The Ergosphere
Saturday, April 10, 2010

Kitson-Still: still obsolete

One of the favorite hobby-horses of a regular GCC commenter is the Kitson-Still locomotive (here's the Wikipedia page).  Built by Kitson and Company in 1926, it used the Still double-acting steam/diesel engine.  One major advantage was the ability of the steam section to apply full torque at zero speed; this allowed the designers to avoid a multi-speed gearbox and clutch, which made the system simpler, cheaper and more rugged.

In normal use, steam was applied to start the locomotive and bring it to a speed of 5 MPH (30 engine RPM).  At 30 RPM steam was turned off and the diesel injection system was activated, which brought the much greater thermal efficiency of the diesel engine to bear.  Exhaust from the diesel engine ran through fire tubes in the boiler, maintaining a head of steam for auxiliary loads and heating of the train in cold weather.  Steam could be applied together with diesel operation for maximum performance.

Despite its mechanical and thermodynamic merits, the Kitson-Still locomotive was not a commercial success.  Some of this may have been due to the design compromises required by the dual-cycle engine (the diesel would probably have operated better at 500-600 RPM instead of the 30-360 RPM set by the single gear ratio), but it doesn't appear to me that this was a major factor.  Kitson and Company went bust in 1934, and the Kitson-Still locomotive was scrapped by the company's receivers.

It is worth taking a look at the problems the Kitson-Still aimed to solve, and why they did not make it successful even at the time:

  1. The thermal efficiency of the diesel system was balanced by the cheapness of coal in pure steam locomotives.
  2. Starting a locomotive under diesel power became easier as transmissions improved.
  3. Eliminating the steam cycle gave operational advantages; it removed the requirement to warm up the boiler and pre-heat the cylinders before starting.
  4. The advent of diesel-electric locomotives also eliminated clutches and multi-speed gearboxes, but did not add the mechanical and operational complexity of an additional steam cycle.  Diesel-electrics are now almost universal.

Adding a bottoming cycle (e.g. exhaust-heat recovery making steam for a combined-cycle system) is a proven way to increase thermal efficiency, but it runs into diminishing returns.  If the bottoming cycle forces design compromises in the engine itself, most of the advantages can be lost while adding difficulties in manufacture, operations and maintenance.  The steam cycles in combined-cycle powerplants restrict the control and lose some gas-turbine efficiency due to increased back pressure, but overall they are well-suited to base-load operation.  Organic Rankine turbines for marine diesels don't alter the main engine and appear to be suitable for automated control; further, the working fluids aren't susceptible to freezing.  Thermoelectric mufflers will need attention to air flow and heat shielding, but the engine won't notice them.  The compromises forced on the Kitson-Still were much larger, and remain roughly the same while the rest of the world has moved on.  It's the past, not the future.

Postscript Monday 4/12:  Somehow, 2 days after I wrote it, this post is now the SIXTH in Google's list of results for "kitson still engine".  I'm not quite sure what to think about this; either it means that posts on the subject are few and far between (the "small pond" effect), or Pagerank's results can be very capricious.

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Obsolete, past? Why? This is an example of an early combined cycle system, built from modules available and well proven at the time such as steam engine, boiler, and diesel engine.
Yes, "early", "at the time".  The labor required to operate one would be prohibitive today, and I suspect that the emissions of lubricating oil alone would make it impossible to certify today.

Thermoelectrics and organic Rankine cycles can turn exhaust heat into work without any of the design compromises or operational complications of a double-acting steam-diesel.  Even the six-stroke diesel engine appears simpler (though I'd worry about steam blow-by causing difficulties with oil contamination).  The Still engine isn't coming back.
No, it is not coming back. It just proves that combined cycle is actually a very old idea. Only now can we truly see how far this concept has evolved, and really benefit from the higher efficiency of CC power plants.

EP, I have started a small blog myself on Feel free to visit it at
if you have the chance
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