In the early years of the nineteenth century, steam engines were at work in a variety of practical uses. However, they were still imperfect in many ways. One particular problem were the boilers, that had a tendency to explode, causing injuries and fatalities. Reverend Dr. Robert Stirling, a Scottish clergyman, was concerned about the death toll from exploding boilers. Based on previous work by George Cayley (known for his pioneering work on aeronautics), Stirling filed his patent for a safer engine in 1816. That makes this year the bicentenary of this engine. The Stirling engine has the highest theoretical efficiency of any thermal engine. It is also a relatively simple machine. Unlike other types of engines, there are no valves, and that makes the mechanical design much simpler.
Principles of a Stirling Engine
In a Stirling engine, as with any other heat engine, there are two zones at different temperature, and the working fluid is moved between them to extract work. In the animation at the right, the cold zone is in blue and contains the power piston. The hot zone is in red and contains another piston called the displacer. Heat is added to the hot zone and the gas expands, pushing the power piston to produce work. When the power piston is about to reach the end of its stroke, the displacer moves to the left, moving the gas from the hot zone to the cold one. The gas is cooled and contracts, and the power piston moves to the left. Then the cycle repeats.