Editing Stirling engine (section)

=== Lubricants and friction ===
[[File:STM Stirling Generator set.jpg|thumb|A modern Stirling engine and generator set with 55 kW electrical output, for combined heat and power applications.]]

At high temperatures and pressures, the oxygen in air-pressurized crankcases, or in the working gas of [[hot air engines]], can combine with the engine's lubricating oil and explode. At least one person has died in such an explosion.<ref name="Hargreaves" /> Lubricants can also clog heat exchangers, especially the regenerator. For these reasons, designers prefer non-lubricated, low-[[coefficient of friction]] materials (such as [[Rulon (plastic)|rulon]] or [[graphite]]), with low [[normal force]]s on the moving parts, especially for sliding seals. Some designs avoid sliding surfaces altogether by using diaphragms for sealed pistons. These are some of the factors that allow Stirling engines to have lower maintenance requirements and longer life than internal-combustion engines.{{citation needed|date=July 2020}}