A useful idea

We ask a chicken-and-egg question about a textbook staple.

Our navigator recently referred to a handbook of Marine Engineering he has in his library, now rather old but full of information.  He wondered about some aspects of reciprocating steam engines, the kind that use (or used) pistons instead of turbines.  He never actually sailed on a piston ship, and in fact they were generally obsolete by the time he went to sea.  The chapter on reciprocating engines was really a holdover from a much earlier edition, kept on (we suppose) out of some sense of completeness.  It wouldn’t be found in any recent edition of the handbook.

But pistons abound in books on thermodynamics, even in the latest editions.  These theoretical versions (without friction, weight, leakage, etc.) are perhaps the clearest way to demonstrate many of the basic concepts of the subject, and indeed a number of other ideas in physics and chemistry.  They are simple enough to grasp easily, yet as useful to science as their real counterparts once were to the Industrial Revolution.  It occurred to us to ask a question: did the theoretical piston appear and persist because of its scientific merit, or because it was actually in use in the engines that scientists were trying to analyze?

There were other mechanical engines in widespread use when steam was first put to work.  Windmills are a form of turbine, and water mills similarly use the motion of a fluid to do something useful.  But the motion of turbulent fluids is a mathematically difficult problem even now, well beyond the capabilities of early nineteenth-century scientists to solve accurately; so these engines made do with practical rules of thumb.  When steam came along the piston became much more prominent, and the improvement of a heat-powered system where a gas expanded in a cylinder occupied the minds of some of the greatest scientists of the day.  Well, the historical development of thermodynamics was long and messy, and we won’t go into it here.  But we do note that the theoretical piston played an important part.

Did the real piston inspire the theoretical piston, and thus give rise to the science of thermodynamics; or did the theoretical piston maintain the dominance of the real piston, because it was a system easy to analyze and improve?  (Perhaps both.)  A piston engine requires quite a bit of accessory gear to turn back-and-forth motion into the round-and-round kind that we mostly desire.  A turbine by its nature is spinning.  On the other hand, turbines are more difficult and complicated to make, and matching turbine speed with propeller speed was for long a major engineering problem.

Even now, over a century after turbines became practical at sea, the piston still rules the land in the form of oil-powered vehicles; and almost all the ships at sea use diesel-powered pistons.  These are more complicated to analyze thermodynamically, though.  And we expect it will be some time before we see a thermodynamics text based on analyzing an electric car.