Rich:
Just a few added notes about modern, commercial ignition systems
(label that "Passenger Car/Truck"), with which I have had intimate dealings
(but not relations) thru my engineering company. Although the following has
very little to do with green amplifiers, I think most of the readers here are
latent motorheads. All American boys are.
First, the capacitive discharge system is not widely in use any more.
The modern method is more efficient and controllable, at the expense of
component count. In this method the primary coil current is ramped up to a
target value, frequently in the 10A. range. The current is then interrupted
and dI/dT of the secondary takes over, resulting in a high voltage transient
which ultimately forms the spark. The only control on the transformer after
primary current interruption is a 2-400V zener to protect the transistor
switch, which will see an additional spike on the primary side due to pri-sec
leakage inductance, which does not reach the spark gap. It is all very dI/dT
controlled and models extremely well on paper.
In most curtain high-performance engines (a note on this later) it is
now becomming de rigeur to use COP - one coil-on-plug per cylinder. Hence, it
is really 10 ignition systems if you like to refer to old-style systems.
These coils are each controlled by a channel in the engine control module
(the 'computer') and a current sense resistor is built into the switch
driver, to allow precise ramping up to the target coil current. We are
currently attempting to even eliminate this Rsense and infer the coil
current by volt-second analysis of the primary voltage, which equates to
total flux. This just requires an additional bank of A/D converters, which
are amazingly cheap.
A final note about modern ignition systems. I don't think that people
outside of the automotive engineering world have a feel for (this is not any
kind of putdown - just a matter of the facts) the extent to which modern
automotive engineering is driven by CAFE standards. Within the companies,
upper management will KILL for every tenth of an MPG improvement they can
conjure. This, of course, is not because they are such good citizens and so
concerned about YOUR welfare. It is simply because they want to spend their
MPG 'allotment' on BS that will let them grab more of your bux, like MORE
weight and BIGGER engines.
In particular, one of the newer developments I have seen (and slightly
hands-on with) is 'programmed' spark, in which the total spark energy is
placed within the overall emissions optimization loop. It can readily do so
by means of that target coil rampup current mentioned above. This means being
able to juice up the spark (quite a bit, in some cases) on demand for a short
period (e.g. during sudden acceleration) while still holding average value,
which IS a power loss, low. Also, NOx emissions can be somewhat tweaked by
ignition temperature, which is also related to that initial dose of flux
dumped into each coil on each cycle.
The days of coil-points-resistor-capacitor are long gone, V=LdI/dT
means more than ever.
Eric K8LV
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