Putt-Put

Transportation is an essential part to all of our lives and a majority of our current transportation revolves around a central theme, that of the heat engine.  A heat engine is “any device that changes thermal energy to work” basically a device that takes a hot gas and by lowering its kinetic energy creates work (Giancoli 396).  However, in order to make heat flow a difference in temperature is needed.  According to Newton’s second law, the efficiency of such an engine can only approach that of 100%.  Greater efficiencies are achieved through a greater difference in temperatures and thus a steeper gradient.  French scientist Sadi Carnot, in an attempt to better the efficiency of heat engines delved into the topic to try to find the maximum efficiency an engine could reach.  His research led to the discovery that the efficiency could be written in terms of the operating temperatures with the formula.

e=(T_H-T_L)/T_H=1-T_H/T_L

Where T_H is the high temperature and T_L is the low temperature.  However, while Carnot was working in the lab on his theoretical engines, the real world was improving its engines along the lines of “reducing engine losses that lowered the utilization of the heat energy in the steam for the production of work on the piston” a.k.a. minimizing the exchange of heat where it shouldn’t be exchanged (Moser, encyclopedia Americana, 645). 

  During the early part of the twentieth century, a new children’s toy emerged.  It was called the putt-putt or pop-pop boat.  It was a toy that was powered by a little candle, a boiler, and no moving parts.  It operates by flash boiling the water in the boiler which causes a higher pressure and forces water out of two tubes leading from the boiler out the back of the boat.  As the steam cools it condenses and draws water back into the boiler, which repeats the process.  The process is said to occur about “ten times per second” (Goris).  However, the actual numbers of pulsations do vary from second to second depending on the amount of heat provided by the burner. 

The steam engine design is similar to that of a pulsejet jet engine of an airplane,  which works by letting air into the combustion chamber and the resulting explosion of the fuel air mixture causes the valves on the front of the engine to shut.  The exhaust can only escape through the rear.  Then the lower air pressure in the chamber allows more air in (Landis).  The putt-putt boat is similar in this manner; however, the flash boiling water is the “air” that is let into the boiler.  It can be seen as an enclosed pulsejet with an external heat source.

Both the pulsejet of the aircraft engine and the pulsejet of the putt-putt boat are both characterized as being inefficient and primitive, especially in the case of the putt-putt engine.  This begs the question as to what exactly goes on in the engine to create the performance it does.  There are two types of boilers for such a boat.  One is a “coil boiler” which uses a coil of copper tubing as the boiler; the other type actually has a boiler built onto the ends of two pipes.  The second boiler also can incorporate a flexible metal top to the boiler to aid in propulsion.  In theory, the flexible metal is to pop up with flash boiling and to pop down forcing water out at a greater rate.  Then both boilers condense and refill with water.  This begs the question as to why there is net movement if water is being alternately sucked and blown from the back of the boat.  Jearl Walker, in The Flying Circus of Physics states that the key is that “the water entering the tube comes from a hemisphere of directions, not from a single direction”.  So the boat is not just sucking its water from where it expelled it.