In the world today, there are many different types of engines that we use to power devices from our cell phones to our cars. The Stirling engine is just an example of one of those types of engines. Today, Stirling engines are not used in cars but only in highly specialized settings, such as submarines or some specific auxiliary power generators.

Robert Stirling invented the Stirling engine in 1816. Stirling was a Scottish minister and responsible for the first external combustion engine. Stirling engines use something called the The Stirling cycle to function.  In the Stirling cylce, all fumes are contained inside the engine and never leave and does not use any explosions to generate energy, this makes the system very quiet. The cycle also uses an external heat source to run, such as solar energy or even energy from decaying plants. In looking at the Stirling cycle, the key to the whole system is that there is a fixed amount of gas sealed inside of the engine at all times. The engine consists of two chambers. When that external heat is applied to the gas in one chamber, the pressure will change and a piston will be forced down which does work. The other chamber is a cooled chamber, and as the heated piston moves up the right one moves down. The heated gas makes its way into the cooled chamber and the gas is cooled and the pressure is lowered. In that cooled chamber, the gas begins to compress and generate heat and is removed by the cooling source of the chamber, a source such as ice. Then the cold piston moves up and the hot piston moves back down, making the gas heat back up and build pressure where the cycle repeats.

A stirling engine pushes hot gas into a cold chamber, depressing pistons.

A common application of the Stirling engine is in submarines and other underwater vessels; due to their lack of emissions and that they do expel a lot of noise, ideal for the highly pressurized and enclosed nature of a submarine. General Motors as well as other Swedish companies and researchers have been involved in advancements in Stirling engine uses.

The Peltier device is another form of engine without internal combustion. In 1834 the scientist Peltier discovered that if one were take a thermocouple (two dissimilar metals that are connected at a junction) and apply a voltage, there will be a difference in temperature between the junctions. This creates a small heat-pump or a thermo-electric cooler. The Peltier device’s main usage is to turn thermal energy into electrical energy. This process can be responsible for turning the heat from our exhaust systems into energy, something that would be very beneficial to the environment. Today, Peltier devices can be used to make mini fans, air conditioners and heaters.

An in-depth look at how a Peltier device works.

It is clear that these devices should be used more often in our world today considering the state of the environment. The fact that these devices do not expel much or any harmful gasses into our atmosphere would seem like a huge draw.
References

http://auto.howstuffworks.com/stirling-engine2.htm

http://en.wikipedia.org/wiki/Applications_of_the_Stirling_engine

http://www.heatsink-guide.com/peltier