The free-piston engine is in practice restricted to the two-stroke operating principle, since a power stroke is required every fore-and-aft cycle.
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These engines are mainly of the dual piston type, giving a compact unit with high power-to-weight ratio. A challenge with this design is to find an electric machine with sufficiently low weight, and control challenges in the form of high cycle-to-cycle variations have been reported for dual piston engines.