[6pack] Ignition (Loooong

[Richard Lindsay]

Sorry Friends,
   I did not mean for the text to read like an ad for Pertronix. That text
was written for a friend having ignition problems and it WAS a
recommendation for him to buy the Pertronix distributor. I have just found
that the module and the full distributer worked so well for me. I should
drop the name from the text and use the generic terms.

Rick

On Wed, Aug 14, 2019, 5:44 PM Richard Lindsay <richardolindsay at gmail.com>
wrote:

> 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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