Hi Miles, great post. Could you answer a couple of questions for me?
Any chance you could list (not endorse) some of the brands that make up
the capacitive discharge and Inductive storage ignitions?
You mention that at certain points a multiple spark discharge ignition
has no spark. Doesn't it only take one of the multiple sparks to ignite
the fuel mixture?
Is there a relationship between the energy of the spark and the power
delivered by the combustion of the fuel mixture? My instincts say yes,
but some how it seems there is only so much "power" in the mixture and
once it combusts you've got all the energy your are going to get.
Larry Hoy
>Jack and all:
>
>Here's my 2 cents. I used to design OEM electronic ignitions for
>Ford/Motorola, and have tested numerous aftermarket units under
controlled
>conditions. Before I comment on MSDs, here as some general comments.
>
>1. There are two basic types of electronic ignitions, Capacitive
Discharge
>(CDI), and Inductive Storage. Inductive is the most common and uses
the
>ignition coil for storing the energy to make the spark. CDI uses a
capacitor
>fired through a coil to make a spark. Inductive are generally capable
of
>delivering more total spark energy to the plug. A CDI can fire at a
higher
>voltage than Inductive (which can burn through a fouled plug, for
example),
>but has a MUCH shorter spark time, resulting in overall lower spark
energy.
>Points and condensor are Inductive storage type. Most electronic
ignitions
>are electronic variations of points & condensor that eliminate the
wear-out
>and failure mechanisms of points & condensor.
>
>2. In today's lean-burning, low emissions engines, keeping the spark
fully
>lit in a swirling chamber is the challenge. This is also the same
challenge
>in high compression, high performance engines. The preferred solution
is a
>high ENERGY igntion, as ALL OEM igntions are designed. Inductive
storage is
>the choice for this as all OEM auto manufacturers employ.
>
>3. Multiple spark igntions (like MSD and others) are a trade-off.
Instead
>of delivering one long, high energy spark, they deliver several to many
short
>sparks. Thus, if the spark blows out, there's another spark to relight
the
>mixture, insuring complete combustion. What this means, though, is
that in
>between sparks, there's NO spark, resulting in overall LOWER energy
than a
>continuous, high energy spark that would occur over the same period of
time.
>
>4. The spark energy is the product of the spark voltage times the
spark
>current, times the time. The voltage is essentially controllled by the
spark
>gap, and the current and time by the available energy source (the
inductance
>of the coil and turns ratio, and how fully charged it is). The time is
>limited by the RPM between spark firings. Thus, in a 4 cylinder engine
you
>have twice as much time to fire as in an 8 cylinder engine at the same
RPM.
>
>5. From my experience, multiple spark discharge ignitions can offer
>performance improvements on certain engines where the mixtures are hard
to
>keep lit, due to cylinder head designs, and/or flow/swirl patterns in
the
>cylinders. For most engines, though, a single high energy spark is
more than
>adequate. Problems typically arise at higher rpms where the time is
>decreased and the overall energy from any system decreases due to the
shorter
>time available to deliver full energy to the spark plug.
>
>6. MSD has done an excellent job of promoting their system through big
time
>racers. However, their expensive, heavy, and complex hardware often is
>outperformed on specific applicaitons by much simpler systems. I have
also
>been disappointed by the reliability of their systems (ever see a pro
racer
>using MSDs with less than 2 systems mounted...1 as a backup)?
>
>7. If the addition of an MSD ignition, or any other aftermarket
ignition
>provides a HP increase on a dyno on your engine, then you can be sure
you
>were not having complete combustion previously. Whatever solution
provides
>the best spark ignition solution for your engine application is the one
to
>use. In some cases an MSD might be an improvement, for others, not.
>
>8. There are other factors that also come into play besides the type
of
>igntion, and that includes the ignition advance curve (mechanical, or
>electronic), the type and heatrange of plugs, the type of wires used,
the
>type of ignition coil, and whether or not you have a distributor. All
of
>these factors can impact the amount of energy delivered to the spark
plug for
>ultimately lighting the mixture in the cylinder.
>
>I hope this sheds some light on the subject for you.
>
>Myles H. Kitchen
>1965 Lotus Cortina Mk1 #128
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