[Diy_efi] EGT detonation sensor (was Re: Timing Advance Curve?)

[Phil Hunter]

> A poor mans detonation sensor for each cylinder could be as
> simple as an EGT sensor correctly positioned without the
> mechanical noise issue prevalent with peizo sensors,
> the cost of pyrometers with associated electronics could
> well be less than the highest spec piezo sensors and
> simultaneuously provide exhaust gas temperature - nice :-)

I'm not sure how to read this, but if you're thinking of using
just one EGT sensor for several cylinders, I don't see how, given
typical EGT probe response times of about a second. Even if
you meant 1 probe per cylinder, you still may be "asking for
trouble" depending...

Consider the hypothetical situation of a step change in temp from
0 to 1000*. You might think that if the response time is 1 second,
in 1 second you'd read 1000*, but that's not the case. The
definition for one "tao" is 63.2% of the temp difference, so
after 1 second you'd only read 632*, the next second would be
63% of the difference (1000 - 632), so after 2 seconds you'd read
632 + 232 = 864*. After 3 tao's you'd be within 10% (actually 5%)
at 950*, but it'd take 5 seconds to get a reading accurate within
1% at 993*.

Folks familiar w/ resistor-capacitor networks will recognize the
curve immediately.

Closer to "real world", a step change from 900 to 1000*,
immediately you'd be within 10%, after only 1 tao you'd be within
5% at 963*, in 3 tao's you'd be within 1%.

So we can see that the bigger the delta in temp, the bigger the
error and the longer you have to wait to get an accurate reading.
I don't have typical temp deltas for detonation, they s/b large
and easy to detect but even if you detected knock within 1 second,
that'd still be 50 "hammer" strokes per cylinder for an engine
spinning 6K RPM.

My point of all this is that there may be more to getting good
readings from EGT thermocouples than folks may have thought of,
a lot depends on "where you've been" and "where you're going",
absolutes, deltas and taos. ;)

Personally, ion-sensing is still the way to go IMO.

HTH's
philh
(digest)

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