gcc, 68K and ignition advance

Tue Nov 29 17:18:33 GMT 1994

Wouter de Waal <wrm at aztec.co.za> Wrote:
| 
| 
| OK, I (finally) got gcc to work. I compiled it under DJGPP, which means
| that it runs on a PC. So now I can develop code on my PC, then stick
| it in my 68K system. After I went through all the pain of building
| gcc, I heard that pre-ported versions are available from ftp.pooh.com.
| So if anybody is interested, you can (definitely) get the stuff from me,
| but it might not work 100% yet, or you can (maybe) get the stuff off the
| net, but I assume that if it's posted, it'll work.

Post this, and send it to fridman at cpsc.ucalgary.ca so he can put it on the WWW 
server.  Am I glad someone finally did this!  I've been meaning to do it under 
linux, from the original source (very easy), but my linux machine is down for 
the time being.  I would have thought it would be more difficult under DOS.  
Anyway, I don't think cross-compilers are pre-built anywhere, and if they are, 
they probably don't exist for DOS, since the FSF code doesn't support DOS.  
You might want to send a message to the GCC groups and 68k groups, I know when 
I was looking for a cross compiler like this a year ago, several people wished 
for a GCC cross compiler under DOS.

is there really a site called ftp.pooh.com?

| The first thing that I would like to start playing with is ignition
| timing. I've been thinking about why ignition advance is required, and I
| can't seem to make my understanding tie up with what is commonly done.
| So would some of you that have thought about this more thoroughly please
| enlighten me?
| 
| My thoughts are as follows:
| 
| * The air/fuel mix takes a constant _time_ to burn.

I thought this also, another poster here said that burn time decreases with 
rpm, but *ignition* time is constant.  So varying the timing is to account for 
the mixture ignition time, not burn time.  Assuming that ignition time is 
constant (and short) compared with burn time, that explains why not as much 
advance is required as it would seem below.

<advance calculations deleted>

| * And the next tricky point - vacuum advance. The vacuum is an indication
|   of engine load. But why is it a good thing to advance the timing when
|   the engine is loaded? By how much should the timing typically be advanced
|   for a given load? (And I mean _typically_, I know that the Bosch systems
|   amongst others use complicated, dyno-derived maps. However, I am more
|   interested in deriving the kind of curve a standard distributor gives.)

I think the purpose of vacuum advance is to advance the timing when the engine 
is *not* under load.  ie when the engine is idling, lots of vacuum, lots of 
advance.  Hit the gas, and the engine would start knocking, so to keep it from 
knocking the vacuum advance retards some.  When you get to cruising speed, 
vacuum returns, the ignition is advanced some more.

I'm not positive about the vacuum advance arguments, so feel free to correct 
me if this is not a correct description.0

| 
| OK. If that's not enough, I have plenty more questions :-)

Lets have them.... :-)

Ok, I looked in my email archive and found this response, if anyone is 
interested.  It's a personal email that I am reposting without permission, but 
I think it's OK....  The heywood book, i believe, says that ignition time is 
constant and burn time varies.

---begin include
Hi Steve, 

Somehow, I doubt this will get thru, as your address looks
"unfriendly" to my mailer (my guess).....

Heywood is an excellent Combustion reference.  The title
is "Internal Combustion Engine Fundamnetals" by John Heywood (MIT).

Two other references:

Very general, with an excellent chapter or two on combustion, and
lots of other source of energy discussion.  Light Thermodynamics:
DaRosa, "Fundamentals of Energy Processes"

Good automotive (general) reference.  Has everything from properties
of auto-glass, to gasoline specs (originally written in German, so
many of the specs are DIN):  "Bosch Automotive Handbook", R. Bosch.

I believe all three are currently in print, and should be available
(order or in stock) in any good tech book store.

The mailing list sounds like a good idea.  Unfortunately, when ever
I join a mailing list, I get flooded, and I end up missing an
important message from my boss or something, and I get teethmarks
on my ass.  

But feel free to forward any question, and I can try to BS my way
through it  :^)

-Eric

PS:  The books I mentioned are not cheap.  Heywood runs in the 
$75 range, Bosch in the $45 range, and DaRosa, I'm not sure,
but probably somewhere in between.
--end include