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OT Heat content of modern petrol v 1930 petrol

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  • OT Heat content of modern petrol v 1930 petrol

    I inhabit a couple of old car forums, and every now and then someone mutters about modern petrol (gas) burning a lot hotter than say 1930s formulations.

    My gut feeling is that that's not right, or if they are there's not much in it. Google wasn't much help on this one, has anyone got the full story?

  • #2
    Lets say soemone was born in 1930, they would be 80 now, so how the h*ll would they know about the calorific content of the fuel?

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    • #3
      Well, I can't speak to your question specifically, but I used to have a 1931 Hupmobile. There was a data plate inside, on the driver's side, just above the windshield as well as on the metal spark plug loom under the hood.

      It read, "Use Ethyl Gasoline - Minimum 74 Octane".

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      • #4
        Originally posted by billtr96sn
        Lets say soemone was born in 1930, they would be 80 now, so how the h*ll would they know about the calorific content of the fuel?
        Do people automatically become stupid at some age? If so, what is that age? Why would an 80 year old not be qualified to comment on the caloric content of 1930's gasoline?

        As to the original question, don't confuse heat content and octane rating, they are not the same. I would suspect that refining in the 30's would be cruder than today's resulting in gasoline closer to kerosene and diesel fuels and actually have a higher caloric content, but a lower octane rating.
        Jim H.

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        • #5
          Since diesel fuel doesn't contain octane it doesn't have an octane rating. It has a cetane rating, not the same thing. Diesel has a lot lower autoignition temperature than gasoline which is why it can be used in a compression ignition engine.

          "Burning hotter" and the total amount of heat produced are not the same thing either. Heat is the amount of kinetic energy contained by a volume of matter while temperature is the velocity of the molecules regardless of the amount of heat.

          Old fuels contained a significant amount of benzene which is no longer permitted in gasoline above very small amounts. The makeup of gasoline now is specified by it's combustion properties rather than by what chemical compounds it contains. Gasoline is not a single chemical, it is a blend of as many as 10 to 20 different chemicals most of which are in the Alkane group.

          The total heat content will vary slightly but not by a great amount. Even the difference between gasoline and diesel on average is just around 5 to 10 percent depending on the specific gravity of the fuels. That is entirely overwhelmed by the actual efficiency of the engine that burns the fuel and a diesel engine is more efficient than a gas engine in large part because of the reduction in pumping losses.
          Free software for calculating bolt circles and similar: Click Here

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          • #6
            Not to mention a good ol helping of Tetra-ethyl lead, or Ethyl as they preferred to call it, Octane ranking [RON]changed when this additive was used, an increase as preignition was not an issue.
            Invented By the same Nutter who later gave us CFCs, Thomas Midgley Jr, an asset to the environment
            mark

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            • #7
              Diesel has a lot lower autoignition temperature than gasoline which is why it can be used in a compression ignition engine.
              Octane and cetane has about the same ignition temperature, which is at about 210°C. In fact, octane is a bit lower.
              You could well use gasoline in a Diesel engine. Problem is, it's burning too fast.
              And gasoline is also self igniting, it's called knocking. It doesn't take to much to get knocking, modern engines are very close to it. That's why they do have knock sensors.


              Nick

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              • #8
                There is a lot of misinformation around about "Octane number". The fuel companies use "Hi-test", "Premium", and "Super" which adds to the confusion.

                Octane number is a measure of 'Resistance to ignition' and is based on percentage of Octane in an Octane/Heptane mixture. So 80% Octane/20% Heptane is the standard used to determine 80 Octane fuel. There is no need for the actual fuel to contain any Octane or Heptane in order to have an octane number. Diesel for example, is roughly 50. The value for diesel is of no use but it has a number none the less. The only reason to use High octane fuel is to prevent ignition prior to the spark which is bad.

                The above is good enough for cocktail parties where people sometimes claim they run Premium every 5th tank to keep their engine new or some such rubbish.

                I'm sure some of you can poke holes in it but for most of us, I think it's close enough to the truth so as to be used to counter grossly incorrect statements about "Hi octane".
                Mike

                My Dad always said, "If you want people to do things for you on the farm, you have to buy a machine they can sit on that does most of the work."

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                • #9
                  Originally posted by Evan
                  ..... because of the reduction in pumping losses.
                  "Pumping losses", what's that? Are we just saying that diesels, as a rule, turn at lower rpm's?
                  Lynn (Huntsville, AL)

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                  • #10
                    and a diesel engine is more efficient than a gas engine in large part because of the reduction in pumping losses.
                    That is almost dead wrong.
                    Diesel's efficency comes most part from the high compression and the (ideally) isobaric expansion cycle.


                    Nick

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                    • #11
                      Thanks for the input so far guys.

                      My thoughts are that a litre/gallon of 1930's petrol/gasoline type hydrocarbons are basically similar to todays hydrocarbons and would contain more or less the same number of BTUs

                      While todays petrol is more highly refined, and contains a lot more volatiles then it used to because we mainly run sealed fuel systems today, I don't know that those factors have increased the BTU capacity of the fuel or indeed raised combustion chamber temperatures by a significant amount.

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                      • #12
                        I know that gas burns my hands a lot more now than in the '30s

                        I thought diesel was more effecient because it contains more BTUs than gasoline.

                        Best regards, Jack

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                        • #13
                          1 Gal Propane = 91,600 Btu's

                          Gal Gasoline (mid grade) = 125,000 Btu's

                          1 Gal of #2 Fuel Oil or Diesel = 139,000 Btu's

                          Cetane number refers to a diesel fuels ignition delay, or how quickly it ignites after it is injected into the combustion chamber.

                          Nick and Evan are both partially right.
                          Running a constant vacuum requires a significant amount of pumping energy. Energy that is wasted. And as Nick said, the high compression of a diesel engine is also responsible for it's higher fuel efficiency compared to a gasoline fueled engine. These two factors plus the high btu content of diesel fuel are all responsible for a diesel engine's ability to deliver lower fuel consumption figures compared to that of a gasoline.

                          I too will go with what JC Hannum said about the gasoline of the 30's having a very slightly higher btu content because of the fuel was not as highly refined then as it is now.
                          Home, down in the valley behind the Red Angus
                          Bad Decisions Make Good Stories​

                          Location: British Columbia

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                          • #14
                            Running a constant vacuum requires a significant amount of pumping energy.
                            That would imply, that the Miller cycle, or the Atkinson-cycle (the modern one, not the initial one) is much more efficient. But it aint.

                            Also, the pumping vacuum has the wanted side-effect of cooling down the inhaled air. Temperature-difference is what counts in efficency.


                            Nick

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                            • #15
                              Nick, the engine's pumping loses of generating a vacuum consumes significantly more energy than the very slight gains from the cooling effect of the vacuum.
                              I will agree though that the high compression ratio of a diesel engine is probably the greatest factor contributing to it's efficiency, but pumping loses and fuel btu content are very definite contributors to the efficiency increase over that of a gasoline engine.


                              Edited for spelling
                              Last edited by Willy; 04-15-2010, 04:36 PM.
                              Home, down in the valley behind the Red Angus
                              Bad Decisions Make Good Stories​

                              Location: British Columbia

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