[6pack] Ignition (Loooong

[Richard Lindsay]

Hello Friends,
   Here is the next piece that I wrote a bit ago. I hope you find them
informative.

   In this work, the function of the ignition condenser is debatable. Some
have considered these words apocryphal. Some have offered alternative
discussion. And some have found them based in elementary physics. You
decide.

Rick

Ignition System

   The ignition system on older British sports cars is comprised of a
number of components, all of which need to be in top operating condition
for maximum performance. Working backwards from the engine, these parts are
the spark plugs, plug wires, distributor, coil, and in some cases, a
ballast resistor.

   Spark plugs, or 'sparking plugs' as they are quoted in the British car
manuals, are the components that ignite the charge. An electric spark from
the center electrode to ground heats the charge, ionizing it and starting
oxidation. The 'flame front' then travels across the combustion chamber. As
it does so, the MEP, or Mean Effective Pressure, increases to a maximum,
pushing the piston down the bore. More on that later but first, here is the
anatomy of a spark(ing) plug.

   The plug is an electrical device. It has a central electrode extending
from a connection at the outside, to a tip inside the combustion chamber.
It is supported by and sealed within a ceramic insulator. The exposed
length of the ceramic insulator sets the 'heat range' of the plug. Short
insulators conduct heat away from the tip more quickly. Long ones cool more
slowly. Too cold and deposits are left on the tip and insulator. Too hot
and the tip material may burn and remain too hot, causing pre-ignition of
the charge. The engine designers understood the heat characteristics of the
various engines and have specified the ideal heat range for plugs in each
application.

   Plug choices are like oil choices. Every manufacturer has a marketing
campaign claiming their plug is the best. And like the oil additive
industry, there are a number of added-on doodads to plug tip design.

   The most basic plug electrode design, and the design for which these
older engines were intended, is a plug with a copper or copper plated
central electrode and a steel ground or earth electrode extending from the
edge of the threads, going up and over the central tip. The gap between the
tip and the ground will be discussed later.

   Arguably, the biggest improvement in spark plug design is the
application of rare metals to the tip design. Platinum and iridium are used
because they withstand high heat better than copper. Copper tends to melt
and burn away with use. These high-end plugs will probably outlast the
engine, unless contaminated.

   Other manufacturers, eager to capture market share, have engineered new
plug designs utilizing multiple ground electrodes in various orientations,
each claiming superior performance, greater power and extended life. They
are a waste of time and money in this author's opinion. Premium platinum or
iridium plugs are the author’s plugs of choice. Old design copper electrode
plugs are perfectly acceptable too, but expect to re-gap or change the
plugs as part of the routine maintenance.
   The 'reach' of a plug is the length of the threaded portion of the body.
It is the bit that screws into the head. The reach must match the engine
maker's design. Too short and the spark doesn't occur within the combustion
chamber. Too long and the plug extends too far into the combustion chamber,
inviting physical damage. Plug reach must match the engine designer's
specification.
   Regardless of the type of plug chosen, assuming the reach and heat range
are correct, the plug threads should be lubricated before installation.
Copper infused thread lubricant, applied sparingly to the new plug threads,
is ideal. This is very important for engines with alloy heads! The plug's
steel threads can easily damage the head’s softer alloy plug-hole threads.
This can happen either by physical damage, such as cross-threading or
over-tightening, or by the action of dissimilar metals placed in contact.
Thread lubricant minimizes head damage. The author uses lubricant even for
iron heads since the copper particles marginally improve heat flow, thereby
cooling the plug. Perhaps that's over-kill but it certainly makes plug
changes easier.
   Before installing the spark plugs, the electrode gap should be set. A
typical gap is 0.025", or 0.64mm, and this is often the gap found, straight
out of the plug's box. However, the engine maker’s specification should be
followed and the gap set properly. In all cases, the gap is set by bending
the ground electrode, never by touching the central electrode. That element
is surrounded by brittle ceramic and may be damaged.
   The physicist in the author requires that he write a little bit about
the anatomy of the spark. The spark is comprised of two phenomena; the
capacitive component that lasts only a few microseconds, and the much
longer electrical component that produces the big, fat, hot spark. It is
argued that a properly tuned and warmed up engine can run on the capacitive
component only, but they never do. Rather, the high voltage electrical
spark is needed to heat the charge, thereby vaporizing any wetted charge on
the electrodes, and further promoting ionization of the charge between the
electrodes. And this is where proper electrode gap becomes important. A
wider gap provides a hotter spark as is required to adequately heat the
charge, especially for cold starts. However, a wider gap requires more
secondary voltage to spark. This topic leads us to the ignition coil.
   High voltage from the coil's secondary winding travels through the
distributor and the plug wires, out to the spark plugs. The distributor and
plug wires will be discussed later but first, here is a discussion of the
coil. The ignition coil is an autoformer. That is, it is a transformer with
one side of the primary winding and one side of the secondary winding
connected together. This side of the circuit is connected to the coil's ( -
) or 'CB' post. The later descriptor means Contact Block and is a better
designation since this is also the connection for older positive earth cars
like the MG T-series of cars. In that case, the ( - ) marking is a misnomer.

   A transformer is a voltage multiplier. Any deeper discussion of
electromagnetic theory is not helpful for our purposes. Stated more simply,
the coil's secondary contains thousands more turns of wire than does the
primary. Both are wound around a soft iron core. The ratio of turns,
primary to secondary, defines how much the primary voltage is 'stepped up'.
An automobile coil typically produces about 20,000 volts at the secondary,
from its 12 volts primary. This is the voltage that is fed to the plugs.

   Some ignition designs incorporate a ballast resistor in series with the
coil's primary circuit. The reason for this design will become apparent
shortly. A conventional coil is designed for 12 volts, applied directly to
the primary. The primary coil winding is typically about 3 ohms resistance.
So at 12 volts, a current of 4 amps, flows. ( I = E /R ) All is well and
good for normal operation but what about start up? During cold weather the
starter has to work harder to spin the engine. And when doing so, it
demands more current from the battery. In fact, the battery voltage is
pulled down to 9 volts or less during these cold starts. We've all seen the
lights dim during starting. Unfortunately, reduced primary voltage also
results in reduced secondary voltage, thereby weakening the spark right
when it needs to be the strongest! The ballasted coil design addresses this
problem.

   Under normal operation, a ballasted ignition works just like a regular
ignition, but the components are different. Designs vary but basically, a
ballasted coil's primary resistance is on the order of 1.6 ohms, rather
than the 3 ohms of a conventional coil. Inserted in the primary circuit
path is a ballast resistor of about 1.4 ohms. The sum of the primary coil
winding and the ballast resistor in series, is still 3 ohms. The ignition
current is the same 4 amps as with a conventional coil. The same hot spark
is provided when the engine is running. The tricky and useful bit of a
ballasted ignition happens at cold start.

   When starting, voltage from the ignition switch is applied directly to
the coil, bypassing the ballast resistor. Assuming the starter is pulling
the available battery voltage down to about 9 volts, the 1.6 ohm coil now
sees 9 volts at it's primary, for a current of 5.6 amps! So even with the
starter pulling down the available voltage, the ignition still provides a
hotter spark for easy starting. Of course, the numbers quoted here are
generic and they vary with manufacturer, but the concept is the same.
Ballasted ignitions are implemented to facilitate cold engine starts.
   The ignition's secondary voltage is not continuous. Rather, it is a
pulsed system operating under the control of the distributor. This device
is actually two devices integrated into one case. The high voltage or 'high
tension' stated in old-speak, is distributed to the spark plugs, via the
plug wires, by this part of the distributor, thus the name. The other part
of the distributor pulses and times the spark by controlling the coil's
primary circuit. Both are now discussed.
   High voltage distribution is achieved by a rotating contactor, aptly
termed the 'rotor'. This device is fed secondary voltage at it's center via
a contact in the distributor cap. Located around the inside of the cap are
electrical contacts leading to the connectors on the outside and to which
plug wires, and therefore, the plugs are connected. The distributor rotates
at one half of the engine crankshaft speed because of the engine's Otto
Cycle or 4-stroke design. The rotor spins around the inside of the cap
allowing a precisely timed spark to occur between the rotor and the
appropriate contact in the cap. That voltage then fires the spark plug on
the correct cylinder at the optimal time to ignite the charge.
   The other function of the distributor is to pulse the coil's primary at
the right times, and to provide a circuit path for the coil's secondary.
The first part of this process is rather well understood by most mechanics,
but the later bit remains shrouded in mystery. Here is an explanation.
   Inside of the distributor is a cam with the same number of 'lobes' as
the engine has cylinders. Riding on the cam and mounted on the 'breaker
plate' are a set of contacts, also called 'points'. The lobes of the cam
cause the points to open and close. When closed, the coil primary is
grounded allowing a magnetic field to grow to saturation in the coil. When
the points open the magnetic field collapses and induces a high voltage in
the secondary winding. It is this voltage that provides the sparks.
Therefore, the spark timing is set at the time the points open, not close.

   Before discussing timing, the purpose of the condenser should be
understood. One side of this device is grounded to the breaker plate. The
other side of the condenser is connected to the coil-side of the points.
When the points open, the coil is electrically isolated from earth ground,
except through the condenser, and thus its purpose. The coil's field
collapses induces a high voltage in the secondary. The purpose of the
condenser is to provide a current path for that circuit. Many workers claim
that the condenser is there to prevent secondary arcing across the points,
but that is a benefit, not the reason for the condenser.
   Of primary importance is when the spark plugs fire, and this timing is
also controlled by the distributor. The breaker plate, described above, is
arranged so that it may rotate a few degrees. The rotor is attached to a
shaft that also may rotate a few degrees, if by a different mechanism. The
former is modulated by a vacuum capsule, or perhaps two of them. The later
is rotated by a set of rotating weights, acting against two springs,
sometimes of different spring constants. In the vernacular, the capsules
are called the 'vacuum advance'. The flying weights are called the
'centrifugal advance'. In both cases, the word 'advance' is the key. And
before discussing ignition timing, the characteristics of the burning
charge will be discussed.
   The goal of any ignition is to arrange for the MEP (Mean Effective
Pressure) caused by combustion, to occur where it can do the most work.
Therefore, timing the ignition is actually timing the MEP to the optimal
piston position. Fortunately the engine designers have worked through the
maths and have done all the experiments to simplify our task to reading the
manual and making a few adjustments! That said, it is still of value to
understand why the timing must be varied.
   The charge does not immediately flash to fully combusted, as our
experiences with flammable liquids might imply. Rather, the charge is
ignited at the spark plug and the 'flame front' then progresses across the
combustion chamber. The MEP is not achieved instantaneously either so the
ignition of the charge has to be in advance of the MEP. This would all
seems easy enough to understand if the engine operated at one speed
only...but it doesn't.
   At slow engine speeds, such as slow idle, the spark timing has to occur
so that the MEP does the most work. In most engine designs this is the
'static timing' and is typically a few degrees BTDC or 'Before Top Dead
Center'. Why before? The spark ignites the charge early enough to allow for
the travel time of the flame front and for the pressure to build to the
MEP. As the engine speed increases, the travel time of the flame front does
not, but the piston is moving faster! Therefore, to place the MEP at the
optimal piston position, the charge must be ignited earlier. And this is
the sole purpose for the timing advance mechanisms in the distributor. The
author could write more about the centrifugal advance curve or how the
vacuum advance mechanism compensates for engine load, but those are topics
better left for later.
  One of the typical failure modes in old British car ignitions is the
points. With use, the contacts tend to burn and the insulating wiper riding
on the cam wears. In both cases, the timing suffers eventually ending in
ignition failure. For about $120 the whole ignition primary can be upgraded
by using a Pertronix Ignitor instead of the points and condenser. This
device includes a circular adapter that fits over the cam. Imbedded in the
adapter are tiny magnets that spin adjacent to the ignition module.
Impulses from the magnets cause the internal circuit to pulse the ignition
electronically, with no moving parts! This system provides a maintenance
free ignition primary.
   Unfortunately, the rest of the old distributor components are typically
worn or damaged. Most old British cars enjoyed by collectors are
40-years-old or older! A lot of wear can happen in 40+ years. Bushings wear
from lack of lubrication, pivots wear in the advance mechanisms, and
previous owners may have done quite nefarious things! The author has even
found ball point pen (biro) springs substituted for the springs on the
centrifugal advance! There are three choices available to remedy these
problems: totally rebuild the distributor - a task beyond the skill set of
most amateur mechanics, buy a period-correct replacement distributor,
either new or professionally rebuilt, or buy a new Pertronix distributor,
already equipped with an electronic primary trigger system.
   Symptoms of distributor wear include the inability to set a smooth, slow
idle, and timing instability at low engine speeds as indicated by a timing
light. The author recommends the Pertronix distributor. His MG TD and TR3b
both have these upgrades and both have beautifully smooth slow idles, and
trouble free ignitions.
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