Turbo boost versus compression ratio ?

[Chris Conlon]

At 04:26 AM 5/10/00, Mike from West Australia wrote:

>ie. He sounds like a very practical guy but is apt to say boost is
>a substitute for low compresson and things such as 'why lower the
>compression ratio when you only need to add an intercooler or something
>else instead of touching the CR at all !


Here are some quick details which I hope help explain some
of this line of thinking.  Disclaimer: These are far from the
only factors which influence detonation... but they are
IMHO the ones most directly related to charge temps and
CR, the topics of discussion.

Compression heats the charge; a higher CR heats the
charge more than a lower CR.

A high initial charge temperature (before compression)
*also* increases the deltaT. In other words, the compression
stroke in a 10:1 CR engine might heat a cold charge by X
degrees, but that same engine would heat a hotter charge
by 1.5X degrees. (Numbers not drawn to scale.)


It's generally true that most flammables and explosives:

ignite more easily;
burn with a higher flame velocity;
detonate more easily (where applicable);
and detonate with a higher velocity (where applicable)

when the temperature is higher.


A higher flame velocity may contribute to a more likely
transition from deflagration (burning) to detonation, via
an increased chance of forming a shock wave. (In general
a shock wave is a very important if not totally essential
component of the transition from deflagration to
detonation. E.g. many explosives can be burned safely in
the open, but will detonate under impact, even heat-free
impact as from a hammer.)


By way of example, a common chem lab rule of thumb is that
the rate of a chemical reaction *doubles* for each 10C
increase in temperature. (For the detailed rule of 8 fingers
and 2 thumbs, see the Arrhenius Law.)  It's not too hard
to see, then, that a higher initial temp, which leads to
a disproportionately higher post-compression temp, can
be *dramatically* more likely to verge into detonation
than an initially-colder charge. Say the intake charge is
40C hotter, and the post-compression charge 70C hotter,
than the hypothetical super intercooled version. You're
now looking at a 128x faster reaction rate. (Again,
numbers not to scale but you get the idea.)


Now all that having been said... unless you have some way
of *really* cooling the intake charge, you can probably
still make lots more HP by dropping the CR some and really
rocking on the boost.


   Hope this helps,
   Chris C.

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