Whippet was built for the new Class B introduced in the 1980 Hot Air Engine Competition , to be judged on power to weight ratio, and weight was eventually reduced to 11ozs - the massive looking bed is actually a very light tinplate shell.
   However, in the Competition it developed 4.7 watts (207 ft/lbs/min) and with a swept volume of 5cc, it actually won the award for specific power.   It is thought that this is still the record for a non pressurised air engine of this size. (?)

  The design is really only a 'sports' version of Phoelix, my 6cc dynamo driving engine, the proportions of which had been very loosely based on a design in a schools metalwork handbook.  The principal changes were the fitting of ball races throughout, a reduction in wall chamber thickness of both air chamber and displacer together with an increase in length of both.  A direct port between chamber and cylinder replaced the sharp angled passage in Phoelix, piston stroke was reduced to reduce swept volume to 5cc and the piston was machined from a single piece of Rulon.  The displacer rod bushes are 'sintered' from some of the swarf produced when machining the piston.  Reciprocating masses were reduced to the minimum and the structural design improved to give greater stiffness.
  To ensure consistent performance electric heating was used and my previous experience with Phoelix enabled me to wind a nichrome coil drawing almost exactly 100watts (when hot) from a 24volt transformer. (thereby ruling it out of Class B with a total weight of 11 ozs + transformer + the National Grid!)

  The two blue caps on the front of the air chamber are covering two valves.  The bottom one is a normal snifter whilst the top one is a non-return valve, used to take peak pressure readings.  This peak, actually 12.5psi when snifting, can then be used to calculate the IMEP and, by comparing this with the measured output, give an indication of the engine's mechanical losses.
 The, fully run in, engine was actually developing 4.9 watts during later tests with a measured torque of 4oz/inches at 1660rpm.  The calculated torque was 5.13oz/inches, giving a mechanical effy of 78%.  Nevertheless, the overall efficiency, with 100watts input, was a mere 4.9%.

  In an attempt to increase power to 5 watts the electric heater was removed and the hot cap run at bright red using a ring type gas burner.  However it stubbornly refused to go beyond its previous best and we must therefore assume that the 100watt electric heater was running it at a similar temperature..

  Following all this excitement the engine was stripped and the cause of a light rustling sound coming from the air chamber found to be due to the mild steel displacer scaling.  Since the displacer had originally been made with a wall thickness of only 0.004" this was deemed to be sufficient cause for retirement; a small firebox was fitted to take a nite-lite candle for demonstration runs and Whippet was put out to grass.

  And he occasionally enjoys a cup of tea ------