Andrew Porter's point regarding rotary engines with an 'energy transfer efficiency' of 70 per cent (Letters, 12 February) had me puzzled.

If Mr Porter is familiar with Carnot's theorem then it isn't obvious from his description. Converting up and down motion to rotary motion actually works quite well, and having a restricted speed range on its own is not going to affect efficiency.

The implication is that a rotary engine will be unrestricted — either at the bottom or top end of the range. Rather odd.

Robin Herrick

Wick, Caithness