Variable Compression, Variable Displacement you decide

[Gary Derian]

Maybe I wasn't clear.  I did not mean the number of stages to be indicative
of efficiency per se, its just that with a given mechanical compression
ratio, adding a stage of expansion means a greater expansion ratio and
therefore more efficiency.

Lets say an engine has a detonation limit at 12:1 compression.  We can build
an atmo 12:1 mechanical compression engine, a 1.5 atmosphere supercharer
boosted engine (7.5 psi) at 8:1 mechanical compression or a 2.0 atmosphere
(15 psi) boost at 6:1 mechanical compression.  These all have 12:1 total
compression.  The first engine has a 12:1 expansion ratio, the second an 8:1
and the third, 6:1.  The lower expansion ratio engines will have less power
and efficiency.

If we used a turbocharger, and the exhaust pressure and boost pressure are
nearly the same, all 3 engines would have 12:1 compression and 12:1
expansion.  Remember, efficiency comes from expansion ratio, compression is
limited by detonation.  To make the same power, the more highly boosted
engines could have less displacement.  At some point the turbocharger makes
more power than the engine.  Why not couple the turbo shaft to the crank and
use it to add power to the engine?  Pretty soon the engine becomes a
combustion chamber for the turbo and we have evolved into a gas turbine
engine.

A turbocharger's power is not free but it comes from exhaust, a high
temperature low pressure source or energy which is not useable any other
way.  There is an increase in backpressure for sure but this takes much less
engine power than a supercharger takes from the crank.  A turbo's turbine
uses expansion at high temperature, an inherently efficient process.

Gary Derian <gderian at cybergate.net>


>> Gary Derian wrote:
>> A supercharged engine has two stages of compression and only one stage of
>> expansion.
>> A turbocharged engine has two stages of compression and two stages of
>> expansion
snip
>I will disagree what what you said
snip
>Also the number of
>compression stages and the number of expansions stages has nothing
>to do with the efficiency
> Roger Heflin