Thursday, April 22, 2010

Engines and Heat

Engines and Heat

Any engine that converts heat to mechanical motion is considered a Heat Engine. An obvious example of this is the internal combustion engine. In an internal combustion engine gasoline and air are injected into a chamber known as the cylinder. Here a spark is introduced inducing a conflagration that turns the mixture into hot gas. The high pressure from the gas forces the movement of a piston, which further supplies motion to gears that ultimately turn the wheels.

Another heat engine that is not so obvious is a nuclear engine. Some military submarines and aircraft carriers run on nuclear power. The basis of the extraction of the energy from the system is actually quite elementary despite the perception. Nuclear power is simply used to heat water and steam. This steam drives, or spins a turbine, which is not much more then a sophisticated fan. The mechanical motion of the turbine is used to spin the prop or propeller of the ship, along with providing electrical energy.

Most of us are familiar with the heat that radiates from engines. It is this very heat that is an indicator of the inefficiency of an engine. Although heat is converted to mechanical much if not the majority of the heat produced in the combustion of the gasoline and air is conducted away to surroundings and is lost or wasted. Typically with automobiles 20%-30% of the heat produced is applied toward useful motion, the rest is wasted heat energy. So much heat energy is wasted in fact that special mechanisms are in place in a standard gasoline engine to help manage and remove the excess heat without the engine destroying itself from its own inefficiencies.

This brings us to an interesting tidbit on thermodynamics. There is a set limit to the efficiency of any heat engine.

As was noted in previous blog posts the temperature of any substance is directly related to kinetic energy of its atoms and molecules. So everything even at room temperature has a considerable amount of energy stored in it. The trick in extracting the energy for use. As it turns out the thermal energy of water at room temperature is about .04 Cal/g which is 5 times the average energy in a battery. So why not extract the energy from water and liberate the world if the reliance on fossil fuels? Well the trouble lies in one of the greatest discoveries in the theory of heat, that is that heat can only be extracted when flowing from a hot region to a cold region. And it is defined by the following equation:


Efficiency is always less then or equal to the solution to that equation when extracting energy from heat. (This does not apply to chemical energy from batteries or solar energy from solar cells).

So to answer the question of why we can’t save the world by extracting the existing energy from room temperature water to supply power we can apply the equation and concepts above. First as we noted energy can only be extracted from heat flowing from a hot region to a colder region. And the equation defines that process. So looking at the equation we can see the Temperature^cold is divided by the Temperature^hot both in Kelvin. Hypothetically if room temperature was 300 Kelvin and the water was at room temperature it too would be 300 Kelvin. Applying those temperatures to the equation we get:


So the efficiency would be zero. The efficiency of extraction of heat energy is dependent on the difference in temperature.

This may lead one to ask, if the temperature of the T^hot was very high, even if the T^cold was room temperature wouldn’t that make for a very efficient engine, surely higher then 20-30% we get from modern car engines? The answer is yes, but that bridge has been crossed. Prior to the 70’s engines such as that of the Volkswagen Beetle in fact ran at a very high temperature greatly increasing the efficiency of the engine. At the time it was also believed to be a benefit as nearly all the carbon was turned into carbon dioxide, making the engine appear to run clean. Unfortunately what was not understood until later was that the high temperatures of these engines turned N2O4 into two atoms of NO2, or Nitrous Oxide, which is a toxic component of smog. Regulation finally regulated the maximum NO2 output of automobile engines and as a result the temperature of engines today run as high as possible while still maintaining suitable levels of NO2 output, but at the expense of decreased efficiency.

Next Blog: Refrigeration, Heat Pumps, and Heat Flow.

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