The example to the left could represent an Otto-cycle or a Lullen-cycle just prior to the fuel/air mix being ignited. Whether the remainder of the cycle is the same or not makes little difference to the piston at this moment. The question becomes "what does it cost in terms of energy used in achieving this situation for power delivery?".
    This is basically the same question as "What's a better means to compress air? A piston compressor or a centrifugal compressor that needs to push it's charge of air through a port and past a valve.

   Aside from just the efficiency of compressing air there is the fact that an Otto-cycle engine devotes half of it's rotation (two out of four strokes) in receiving air and then compressing it. 

   Also, To get to this point a typical four stroke engine will have mixed some of the new fuel/air mix with that of the previous cycle. Given the fact that a Lullen-cycle motor has no combustion chamber each fuel/air charge is pure and undiluted.
A simplified illustration showing a common event in both an Otto-cycle and a Lullen-cycle.

© 2010 - 2011 by Walter E. Parrish. All Rights Reserved