Gasoline Distillation Curves

[Duke W.]

When new, vintage Corvettes were not immune from problems due to excess fuel vaporization from the fuel bowls or percolation (actual boiling - formation of bubbles in the liquid that can be powerful enough to eject liquid fuel from the bowls vents). These events cause an excessively rich mixture and the symptoms may be rough idle and hard hot starting. Excess fuel vaporization can also cause "vapor lock", which is the formation of vapor bubbles in the fuel supply line, and since the fuel pump will not move vapor, the result can be engine stalling due to fuel starvation.

As you can see from the attached graph of distillation curves, modern E10 gasoline has a greater percentage of its components at or above the boiling temperature at 180 deg. F, and fuel lines and carburetor bodies can reach this temperature based on IR gun readings submitted to the TDB in various threads over the years.

For example, at 160F the average modern E10 summer blend has about 40 percent of its components at or above the boiling point compared to only about 20 percent for an average traditional non-ethanol summer blend. The primary culprit that lowers the boiling point in the lower temperature range is ethanol, which boils at 170F.

The "vapor pressure" listed for each blend is Reid vapor pressure (RVP), which is measured at 100F. It's an important criteria for cold starting, the higher the better, but higher RVP also results in more evaporative emissions from fuel supply systems - even from modern vehicles with vapor recovery systems, so over the years it has been lowered, and this is evident in the E10 summer blend higher boiling point at and below the ten percent point.

Modern fuel injection engines will cold start easier with lower RVP than carbureted engines due to the fine atomization achieved with typical port injection pressures of 40-80 psi. Direct injection pressures on the order of 1500+ psi provide even finer atomization.

The chart is extracted from Chevron's "Motor Gasolines Technical Review" document. You can download a pdf of this document from the following link, and I highly recommend you do so and read the entire report:

www.chevron.com/documents/pdf/MotorGasTechReview.pdf

I believe this graph makes clear why excess fuel vaporization/percolation or vapor lock is more of a problem today than it was back in the day.

There are many threads in the archives on how to mitigate this problem such as wiring the heat riser valve open or partially blocking it with shims with small holes to limit heat riser exhaust flow, plugging the "heat slot" under the front of the carburetor on some cast iron manifolds, and insulating fuel lines. These have proven successful in significantly lowering fuel system temperatures that eliminate or significantly mitigate typical excess fuel vaporization/percolation problems like hard hot starting due to heat soak.

Thanks to NCRS President Dr. Mike Ingham, co-author of the referenced document, for providing me a bitmap of the graph and to a TDB member (name withheld at his request) for converting it to a jpg file that was small enough to allow posting on the TDB.

Duke

[Michael J.]

Re: Gasoline distillation curves

Thanks Duke, very helpful to visualize what is going on with modern ethanol laced gasolines in our old cars. I have a friend with a 427 Cobra with Webers, which run at about 3 psi inlet pressures with the regulator they need. He was always having vapor lock issues in the summer. He installed a return line from the back end of the fuel rail to the gas tank, which solved his problems.

[Richard M.]

Re: Gasoline distillation curves

Duke, Impressive document. Thanks for taking the time to compose and post this. I scanned it quickly but will read in more detail when time allows.

Of particular notation was this sequential pair in the Q & A's......

What is oxygenated gasoline?

Oxygenated gasoline is conventional gasoline to which oxygenates have been added toincrease octane and, as required by regulation, to reduce emissions or to comply withrenewable fuel standards. Oxygenated gasoline is required in the U.S. during winter inthose metropolitan areas that do not meet the federal air quality standard for carbonmonoxide (carbon monoxide nonattainment areas). Reformulated gasolines (RFGs) in theU.S. are generally oxygenated year-round, not just during the winter months. Commonoxygenates are ethanol and methyl tertiary butyl ether (MTBE). Other ethers that may beused are tertiary amyl methyl ether (TAME) and ethyl tertiary butyl ether (ETBE).


Will oxygenated gasoline perform as well as conventional gasolinein my vehicle?

Oxygenated gasoline will perform as well as conventional gasolines in modern vehicleswith engine control systems that adjust the air-fuel ratio (A/F). Oxygenated gasoline maycause some driveability problems in carbureted cars and fuel-injected cars without enginecontrol systems because the air-fuel mixture is more fuel lean. The use of oxygenatedgasoline can reduce fuel economy up to 3 percent, depending on the level of oxygenatein the fuel.

Thanks,
Rich

[Steve G.]

Re: Gasoline distillation curves

Duke,

I looked at the charts and the one thing that strikes me is that, except for the very low end (beginning), the ethanol laced fuel maintains a lower boiling point than the reg summer fuel. In fact, lays near flat for 20 to 40% evap. Seems contrary to what people here were saying about their experience with ethanol blends.

I have never disputed possibility of the change in the fuel characteristics over the years. I don't know that, and I still don't know that after studying your charts. We only have the 2008 data set. Nor do I deny that there may be driveability issues associated with the change.

One of the issues I see is the huge disconnect in the diagnosis of the problems posted on here. We have a perfect example in the recent post on here.

Complaint is a Holley carb dripping fuel down the throat and hard hot starts. Diagnosis, percing fuel. Correction, tie back the heat riser, alter timing management and settings. Take other steps to reduce underhood temps.


I see several things wrong with this diagnosis and recommended repair.

Hard starting is caused by the dripping fuel. Dripping fuel issues have been around as long as carburators. How did we go from the checking all the things we checked back in the day to the first assumption that a dripping carb and hard start are the result of the change in the fuel?

I like to do my diagnosis, whenever possible, by starting at one end and working in a logical order towards the other end. We have fuel dripping. Where, specifically, in the throat is it dripping from and how is getting there from the bowl? In the field when there was nothing but carbs, almost all were the result of high fuel level in the bowl, climbing above the spill over height in the throat. Probably 99% of the time. Look down the throat. Is it coming from a point high in the throat? Yes, go to fuel level check. No, it's low in the bowl. Question, how is it getting over the high spill point in the throat to leak down the idle circuit and still not flow out the higher points? Most probably cause with the Holley is leakage at the metering block to body that's allowing fuel leaving low in the bowl to leak across and out the idle circuit without going up and over. If you believe rapidly boiling fuel spraying out the vent into the throat is the cause of the drip, you would have seen that first when you took the air cleaner off. Fuel vapour out the nozzles in a high enough volume to condense and drip? You would see that too. Still not convinced, take the sight plug out and look at the surface of the fuel in the bowl.


What's missing from your graph is the curves from 1967. Fuel has boiled in carbs forever. They knew about it in 1963. They knew it caused a problem. They designed in a compensator. They didn't deal with it by eliminating the heat riser and changing the timing parameters. They knew that the only problem was the venting of fuel into the airstream. The fix, vent it to atmosphere (or charcoal canister) when conditions exist that will cause it to boil. They didn't care if it boiled. They knew it was not causing fuel to drip so they didn't fix that. The only product of the boil was the vapor. They weren't worried about liquid fuel spraying out of their boiling fuel fix, the idle compensators. So how did it go from fuel boiling in the bowl in 1967 that wasn't splashing out the vents to now where we suspect fuel boiling in the bowl will splash out?

They didn't eliminate the heat riser to deal with the boiling fuel because a) it wasn't necessary and b) the heat riser and intake manifold heat were crucial to maintaining consistant A/F ratio across the spectrum of temps the cars were designed to operate. They still are today.

I think it needs more than the boiling points of fuel in 2008 and underhood temp reported. If not for the fact that we know fuel boiled in carbs back in the 60's, it might be enough to cause questions that could lead to the next step, confirmation of the theory. Why don't we try and find pertinent 60's data?

Steve

[John M.]

Re: Gasoline distillation curves

On a quick scan of the graph it looks like there's little difference in the curves at the 10% level for the summer blends. It would be a good plan.

[Michael J.]

Re: Gasoline distillation curves

Correct me if I am wrong, but I think all these gasolines have 10% ethanol or so, summer and winter blends have different additives (winter blend has more butane for instance), but both are ethanol laced since MBTE is no longer available.

[Robert D.]

Re: Gasoline distillation curves

When i used pump gas in my car i had very hard hot starts and fuel would leak down the manifold out of the carbs. if the RVP of summer gas is 7.9 then 50% of the gas would boil off at 200 degrees which is nothing for a big block engine compartment. If 50% of the gas in my lines were vapor and boiling i can understand why these things would happen. i then changed to vp vintage gas which has a RVP of 6.70 and it totally eliminated my hard hot start problems, the fuel stains on my manifold were still there but about 50% less. vp fuels makes a fuel called c-10 which has a RVP of 1.9. this should theoretically eliminate these problems if the boiling point of the fuel is raised above 200 degrees. i spoke with a rep from vp and he said it should only affect cold starts at very low temps 10 degrees F or less which my car will never be driven at temps that low. im going to try it and post my results. i have gone through the carbs rebuilt them and cannot find any other cause of the fuel leakage other than the boil over.
Bob

[Steve G.]

Re: Gasoline distillation curves

Was looking for data on 1960's fuel and came across this:

"Gasoline which is not volatile enough (a commonoccurrence in the 1960s) results in poor cold start and poorwarm up driveability as well as unequal distribution of fuel tothe cylinders in carbureted vehicles. These fuels can alsocontribute to crankcase and combustion chamber depositsas well as spark plug deposits.Gasoline which is too volatile (typical of the mid 1980s),vaporizes too easily and may boil in fuel pumps, lines or incarburetors at high operating temperatures. If too muchvapor is formed, this could cause a decrease in fuel flow tothe engine, resulting in symptoms of vapor lock, includingloss of power, rough engine operation, or complete stoppage.Fuel economy could also deteriorate and evaporativeemissions could increase"

The whole thing is here.

It appears there's much more to it than the distillation curves. Good read here as well.

Steve

[Steve G.]

Re: Gasoline distillation curves

When i used pump gas in my car i had very hard hot starts and fuel would leak down the manifold out of the carbs. if the RVP of summer gas is 7.9 then 50% of the gas would boil off at 200 degrees which is nothing for a big block engine compartment. If 50% of the gas in my lines were vapor and boiling i can understand why these things would happen. i then changed to vp vintage gas which has a RVP of 6.70 and it totally eliminated my hard hot start problems, the fuel stains on my manifold were still there but about 50% less. vp fuels makes a fuel called c-10 which has a RVP of 1.9. this should theoretically eliminate these problems if the boiling point of the fuel is raised above 200 degrees. i spoke with a rep from vp and he said it should only affect cold starts at very low temps 10 degrees F or less which my car will never be driven at temps that low. im going to try it and post my results. i have gone through the carbs rebuilt them and cannot find any other cause of the fuel leakage other than the boil over.
Bob

Robert,

Fix your internal fuel leak then go back and try pump gas. Not every one of those engines drips fuel on pump gas.

Steve

[Domenic T.]

Re: Gasoline distillation curves

This pump fuel thing that is sooooo much of a problem was something I solved years ago!! I have FACTORY timing, no pinging, no dripping, no garage gas smell, just the rated HP, but less money in my wallet. NO varnish problems, no other map curves to figure out, just simple settings out of the original MANUAL, and the gas will stay fresh for ever. NO heart burn either till I get my AV gas bill.
I would wish all could easily get it because it makes a driver into a runner...

DOM

[Robert D.]

Re: Gasoline distillation curves

I cant find any internal leaks where would it be leaking from? timing is where it should be carbs are newly rebuilt there are no warped surfaces the butterfly rods have no extra play the floats are set properly when cold, take the site plug off when hot and fuel pours out of the sight hole. spend 15 20 minutes staring at the carbs after i shut the car off hot no leaks. come back the next day and there is dry fuel on the manifold. if it is leaking where do you think its coming from i have looked at this in a systemic way any ideas what im missing?

[Duke W.]

Re: Gasoline distillation curves

Duke,

I looked at the charts and the one thing that strikes me is that, except for the very low end (beginning), the ethanol laced fuel maintains a lower boiling point than the reg summer fuel. In fact, lays near flat for 20 to 40% evap. Seems contrary to what people here were saying about their experience with ethanol blends.

You're not interpreting the distillation curves properly. At any given temperature, the greater the percentage of the fuel's components that are near, at, or above their boiling points, the more rapid the rate of vaporization from the surface of the liquid as I illustrated at a carburetor temperature of 160F in my initial post.

Fuel vapor is heavier than air, so it will displace air in the carburetor and continue to displace air from the air cleaner interior until it escapes to the surrounding area and creates the "fuel stink" that owners report after bringing the car back to the garage from a drive on a warm to hot day. If a hot restart is attempted after a few minutes the engine just ingests fuel vapor and considerable cranking time is required to purge excess vapor and ingest enough oxygen to form a combustible mixture, and then rough operation will usually occur for several seconds before the excess vapor is purged and a more normal A/F ratio is established.

Violent percolation that ejects liquid fuel from the bowl vents and/or booster venturi nozzles is the extreme case, but a high rate of vaporization from the fuel's surface without actual boiling in the bowl is all that is required to create the problems.

The long-dash line of the summer non-oxygenated gasoline distillation curve is representative of average distillation curves from decades ago includilng the sixties.

The first link you posted is a good tutorial on gasoline, but it does not address the difference in distillation curves between traditional non-ethanol gasoline and current E10, nor does it discuss the driveability problems that can arise from E10 on vintage cars with non pressurzied fuel systems.

The second link is a good tutorial on refining, but does not address difference distillation characteristics of modern E10 compared to traditional non-ethanol gasoline blends.

Both are good references for someone who wants to take a deep dive into the technology, but neither helps in understanding current driveability problems on vintage cars with non-pressurized fuel systems.

That's the beauty of the Chevron graph that compares the distillation curves of modern E10 with traditional non-ethanol gasoline, like what was typical in the sixties. There's no need to complicate it. The answer is there at a glance in a couple of simple curves, and the archives are rich will simple methods to mitigate the problem.

Duke

[John H.]

Re: Gasoline distillation curves

I cant find any internal leaks where would it be leaking from? timing is where it should be carbs are newly rebuilt there are no warped surfaces the butterfly rods have no extra play the floats are set properly when cold, take the site plug off when hot and fuel pours out of the sight hole. spend 15 20 minutes staring at the carbs after i shut the car off hot no leaks. come back the next day and there is dry fuel on the manifold. if it is leaking where do you think its coming from i have looked at this in a systemic way any ideas what im missing?

Robert -

There's no mystery here - hot-soak-after-shutdown percolation has been going on for about 80 years, but is more prevalent now due to higher underhood temperatures and the lower boiling point of today's E10 ethanol-laced gasoline. Fuel boiling in the float bowl is forced out either through the vent or the boosters, dribbles on the primary throttle shaft and butterflies, and some wicks out through the throttle shaft bushings and drips on the intake manifold. Anything you can do to insulate the carburetor from convection shutdown heat will help - there are lots of techniques discussed in the archives.

[Steve G.]

Re: Gasoline distillation curves

Robert -

There's no mystery here - hot-soak-after-shutdown percolation has been going on for about 80 years, but is more prevalent now due to higher underhood temperatures and the lower boiling point of today's E10 ethanol-laced gasoline. Fuel boiling in the float bowl is forced out either through the vent or the boosters, dribbles on the primary throttle shaft and butterflies, and some wicks out through the throttle shaft bushings and drips on the intake manifold. Anything you can do to insulate the carburetor from convection shutdown heat will help - there are lots of techniques discussed in the archives.

John,
There's a lot of mystery here. How heat soak forces fuel through the nozzles with no pressure behind it, how it gets out the vents with no one seeing it, how underhood temps of 50 year old cars got hotter today (global warming accepted) than they were when they were new. Why the idle compensators that were designed to overcome the effects of fuel boiling in the bowl no longer work. This article that contends volatility of gasoline today is lower than it was in the mid 80's further adds to the mystery.
In my opinion, heat soak is more often an excuse than it is a cause.
Steve.

[Paul J.]

Re: Gasoline distillation curves

...
What is oxygenated gasoline?

Oxygenated gasoline is conventional gasoline to which oxygenates have been added toincrease octane and, as required by regulation, to reduce emissions or to comply withrenewable fuel standards. Oxygenated gasoline is required in the U.S. during winter inthose metropolitan areas that do not meet the federal air quality standard for carbonmonoxide (carbon monoxide nonattainment areas). Reformulated gasolines (RFGs) in theU.S. are generally oxygenated year-round, not just during the winter months. Commonoxygenates are ethanol and methyl tertiary butyl ether (MTBE). Other ethers that may beused are tertiary amyl methyl ether (TAME) and ethyl tertiary butyl ether (ETBE)....

Thanks,
Rich

[/QUOTE]

Hi Rich,

There's a lot of history in that first question, and it has little to do with what is happening now. There are very few CO nonattainment areas, so the winter requirements for oxygenated fuels have pretty much been superseded by the summer requirements concerning ozone nonattainment areas. This has occurred over time, as has the ban on MTBE. To further complicate matters, a few years ago Obama mandated the use of ethanol blends to help reduce dependency on foreign oil. Last year, he also directed the EPA to move forward on establishing new, lower ozone ambient standards. This will place many more areas of the country in ozone nonattainment, and require the use of oxygenated fuels year round. Despite studies showing the inefficiency and problems with ethanol, many more of us will be using it in years to come.

Paul