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  • #16
    all engines are more efficient at colder temps. has something to do with air density...

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    • #17
      Originally posted by TeeVee View Post
      all engines are more efficient at colder temps. has something to do with air density...
      Quite correct. A car engine requires a defined ratio of fuel to air to operate. In cold weather the air is denser, as is the fuel....so the engine uses less fuel over a given distance/time in cold weather compared to more fuel for the same distance/time in hot weather.

      Anyone got a house in Alaska that they want to sell ?
      If it 'ain't broken........ Don't try to mend it !

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      • #18
        Originally posted by brianw999 View Post
        Quite correct. A car engine requires a defined ratio of fuel to air to operate. In cold weather the air is denser, as is the fuel....so the engine uses less fuel over a given distance/time in cold weather compared to more fuel for the same distance/time in hot weather.

        Anyone got a house in Alaska that they want to sell ?
        I'm not sure how you get from "the air is denser as is the fuel" to "the engine uses less fuel", but in any case that's not exactly the case.

        In an airplane piston engine, the time when the engine is most efficient at a given power is at an altitude where you get that power with the throttle fully open (as the engine loss power capacity with altitude, you have to keep opening the throttle to maintain a given power, once the throttle is fully open you can't hold that power if you keep climbing). And in that condition, the air is LESS dense, not denser, than at sea level (while the temperature is colder because it diminishes with altitude, the pressure diminishes with the altitude faster than the temperature).

        The cold temperature by itself raises the engine efficiency because it enlarges the "step" in temperatures between the "hot source" (in the combustion chamber) and the "cold source" (the ambient).

        As a reference, the max theoretically possible efficiency that can be obtained from a thermal machine without violating the second principle of Thermodynamics is 1-Tc/Th (Tc = coldest temperature along the cycle, Th= hottest temperature along the cycle).

        --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
        --- Defend what you say with arguments, not by imposing your credentials ---

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