Editing Stirling engine (section)

=== Size and temperature ===
Very low-power engines have been built that run on a temperature difference of as little as 0.5 K.<ref name="Senft-1996" /> A ''displacer-type Stirling engine'' has one piston and one displacer. A temperature difference is required between the top and bottom of the large cylinder to run the engine. In the case of the ''low-temperature-difference'' (LTD) Stirling engine, the temperature difference between one's hand and the surrounding air can be enough to run the engine.<ref name="Romanelli-2020" /> The power piston in the displacer-type Stirling engine is tightly sealed and is controlled to move up and down as the gas inside expands. The displacer, on the other hand, is very loosely fitted so that air can move freely between the hot and cold sections of the engine as the piston moves up and down. The displacer moves up and down to cause most of the gas in the displacer cylinder to be either heated, or cooled.{{citation needed|date=July 2020}}

Stirling engines, especially those that run on small temperature differentials, are quite large for the amount of power that they produce (i.e., they have low [[power density|specific power]]). This is primarily due to the heat transfer coefficient of gaseous convection, which limits the [[heat flux]] that can be attained in a typical cold heat exchanger to about 500&nbsp;W/(m<sup>2</sup>·K), and in a hot heat exchanger to about 500–5000&nbsp;W/(m<sup>2</sup>·K).<ref name="Organ-1997" /> Compared with internal combustion engines, this makes it more challenging for the engine designer to transfer heat into and out of the working gas. Because of the [[thermal efficiency]] the required heat transfer grows with lower temperature difference, and the heat exchanger surface (and cost) for 1&nbsp;kW output grows with (1/ΔT)<sup>2</sup>. Therefore, the specific cost of very low temperature difference engines is very high. Increasing the temperature differential and/or pressure allows Stirling engines to produce more power, assuming the heat exchangers are designed for the increased heat load, and can deliver the convected heat flux necessary.

A Stirling engine cannot start instantly; it literally needs to "warm up". This is true of all external combustion engines, but the warm up time may be longer for Stirlings than for others of this type such as [[steam engine]]s. Stirling engines are best used as constant speed engines.

Power output of a Stirling tends to be constant and to adjust it can sometimes require careful design and additional mechanisms. Typically, changes in output are achieved by varying the displacement of the engine (often through use of a [[swashplate]] [[crankshaft]] arrangement), or by changing the quantity of working fluid, or by altering the piston/displacer phase angle, or in some cases simply by altering the engine load. This property is less of a drawback in hybrid electric propulsion or "base load" utility generation where constant power output is actually desirable.