The Stirling Engine


(YouTube Link)


The above video shows Stirling engine enthusiasts motoring around on the River Thames. This engine, invented in 1816 by Robert Sterling, consists of, at minimum, two pistons, one of which is heated. Karim Nice of How Stuff Works explains the cycle:

1. Heat is added to the gas inside the heated cylinder (left), causing pressure to build. This forces the piston to move down. This is the part of the Stirling cycle that does the work.

2. The left piston moves up while the right piston moves down. This pushes the hot gas into the cooled cylinder, which quickly cools the gas to the temperature of the cooling source, lowering its pressure. This makes it easier to compress the gas in the next part of the cycle.

3. The piston in the cooled cylinder (right) starts to compress the gas. Heat generated by this compression is removed by the cooling source.

4. The right piston moves up while the left piston moves down. This forces the gas into the heated cylinder, where it quickly heats up, building pressure, at which point the cycle repeats.


The Stirling engine never caught on as well as internal combustion engines did, but has in recent years caught the attention of solar energy developers.

Information Link and Fanclub Link via View from the Porch

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Some possibilities where the Sterling engine would be great in a car are to power the water pump, cabin refrigeration (two sterling engines with one powering the other to generate cold air), and maybe the alternator. All of this off of the car's engine waste heat with the idea of getting these items off of the engine directly to help improve gas mileage.

Sterling engines where once used to power fans off of burning kerosene.
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So... the linked article doesn't actually contain muchinformation about how a stirling engine is *different* from a regular car engine (Otto or Diesel cycle). It mostly just explains how a heat engine works in general.

The big reasons we're not using Stirling engines everywhere: they can't change their output as quickly as a car engine needs to, and (because we've spent less time and money developing them) they cost more to build. To change that equation you need 1) fuel to be more expensive (stirling engines are sometimes used in solar thermal power, and may end up in fossil fuel power plants if coal and natural gas get expensive enough), and/or 2) a fairly constant power requirement (which would work in a serial hybrid like the Volt where the fuel charges a battery that powers the car).

Actually, a constant power requirement is good for *any* engine because it can always run at it's most efficient rate of output.
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You can build a single piston sterling engine... a heat source at the base and a cooling source at the top. The single piston isn't as effective but still possible.
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...Or even more simply, a mass of air is moved between a hot chamber and a cold chamber, causing the air to expand and contract, which runs the engine.
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