<VV> BMEP was octane

[Padgett]

>With lower octane fuel (say 87 from "regular"
>gas) the electronic control system would limit the opening of the throttle 
>valve
>so that it was somewhat less then 100% and the resulting chamber charge would
>be less than maximum.
>        But how does the manifold vacuum come into this setup?

Good question. At WOT (or relatively small throttle openings from idle) a 
normally aspirated engine has the manifold vaccuum go to zero and you get 
maximum charge filling in the cyl. (see Brake Mean Effective Pressure).

Now generally we see a correlation between compression ratio and octane 
requirement. Higher comperssion requires higher octane but what we are 
really saying is that higher BMEP requires higher octane (for a given 
chamber geometry - best is a spherical chamber with the spark in the 
middle). So if we can limit the BMEP, we can also limit the octane 
requirement because it is acting like a lower compression engine.

Now this is also going to require precise control of the ignition because 
hi-test burns slower than regular (and has a higher flash point - see also 
"flame propagation rate") but the whole idea is to have max chamber 
pressure occur at about 7-10 degrees ATDC. *Everything* in engine controls 
is a kludge to achieve this and is why for lower octane gas you retard the 
spark (burns faster so same peak pressure point). Important point is that 
if you retard the spark too much, the pressure peak occurs later and is not 
as effective. So for the same load, you use more gas and temperatures go 
up. When everything is in sync you use the least amount of gas for a given 
load and mpg goes up while temps go down.

Can get into some really complicated math here since as the rpm goes up, 
the BMEP goes down (usually, are some special cases) so the advance really 
needs to go up with RPM but is not a linear relationship - why the fuel and 
advance maps are three dimensional in modern cars.

Shortening things by a couple of hundred pages (see "The High Speed 
Internal Combustion Engine" by Sir H. R. Ricardo), this is basically why if 
you can control manifold vacuum so that it never drops to zero, you can run 
a lower octane fuel. Is a much better control than just retarding the spark.

Padgett