Sample Questions

Chiefs Engineers Motor EKs


December 2002 Question 3.

With reference to a piston capable of rotating in a cylinder:

  1. sketch a cross section through such a piston; (8)

  2. state the purpose of this facility; (2)

  3. state, with reasons, how rotational movement is restricted: (3)

  4. outline the practical difficulties arising from this design. (3)



Advantages of the rotating piston are:

  • Each stroke of the piston presents a newly wetted portion of the skirt to the side of the liner absorbing the thrust, reducing wear.

  • The piston rings rotate with the piston, thus any blow by through the ring gap is not blowing onto the same portion of the liner, avoiding localised overheating and burning of the LO film.

  • The load on the spherical bearing is uniform, eliminating distortion of the piston.

  • The piston is symmetrical in shape, therefore expansion is uniform; The piston can be manufactured with  smaller clearances, avoiding piston slap at low loads.


Rotational movement of the piston is restricted by the ratchet teeth, which engage with slightly offset pawls. There are an uneven number of teeth and during each swing of the conrod the piston rotates by one half of a tooth pitch, i.e. one tooth /engine revolution, the movement being transmitted by an annular spring. The piston revolves slowly (about 6rpm). This small movement ensures even wear of the piston skirt, helping maintaining it's circular form.


Because of the smaller clearances, if the piston was to stop revolving, then there is a danger of seizure due to uneven expansion. These pistons have proved very reliable in service, however, and maintenance periods for the piston, rotating mechanism and spherical bearing are set at 48 - 60 000 hours. Additional costs must be borne in mind when overhauling the piston due to the cost of replacing the worn ratchet ring and pawls.

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