Thermo 101, Part II (Boost vs Compression Ratio)

[rdh@sli.com (Robert D. Houk)]

Regarding the previous thread on Boost, Compression Ratio, etc, and so
forth: I managed to finally remember where I had read some interesting
stuff.

The magazine "Turbo and Hi-Tech Performance", November 1994 issue, went
into some details on "more power". (Actually, "More Power!" might well
be the magazine's sub-title, if it had one.)

Herewith, some extractions.

Enjoy (heh heh heh)

					-RDH

Typical auto engines operate in the 20 to 30 percent thermal efficiency
range. This means 70 to 80 percent of the heat energy available to make
power is lost either out the exhaust or through the emission of heat
through the engine or radiator.

[...]

Compression ratio increases are aimed at improving the thermal effi-
ciency of an engine. One of the rules most often quoted is: expect a
4 percent increase in horsepower for every full point of increase to
the compression ratio. More exactly,

   delta power =
	100 * (SQRT (1 - (1 / rnew ** .4)) / (1 - (1 / rorig ** .4)))

where "rnew" and "rorig" are the new and original compression ratios.

Example: going from 8.5:1 to 11.0:1 yields 7.3 percent power increase.

[...]

Preignition occurs when the gasoline and air mixture ignites without
the aid of a spark due to excessive pressures and residual heat in the
cylinder and combustion chamber. This causes an early and rapid burn
of the fuel, which not only hurts performance, but can cause severe
engine damage.

[...]

>From a thermodynamic standpoint, turbocharging is a great solution.
Forcing compressed air into the engine increases the engine's effective
compression ratio (see chart). This can also be accomplished by means
of supercharging where the engine uses some horsepower to drive "the
blower". With a turbocharger, the energy present in the exhaust gas,
which would have otherwise been lost to the atmosphere, is used to
power a turbine. Often people have referred to turbocharging as free
horsepower, but this is only partially true. On turbocharged appli-
cations, the turbine of the turbo acts as a restriction to exhaust
flow, killing off some horsepower in itself. The beauty of either a
supercharged or turbocharged application is that they produce much
more horsepower than they use.

			Effective Compression Ratio

   static     |			boost level
  comression  |
   ratio      |	2psi	4psi	6psi	8psi	10psi	12psi	14psi	16psi
  ------------+--------------------------------------------------------------
	      |
    7.0 : 1   |	 8.0	 8.9	 9.9	10.8	11.8	12.7	13.6	14.5
	      |
    7.5 : 1   |	 8.5	 9.5	10.6	11.6	12.6	13.6	14.6	15.7
	      |
    8.0 : 1   |	 9.1	10.2	11.3	12.4	13.4	14.5	15.6	16.7
	      |
    8.5 : 1   |	 9.7	10.8	12.0	13.1	14.3	15.4	16.6	17.8
	      |
    9.0 : 1   |	10.2	11.4	12.7	13.9	15.1	16.3	17.6	18.8
	      |
    9.5 : 1   |	10.8	12.1	13.4	14.7	16.0	17.3	18.5	19.8

         [ Chart data courtesy of Weiand Automotive, Supercharger Division ]

Note that "12.0:1" is the generally-recognized maximum safe operating
zone for vehicles running "moderate timing with 92-octane pump gas".

...

In addition to increasing compression ratio or using a form of forced
induction [...] engineer's rule of thumb is for every 11 degrees Fahren-
heit (6 degrees Celsius) you can reduce the inlet air temperature you
will obtain a one-percent increase in horsepower.