Subject: why is some energy put into engine not converted to work?

2nd law of thermodynamics says Q put into a system during one cycle can't 
completely be converted to W, some is exhausted; if there is no friction in 
the engine parts this is still the case.  My text proves why this is true 
by saying if heat transfer was completely converted to work, we could make 
a perfect engine, but it doesn't give a clear reason why this is true.  
Their proofs are rather circular.  Can you explain, on a microscopic 
molecular scale, what is physically happening to the gas molecules in a 
simple engine that would prevent complete conversion of Q to W.  Why does 
some Q have to be exhausted?

Re: why is some energy put into engine not converted to work?