The coffee cup engine

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am1jon2
Posts: 1
Joined: Thu Apr 04, 2002 12:01 pm
First Name: Johnny
Last Name: Nipper

The coffee cup engine

Post by am1jon2 »

Dear sirs,
I saw my first coffee cup engine to day for the first time. I thought
it was really fasinating could you please respond with the theory of
how the engine works. We ran the engine with hot coffee and later on
with ice water. The only difference I noticed was that the engine ran
in opposite directions with cold and hot and also the engine seemed
to run faster on the hot thank you for addressing my question

Thank You,
Johnny Nipper
info74
Posts: 70
Joined: Thu Aug 02, 2001 7:41 pm
First Name: Brent
Last Name: Van Arsdell

Response to The coffee cup engine

Post by info74 »

I will assume that you have read the FAQ so let me comment on why the engine runs faster on "hot" than on "cold".

The temperature difference between the hot side and the cold side is greater when running on hot water. Let's use degrees C for convenience. When running on steaming hot water the hot side is perhaps at 100 deg. C and the cold side is perhaps at 23 deg. C. for a difference of 77 deg. C.

When the engine is running on cold it is probable that the cold side of the engine is about 0 deg. C and the warm side is about 23 deg. C for a difference of 23 deg. C.

Now it just so happens that the air inside the engine doesn't know a thing about the degrees that we use to measure it's temperature. It works on ratios of absolute temperatures. Absolute temperature scales are ones like Kelvin or Rankine. Let's do the same thing only this time we will use the Kelvin temperature scale. 100 deg. C = 373 K. and 23 deg. C = 296 K. 373/296 = a ratio of 1.26 running the engine on steaming hot water. Running on melting ice chips the hot side of the engine will be 23 deg. C = 296 K and the cold side at 0 deg. C = 273 K. 296/273 = a ratio of 1.08.

The greater ratio of absolute degrees is the real reason why the engine runs faster on "hot" than on "cold". If you were to cool the cold side so that the engine had a ratio of 1.26 again it would run just as fast on that "cold" source as it does on steaming hot water.

If you want to do something fun try adding 50 deg. C to the hot side vs. substracting 50 deg. C from the cold side and see which one increases the ratio of absolute degrees more. I think you will be surprised.
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