Zachary Carter Wrote: 
There is no question that rotary engines will never equal
reciprocating engines in the realm of combustion efficiency, 
for the reasons others have already stated.

However, it is important to remember that the thermodynamic 
efficiency of the rotary engine cycle is considerably better 
than that of the piston engine cycle.

This is best illustrated with an excerpt from Richard Ansdale's
book "The Wankel RC Engine", A.S. Barnes & Company 1969 page 5.

"Before elaborating essential features of rotary piston 
engines it may be opportune to recall that any heat 
engine is a mechanism capable of converting the 
potential heat energy contained in the fuel supplied 
into measurable mechanical work. Furthermore, it may be 
reiterated that according to the first law of 
thermodynamics energy can neither be created nor 
destroyed. It must, therefore, be true that the energy 
supplied - in the form of fuel, i.e. in thermal 
units/lb., of fuel - is equal to the energy converted 
into mechanical work plus heat rejected with the exhaust 
gases plus heat lost to coolants and the environment. 
Ideally, all the potential heat energy ought to be 
converted into mechanical work unfortunately this is 
impossible as friction must be overcome. Heat is lost by 
radiation and indispensible auxiliaries such as the oil 
pump, generator, fan and water pump must be driven and, 
therefore, require power.

*****************
Consequently it is necessary 
to (differentiate between the thermal efficiency (the 
relationship between the heat converted and the heat 
supplied) and brake thermal efficiency, that is the 
energy available to do external work. [out of the e-shaft!!!]

******************* 
If anything, RC engine designs, for example the 
NSU/Wankel, are better off in this respect as they are 
essentially low friction engines, the number of their 
moving parts being small (Fig. 2.1), and the forces on 
their contact areas, where relative motion occurs, are 
also modest. Further gains are possible if the 
auxiliaries, equally indispensible for both types of 
engines, are specifically designed for RC engines and 
not simply inherited from recipro- cating engine 
practice. Indicated horsepower (i.h.p.) may be defined 
as the work done by the gases on the piston and brake 
horsepower (b.h.p.) as the measurable work transmitted 
by the crankshaft output power."

End of Mr. Ansdale's contribution.

The chart below demonstrates the low friction of the
rotary engine as compared to a four cylinder piston
engine. This is mainly due to a lack of a cam and
valve train.  friction.gif

The chart is from the first "ROTARY ENGINE" book by
Ken Yamamoto published in 1969. Page 93.

Paul Lamar  (Good work Paul. Let's hear it for Paul Lamar)
 
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