At 05:14 PM 8/19/98 EDT, DANMAS@aol.com wrote:
>.... Distributed capacitance is real, .... the output stage in the radar
systems I used to maintain for the Air Force had neither descrete
capacitors nor inductors. .... They machined a cavity into a solid chunk of
steel (or perhaps aluminum, I don't remember now) - the walls of the cavity
formed the capacitor, and the surface of the cavity formed the inductors. ....
Sounds to me like they built a capacitor and an inductor. I wouldn't call
that distributed capacitance, it's definitely all in one identifiable place.
>.... Suppose you have a capacitor between the secondary winding and the
case of the coil (you do, as a matter of fact - I just don't know the value
of it). The case is grounded, so you have another capacitor, in addition to
the one we normally think of, directly between ground and the secondary. If
this capacitance is large enough, wouldn't it (chuckle, chuckle) make a
nice return path from the plugs to the secondary?
>
>.... I have read, though, in at least one text book, references to this
capacitance as a return path. ....
>
>.... According to Herbert E. Ellinger, .... the spark is initiated by
capacitor discharge, and maintained by the magnetic field decay. .... he
very clearly and precisely identified this capacitance as the (chuckle,
chuckle) distributed capacitance, mostly between the windings of the
secondary. ....
>
>Maybe I'm wrong (or maybe Ellinger is wrong), but it seems to me that if
this capacitance is large enough to generate a spark, the coil-to-ground
capacitance just may be large enough to provide a return path for the
secondary.
As others here have recently stated, the engine will run nicely without the
coil being grounded. Apparently this capacitance path from secondary to
ground is not too significant.
>.... According to P. M. Heldt, .... some of the earlier ignition coils had
the secondary winding connected directly to ground, rather than to the
points as in modern systems, ie the coil had four terminals rather than three.
I'm glad you have found a reference to the four terminal coil not needing a
capacitor. As I stater earlier, the modern waste spark ignition system
uses a four terminal coil where the secondary is connected to ground on
both sides, through two spark plugs.
>If capacitor discharge is vital to producing a spark, then the capacitance
would have to be (chuckle, chuckle) distributed capacitance, as there would
be no other capacitance in the circuit.
Capacitor discharge is not vital to producing a spark with a four wire
coil, but it does provide a path to complete the HT circuit with a three
wire coil. Three wire coils are (or were) popular because they are more
efficient than a four wire coil (Read that "Less material means lower cost,
and nobody cares about the electrical efficiency".) Four wire coils have
made a come back because they can eliminate the need for a mechanical
distributor, and multiple coils is apparently cheaper than one coil and a
distributor. Otherwise, I wouldn't know what motivates car manufacturers
to use the waste spark system.
Barney Gaylord
1958 MGA with an attitude
|