Variable Compression, Variable Displacement you decide

[Gary Derian]

	An internal combustion engine gets its power by expanding gases at a higher
temperature than it compresses them.  More compression means a greater
difference between compression temperature and expansion temperature which
means more power.  A spark ignition engine is limited by the onset of
detonation which limits compression.  Efficiency is determined by the
expansion ratio.  The greater the expansion, the more energy is extracted
from the hot gasses.  The ultimate in efficiency would be to expand the hot
gasses all the way to atmospheric pressure.  In a conventional piston
engine, compression ratio and expansion ratio are about equal.  It doesn't
have to be this way (Miller Cycle).  Limiting compression to the detonation
limit yet using a high expansion ratio is a good way to improve efficiency.

	A supercharged engine has two stages of compression and only one stage of
expansion.  This is not efficient under boost.  The displacement of the
supercharger determines the actual engine size (in theory).  Therefore a
supercharged engine is like a non-supercharged engine with higher
displacement and compression but no higher expansion.  These engines are not
efficient while running under boost but they make lots of power.  If the
superchargers are bypassed during cruise, then you do have, in effect, a
variable displacement engine.  While bypassed, this engine gains back its
efficiency.  This works well if most of the time, the engine operates at
part throttle and only uses boost for quick passing maneuvers.

	A turbocharged engine has two stages of compression and two stages of
expansion.  This is efficient under boost and turbocharged engines can make
lots more power than supercharged engines.  Again, boosting pressure has the
same effect as increasing engine displacement.

	Because the mechanical compression ratio in either super or turbo charged
engines is limited due to detonation, either engine looses some efficiency
at part throttle to a naturally aspirated engine.  What is best for a
particular car is not so straightforward, though.  There are many other
factors to consider.

Gary Derian <gderian at cybergate.net>

> -----Original Message-----
> From: owner-diy_efi at efi332.eng.ohio-state.edu
> [mailto:owner-diy_efi at efi332.eng.ohio-state.edu]On Behalf Of bruce
> plecan
> Sent: Tuesday, February 17, 1998 2:31 PM
> To: diy_efi at efi332.eng.ohio-state.edu
> Subject: Re: Variable Compression, Variable Displacement you decide
>
>
> I'd like to hear from a ME type, and know which/why is most
> accurate to describe a super/turbocharge engine.  While
> compression ratio is a mechanical calculation, and the
> engine size doesn't increase due to "boost" the VE does,
> so what term most accurately describes what happens to the
> engine?.  A rather well known engine builder keeps calling them
> variable compression motors, is this accurate?.
>   Also what is the limit for VE?.  Seems like a turbo street engine
> ala Buick GN gets to 120% rather easily?.
>   Would this VE be as easy as using a bar conversion and saying
> at 1.5 bar boost it would be 150% VE (turbo application),  Yes we
> are ignoring intake air heating, but just as a place to start thinking
> about what's going on.
>   Something else I'm curious about is once you exceed peak
> torque at WOT and are approaching max HP does the pulse
> length for the injector really need to be much longer?.  Or
> since there are more of them per amount of time, things just
> kinda work out?.
> Bruce   The guy with the cone shaped hat, and chin strap
>             way too tight.
>