Useless trivia

[kb426]

At Louisville, we had some discussion about fuel and vapor locking. I found some info that might be useful someday.


Tests were performed to compare the evaporation rate of 10 volume percent (vol%) ethanol-blended gasoline (E10) with the evaporation rate of its base gasoline. Weight loss, temperature, pressure, and humidity were monitored as lab-blended E10 and base gasolines were evaporated concurrently from glass cylinders placed on balances located side by side under an exhaust hood. The averaged results of four tests at about 70°F showed that the E10 lost more total weight to evaporation than the base fuel, but less gasoline. The increased weight was due to ethanol, which was present in the E10 evaporative emissions at concentrations of about 13 weight percent (wt%). In two-hour tests at temperatures near 70°F, during which 4.5 to 5.3 wt% of initial fuel samples were evaporated, E10 fuels lost an average of about 5% less gasoline than their base fuels. A similar result was obtained for a one-hour test, during which about 2.4 to 2.5 wt% of the initial fuel samples were evaporated. Gas chromatography (GC) component analysis indicated that the compositions of the ethanol-free emissions from the two fuels were similar. Reid vapor pressure (RVP) measurements made using a Grabner CCA-VPS according to ASTM D5191-91 indicated that E10 fuels underwent an approximate 5% greater RVP reduction than their respective base fuels.

Global ethanol trade is forecast to increase 25-fold by 2020. Most of it will be blended with gasoline to make biofuel. However, blending ethanol with gasoline has a profound effect on the evaporation characteristics of the mixture. In particular, the thermodynamic properties of the blends can be significantly different than the constituents. A clear understanding of the blend's properties is essential for optimizing engine design, e.g. utilizing charge cooling effect. Data available in the literature is very limited, considering ethanol-gasoline blends will be used as a fuel in large scale worldwide. In this work, comprehensive measurements of vapor pressures were carried out. The enthalpies of vaporization were derived from vapor pressure data using the Clausius-Clapeyron equation. Maximum vapor pressure occurs with 20% ethanol-gasoline blend at which a positive azeotrope is formed. The trend is different in enthalpy of vaporization. The results presented here contradict previous claims that the enthalpy of vaporization is a linear function of ethanol content. Such a trend is true up to 20%; the value then decreases a little and appears to flatten out between E30 and E60. After E60, it begins to increase again. The ethanol-gasoline blends were tested in a port injected SI engine. The evaporation characteristic of the fuel was assessed by measuring the cycle-resolved temperature in the cylinder. The results have shown that low ethanol blends (0–30%) tend to evaporate readily, but the evaporative cooling is limited by the relatively low fuel flow rate (at fixed stoichiometry) and enthalpy of vaporization. High ethanol blends (>50%) with reduced vapor pressure cannot evaporate readily, but if they do vaporize, then they absorb large amounts of energy per unit mass. The implication of these competing effects is that maximum cooling is achieved using a blend with about 50% ethanol. A parameter "potential evaporative power" is introduced to estimate these effects.

I found this document on my local Oil Refinery's web site and it may help explain some of the problems with vapor lock and hard start.
In this document is a chart for Seasonal Gasoline Specifications. Gasoline vaporizes according to fuel system pressure.

December - February gas mixture begins to vaporize at 11-15psi at 105 deg F

April gasoline is 9-11.5 psi at 124 deg F

and May gasoline is 7-9 psi at 140 deg F

If you had winter gasoline in your Antique car on a 100 deg day you would only be about 5 degrees from vapor bubbles in the fuel. A vapor lock condition would be caused.

If you purchased April gas when you take your Antique out of winter storage it is only a month until the switch from 124 deg F to 140 deg F. This might explain a lot of problems in early summer.

I Have problems with the fuel temp in my 1937 Buick. I can start my car instantly after sitting in a cool garage at about 75 deg F. Drive 30 miles and then park for 20 minutes. As the car is parked Temperature

Will rise to above 180 degrees on the Temperature gauge. Much of the excess engine heat will end up soaking the fuel system under the hood. The fuel pressure is at 1 - 3 psig since the pump is not running.

The summer gasoline vaporizes in the line and the heat soak is quicker and longer on hot day's. If I wait a hour for the system to cool to ambient temperature then it starts easy.

I have a electric pump at the tank to try and speed up the fuel cooling process but if you have a 100 deg F day you may have fuel near 100 deg F in the tank.

If I could get 75-85 octane, non-ethanol, Regular gasoline that vaporized at above 180 deg F, I think my car would run fine all the time.




1.) The vapor pressure is a property of the substance and is constant at a given temperature. It increases when temperature increases.
2.) The boiling point of a substance is the temperature at which the vapor pressure of the liquid equals the pressure surrounding the liquid.

-If you create a vacuum over the liquid you decrease the pressure surrounding the liquid. As soon as it falls below the vapor pressure the liquid will start to boil (you do not have to increase the temperature)!
-If you seal the container, the pressure over the liquid will always be the same as the vapor pressure. Thus no matter how much you heat your liquid in order to increase the vapor pressure it will never boil (provided you have a perfect seal and your container can withstand the pressure i.e. it doesn't blow up!). I.E. your boiling point will increase if you seal the container.

[enjenjo]

I don't think this is useless trivia. Those of us that are still running carburetors should be aware that the oil companies no longer adjust vapor pressure with the weather like they used to before fuel injection became common. It can cause problems in your car that never experienced problems in the past. EFI is generally under enough pressure that it is not a concern.

One of the reasons that the manufacturers started running a return line on fuel pumps was to combat vapor lock as the fuel would return to the tank rather than setting up front getting hotter. The problem has only gotten worse over the years. So if a fuel pump with a return line is available for your engine it may be a good idea to use it. If you can't get that type of pump, using a fuel regulator before the carburetor with a return line will work too. If you do use a return line make sure it reaches the bottom of the tank so it gets a chance to cool. Another way to help is a fuel pump near the tank so the fuel in the engine bay is always under pressure. When I had a problem a few years ago I was able to cure it with a cheap parts store electric pump spliced in at the tank.When routing your fuel line keep it as far from the exhaust as you can, and add insulation to the line if you have any doubt. Rubber fuel line holds heat much more than a steel, plastic, or aluminum line, so keep the rubber to a minimum.

KB426 brought up the subject of fuel systems with a low pressure pump to a surge tank where the high pressure pump creates the pressure for the EFI. This type system can also have vapor lock problems, particularly if the high pressure pump is not matched to the injector size. If your require 100 PPH and your pump supplies 400 PPH the fuel will be heated by the return from the injectors.