This is a category for internal combustion engine designs which have been proposed and have received sufficient development and/or publicity to warrant a Wikipedia article, but which have not yet found any commercial application.
They may be at many stages of development. They may be patented and/or have been demonstrated. Development may be under way, abandoned, or being actively promoted.
They may describe mechanisms that have other current or proposed uses, for example as steam engines or as pumps.
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