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I would like to know how much thermal energy is converted to kinetic energy in a steam engine, or a more efficient means if available.

I have done some research and found out that Carnot made some equations to determine efficiency. It is also mentioned that as part of the Carnot cycle it is important that the cold temperature reservoir be colder, but I don't understand how that would help increase efficiency of the conversion of thermal energy to kinetic energy. Here is a paragraph taken from Wikipedia at http://en.wikipedia.org/wiki/Carnot_cycle

"Lowering the temperature of the cold reservoir will have more effect on the ceiling efficiency of a heat engine than raising the temperature of the hot reservoir by the same amount."

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  • $\begingroup$ Fixed title since the question actually has nothing to do with steam engines. $\endgroup$ – Olin Lathrop Dec 9 '13 at 20:14
  • $\begingroup$ A steam engine does not work on the Carnot cycle. A Carnot cycle is an idealized, impractical cycle consisting of two reversible isothermal processes and two isentropic processes. $\endgroup$ – Pranav Hosangadi Dec 10 '13 at 0:06
  • $\begingroup$ As for your question "how much thermal energy is converted to kinetic energy in a steam engine": This isn't an objective answer. It depends on the cycle employed in the steam engine. $\endgroup$ – Pranav Hosangadi Dec 10 '13 at 0:07
  • $\begingroup$ I realize there are various types of steam engines, but picking anyone at all as an example would be an acceptable answer for me as a base answer for me to build on. $\endgroup$ – Klik Dec 10 '13 at 2:10
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Look up something called the Carnot efficiency. That is the theoretical limit of how effecient any heat engine can be at converting heat power to some other form. This maximum possible efficiency is

   Carnot efficiency = Tdiff / Thot = (Thot - Tcold) / Thot

By simple 8th grade algebra, you can see that you get a higher value by decreasing Tcold (the cold side temperature) than by increasing Thot (the hot side temperature) by the same amount.

For example, the Carnot efficiency of 100°C to 0°C is 100°K / 373°K = 26.8%. Adding 10 degrees to the hot side you get 110°K / 383&degK = 28.7%, but decreasing the cold side by the same 10 degrees yields 110°K / 373°K = 29.5%.

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  • $\begingroup$ Thanks for the answer! I did come across the Carnot efficiency formula, but what I find difficult to understand is why temperature difference is important. Consider the following: when steam from a boiler goes into a steam engine and pushes a piston it is doing work. How is it able to do so? It is the pressure difference which allows the steam to push on the piston. I understand pressure is related to temperature, but isn't the pressure difference the actual driving factor? $\endgroup$ – Klik Dec 9 '13 at 21:03
  • $\begingroup$ But the steam can only get out of the way of the piston on the return stroke, if it is vented or condensed in a cold reservoir. If the steam outlet ran directly back into the boiler, the engine wouldn't run... $\endgroup$ – DJohnM Dec 9 '13 at 21:26

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