go a 1 psi "waiver" for 10% ethanol
fuel. That is, they can pipe standard fuel and then simply add 10% ethanol at
the distribution point, increasing the vapor pressure over what would have been
allowed, or customary. So I looked up the vapor pressure of neat ethanol. It
is about zero at 0F and it boils at 156F (vapor pressure = 14.7 psi). Turns out
it is about 1 at room temp and that is about the vapor pressure increase
allowed. It doesn't follow that dissolved ethanol will increase the vapor
pressure of the fuel by the vapor pressure of ethanol itself, but it's not that
far off. So, what does this mean? Adding ethanol will cause the vapor pressure
of the fuel to increase rapidly with temperature. At 100F the vapor pressure is
over 2.5 psi. That would be just a normal hot day, not counting much fuel
heating. At 112 the pressure jumps up to 4.3 and at 128F it is 6.8 psi. The
bottom line is that while ethanol does do much at normal warm temps, when the
fuel is heated, as in the fuel line as it goes to the engine, it can cause real
problems. My conclusion is that an ethanol blend is MORE SENSITIVE to heating
than a standard fuel with the same nominal RVP. Maybe that's why the major
reported problem is a "vapor lock" upon acceleration after an idle period. When
idling, the fuel flow is very low and consequently will increase in temperature
around the engine. Then the fuel pump tries to pump a lot of fuel on
acceleration and - boom - instant vapor lock.
Gary Casey
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| Re: [GMCnet] Ethanol vapor pressure [message #99227 is a reply to message #99222] |
Fri, 10 September 2010 09:40  |
John Sharpe
Messages: 489
Registered: February 2006
Location: Texas
Karma: 1
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Senior Member |
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| Quote: |
... At 112 the pressure jumps up to 4.3 and at 128F it is 6.8 psi. The
bottom line is that while ethanol does do much at normal warm temps, when the
fuel is heated, as in the fuel line as it goes to the engine, it can cause real
problems. ....
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Gary, I knew the result. Now I know the cause. Thanks for the 'rest of the story'.
John Sharpe
Humble,TX
'78 Eleganza TBI
'89 Spectrum 2000 MPI V-10
'40 Ford Panel Delivery TPI
johnasharpe@gmail.com
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