Gasoline Distillation Curves

[Steve G.]

Re: Gasoline Distillation Curves


If I am allowed to question any of this further, how do we know the temp of the fuel vs the temp of the container?

Why doesn't the fuel start to boil when it hits 100* F and continue boiling right on up through the range?

In stock form with exhaust gas right at the base of the carb even in the dead of winter I'm sure we can hit 150*. Why is our fuel not boiling all the time?

At 175* even the fuel without ethanol has 35% of it's fuel with boiling points below that number. Is 35% the threshold for trouble?

Curious minds want to know.

Steve

[Peter S.]

Re: Gasoline Distillation Curves

If I am allowed to question any of this further, how do we know the temp of the fuel vs the temp of the container?

Why doesn't the fuel start to boil when it hits 100* F and continue boiling right on up through the range?

In stock form with exhaust gas right at the base of the carb even in the dead of winter I'm sure we can hit 150*. Why is our fuel not boiling all the time?

At 175* even the fuel without ethanol has 35% of it's fuel with boiling points below that number. Is 35% the threshold for trouble?

Curious minds want to know.

Steve

Steve,

I would expect your fuel to not boil because, when running, you have a constant flow. This is why the issue exists during start-up conditions - because the fuel has been sitting in your fuel bowl in a hot environment.

Regarding fuel temp vs. the temp of the container, all liquids take time to heat up. This relates to the specific heat of the materials. When you put a pot of cold water on a hot stove, the you'll find that the iron pot gets hot very quick, whereas the water inside the pot takes much longer. Iron has a specific heat of 0.11 btu/lbF, whereas water has a specific heat of 0.938 btu/lbF. Basically, it takes 9 times as much heat to increase 1 lb of water 1 degree F than it does to increase 1 lb of cast iron by 1 degree F. Plus, you have heat transfer that must occur between the iron pot and the water.

Same applies to gasoline. The heat from the engine compartment area must transfer to the carburetor, then it must transfer to the gasoline. Gasoline has a specific heat of 0.53 btu/lbF. So if your car has been sitting after shutdown for 10 minutes, there is a good chance that the small amount of fuel in your carburetor bowl has absorbed the necessary heat from the engine bay through the carburetor to increase the rate of vaporization so that starting the car is an issue.

I cannot answer your last question. But, there is a graph in that paper that Duke posted that I found interesting:

[Steve G.]

Re: Gasoline Distillation Curves

Steve,

I would expect your fuel to not boil because, when running, you have a constant flow. This is why the issue exists during start-up conditions - because the fuel has been sitting in your fuel bowl in a hot environment.

Regarding fuel temp vs. the temp of the container, all liquids take time to heat up. This relates to the specific heat of the materials. When you put a pot of cold water on a hot stove, the you'll find that the iron pot gets hot very quick, whereas the water inside the pot takes much longer. Iron has a specific heat of 0.11 btu/lbF, whereas water has a specific heat of 0.938 btu/lbF. Basically, it takes 9 times as much heat to increase 1 lb of water 1 degree F than it does to increase 1 lb of cast iron by 1 degree F. Plus, you have heat transfer that must occur between the iron pot and the water.

Same applies to gasoline. The heat from the engine compartment area must transfer to the carburetor, then it must transfer to the gasoline. Gasoline has a specific heat of 0.53 btu/lbF. So if your car has been sitting after shutdown for 10 minutes, there is a good chance that the small amount of fuel in your carburetor bowl has absorbed the necessary heat from the engine bay through the carburetor to increase the rate of vaporization so that starting the car is an issue.

I cannot answer your last question. But, there is a graph in that paper that Duke posted that I found interesting:

Sorry Peter, what I meant to ask is why we don't have the perc problem anytime we shut the engine off with any type of gas, ethanol blended or not. Underhood and carb temps are usually above the temp that is being cited as the problem area.

I understand about the flow of heat, but if we are to draw corelations to the chart don't we need to know what temp the fuel is and not the container? The assumption previously was that the temp read on the outside of the container is what the fuel will be. From what you've just written, that doesn't seem likely.

So as we drive the car cool fuel keeps entering the bowl and keeping the fuel temp below it's boiling point. When we shut it off the fuel stops entering the bowl and what's in there starts to warm up. What is the temp that it starts to boil? I think you said earlier that it wasn't necessarily the bp's shown on the distillation chart, but rather that it was a blended temp dependant on the blend of the compounds. In the distillation chart roughly 60% of the gas has boiling points between 200 and 400F, the remaining 40% between 100 and 200F. Does that give us any indication of what temp the mixture will start to boil?

Steve

[Timothy B.]

Re: Gasoline Distillation Curves

Check out this little goody I saw on ebay.

http://www.ebay.com/itm/GM-Rochester-CarbAIRator-Add-On-Hot-Idle-Compensator-NOS-Vintage-Oldie-NOS-/161689417079?hash=item25a570b577&vxp=mtr

[Steve G.]

Re: Gasoline Distillation Curves

[/QUOTE]

Peter,

Am I reading the chart wrong? It seems to be saying that loss of volume in the bowl starts to be an issue at 150F , but doesn't become a driveability and vapour lock issues until 200F. I'm assuming they are meaning fuel temp, not container temp.

Loss in the bowl is likely to be a wash when it comes to hot start. The fuel that was lost to vapour and vented in the throat is likely offset by the lower head pressure of the fuel in the bowl. Either way, they don't describe it as a driveability issue.

Steve

[Steve G.]

Re: Gasoline Distillation Curves

Check out this little goody I saw on ebay.

http://www.ebay.com/itm/GM-Rochester...70b577&vxp=mtr

Yup, fuel percs been around along time. I guess that's how they dealt with it before they started building those things into the carbs.

Instructions on wiring the heat riser must be on the back side of the sheet.

Steve

[Jim L.]

Re: Gasoline Distillation Curves

Yup, fuel percs been around along time.

So, after 21 postings in this thread and who-knows-how-many-posts in other threads, maybe I can distill this down to its essence:

1. Engine gets hot
2. Fuel gets hot
3. Fuel perc results

I think that pretty much covers it.

Jim

[Steve G.]

Re: Gasoline Distillation Curves

So, after 21 postings in this thread and who-knows-how-many-posts in other threads, maybe I can distill this down to its essence:

1. Engine gets hot
2. Fuel gets hot
3. Fuel perc results

I think that pretty much covers it.

Jim

I think it looks a lot more like this.

What we currently hear.

Engine gets to operating temp
Fuel drips down throat of carb
Fuel is percolating
Tie back the heat riser
or
Engine runs poorly when at operating temp
fuel is percing
wire back heat riser

When what perc has been since the 60's looks like:

Fuel leaves tank at a high temp on a sweltering hot day
Fuel picks up more heat from the engine, particularly at idle
fuel percs
hot idle compensator does it's job and operator never knows the fuel was percing

The question was never does fuel perc. The question is, is it any more prevalent today and are the driveability symptoms any different?
Steve

[Jim L.]

Re: Gasoline Distillation Curves

The question is, is it any more prevalent today and are the driveability symptoms any different?

I can't speak to the issues facing carbureted engines. I'm stuck with those Rochester FI thingys.

I can assure you that hot start, hot idle, and low speed driveability have all gotten worse in the 35+ years I've had these contraptions.

[Steve G.]

Re: Gasoline Distillation Curves

I can't speak to the issues facing carbureted engines. I'm stuck with those Rochester FI thingys.

I can assure you that hot start, hot idle, and low speed driveability have all gotten worse in the 35+ years I've had these contraptions.

I have no experience with old gasoline FI systems, so no comment on that.

But I can tell you I have no more difficulty making carberated engines run properly than I did when I got my Journeyman's ticket almost 40 years ago. And that seems to be the case with a great many of the carberated cars out there using the same fuel as the ones diagnosed with "chronic perc".