17 JAN. 2017 LEARNING physics. heat engine.

17 JAN. 2017
LEARNING physics.
heat engine.

In thermodynamics, a heat engine is a system that converts heat or thermal energy—and chemical energy—to mechanical energy, which can then be used to do mechanical work.
It does this by bringing a working substance from a higher state temperature to a lower state temperature.

Heat engine - Wikipedia

https://en.wikipedia.org/wiki/Heat_engine

Engines in theory

The pioneers of engines were engineers, not scientists. Newcomen and Watt were hands-on, practical "doers" rather than head-scratching, theoretical thinkers. It wasn't until Frenchman Nicolas Sadi Carnot (1796–1832) came along in 1824—well over a century after Newcomen built his first steam engine—that any attempt was made to understand the theory of how engines worked and how they could be improved from a truly scientific perspective. Carnot was interested in figuring out how engines could be made more efficient (in other words, how more energy could be obtained from the same amount of fuel). Instead of tinkering with a real steam engine and trying to improve it by trial and error (the kind of approach Watt had taken with Newcomen's engine), he made himself a theoretical engine—on paper—and played around with math instead.
The Carnot heat engine is a fairly simple mathematical model of how the best possible piston and cylinder engine could operate in theory, by endlessly repeating four steps now called the Carnot cycle. We're not going to go into the theory here, or the math (if you're interested, see NASA's Carnot Cycle page and the excellent Heat Engines: the Carnot Cycle page by Michael Fowler, which has a superb flash animation).
What is worth noting is the conclusion Carnot reached: the efficiency of an engine (real or theoretical) depends on the maximum and minimum temperatures between which it operates. Making the temperature of the fluid inside the cylinder higher at the start of the cycle makes it more efficient; making the temperature lower at the end of the cycle also makes it more efficient. In other words, a really efficient heat engine operates between the greatest possible temperature difference. That's why real engines—in cars, trucks, jet planes, and space rockets—work at such enormously high temperatures (and why they have to be built from high-temperature materials such as alloys and ceramics). It's also why things like steam turbines in power plants have to use cooling towers to cool their steam down as much as possible: that's how they can get the most energy from the steam and produce the most electricity.
Photo: Steam engines are inherently inefficient. Carnot's work tells us that, for maximum efficiency, the steam in an engine like this needs to be superheated (so it's above its usual boiling point of 100°C) and then allowed to expand and cool down as much as possible in the cylinders.