Heyday of the Stirling Hot Air Engine

Unique principle led to rise of the Stirling hot air engine in varied applications

| January 2012

Almost 200 years ago, in 1816, a 26-year-old Presbyterian minister in Scotland patented a revolutionary external combustion engine. The closed-cycle Stirling hot air engine was designed as a safe, economical and efficient alternative to steam. Although the hot air engine never achieved success in industrial applications, Robert Stirling’s invention met the needs of low-power domestic applications from the 1860s to the early 1900s.

How Stirling engines work

The Stirling engine receives its heat supply through the cylinder walls or a heat exchanger in contact with the heat source. Alternately heating and cooling the air causes expansion and contraction, which creates the power stroke that moves the piston. Every Stirling engine has a sealed cylinder with one part hot and the other cold. The working gas inside the engine (which is often air, helium or hydrogen) is moved by a mechanism from the hot side to the cold side. When the gas is on the hot side, it expands and pushes up on a piston. When it moves back to the cold side, it contracts.

Stirling’s “Heat Economizer” (basically, a heat exchanger) is one of the earliest examples of engines operating with the heat regeneration principle. To prevent heat waste, gas flowed through a porous material such as steel wool or tubing.

Stirling’s invention was well ahead of its time. About eight years later, in 1824, Nicolas Leonard Sadi Carnot of France discovered that heat could not be transmitted from a cold object to a warm one, and that the efficiency of an engine relies on the amount of heat it is capable of employing.

Existing materials posed another major problem. Because the power produced by a Stirling engine is directly related to the extremes between heat and cold, extremely hot air was needed for efficient operation. Metals commonly available in the early 1800s, however, could not withstand such heat.

Although a Stirling engine was used as early as 1818 to pump water in a quarry, steam engines (though still considered complex and potentially dangerous) remained the first choice for heavy applications. For light work such as pumping water for domestic use, air for church organs and powering toys, the hot air concept was an intriguing option.