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Written by Richard McCuistian
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Thursday, 13 December 2007 |
No engine will run without the proper amount of homemade lightning, and it takes a lot of voltage to punch through plain old atmosphere, let alone the tightly packed molecules in a combustion chamber as the piston in question nears Top Dead Center with both valves closed. The ‘pop’ of an ignition spark in the open air is actually thunder on a tiny scale, since the stream of electrons in an ignition spark displaces air molecules the way lightning does when it stabs through the air in a thunderstorm. So how do we make artificial lightning and harness it for our purposes? The step-up transformer we all know as an ignition coil has been the heart of gasoline ignition systems for decades, and the principle is fairly simple:Magnetism induces voltage in copper. When very thinly insulated copper wire is wound in significant volume around a metal core and is saturated with a magnetic field, the collapse of that field produces a voltage spike that can be very powerful and very useful if properly controlled and timed. Ignition coils contain two windings: Winding # 1 is known as a ‘primary’ winding, and one end of the primary winding is connected, either directly or indirectly to the ignition switch. For decades, the ground side of the ignition coil primary winding was switched on and off by contact breaker points wired parallel with a capacitor and located in the base of the distributor. The distributor on these archaic old systems rotates at camshaft speed (half crankshaft speed). The breaker points on older ignition systems open once each cycle per cylinder, i.e., four times for a four cylinder, six times for a six cylinder, and so on. The primary and secondary windings in the old oil-filled coils were connected at the positive end (fed from the ignition switch through a voltage-dropping ballast resistor), but newer coils are ‘potted’ with both windings wrapped around a metal core to amplify the magnetic saturation.Winding #2 turns out to be the ‘secondary’ winding and it consists of a much greater number of windings than the primary. When the breaker points or ignition module circuitry interrupt the current flow through the primary circuit, the magnetic field that has saturated both the primary and secondary windings collapses, causing a several hundred volt spike in the primary winding and spike in the secondary winding that is downright intimidating. The old oil-filled coils generally popped about 50,000 or so volts, but newer coils are routinely capable of murderous voltages that can actually be fatal if they cross a person’s chest cavity in search for a ground path, so be careful!While ignition coils are the muscle that makes the spark, the hardware that fires the coils has everything to do with how well the coils perform, and no article on ignition coils would be complete without a bit of a history lesson on what manufacturers have done to trigger them. Breaker points were initially replaced by magnetic distributor pole pieces that sent an AC current to electronic ignition modules that used transistors to control the coil’s primary current. In this new setup, reluctor in the distributor whirled past a wire-wound magnet to make the signal; a four cylinder had four reluctor teeth, a six cylinder had six, and so on and while Chrysler and Ford held on to the oil filled coil for years even on their electronic ignition systems, GM introduced the hotter-sparking 80,000 volt HEI coil in 1975, complete with a tiny distributor-mounted ignition module that would fit in the palm of your hand, and by 1981, the Engine Controller would be in charge of ignition timing on many, if not most cars on the American road. With the new laminated core ignition coils came the need to limit the current that flowed through the primary windings so as not to damage the delicate primary windings. The old oil-filled coils were a bit more robust in that regard, and didn’t need current limiting, although they did operate on lower voltages.Shop Tip | Distributor cap carbon tracks were once a serious problem, but the folks who made distributor caps got sharp and started using different materials that resisted tracking. On that same order, one problem with DIS and the powerful spark it produces is that a faulty spark plug with high internal resistance can cause the spark to run down the outside of the ceramic to hit the metal shell, making an external track on the ceramic that looks suspiciously like a crack, but with a matching carbon track on the inside of the spark plug boot. Replacing the plug only temporarily repairs this problem because the spark loves to follow the carbon track inside the boot and it will do so at every opportunity. This boot-tracking anomaly never happened on non DIS ignition coil equipped systems. |
The Hall-Effect switch took hold in the early eighties on Ford vehicles, providing a nice square wave signal that triggers a distributor-mounted Thick Film Ignition (TFI) module that takes its timing orders from the Engine Controller and fires a potted coil similar to GM"s HEI, but in 1985, GM had forged ahead and introduced the distributorless ignition system whereby companion cylinders shared the same coil, meaning a four cylinder would have two coils, a six cylinder would have three, and so on. In a DIS system, one spark event happens near the end of the compression stroke in the firing cylinder and the another happens on exhaust in that cylinder’s companion TDC stoke, but the very nature of secondary ignition dictates that the stronger spark will always happen in the cylinder that needs it, so very little spark is ‘wasted’ in the exhausting chamber. In the absence of a distributor, GM’s system began using a couple of different types of crankshaft sensors.GM’s Quad Four engine platform took the DIS concept to a new level by eliminating spark plug wires and letting the coils sit right on top of the spark plugs, but there were still only two coils under there, and as the spark plugs wear out, the powerful ignition coils tend to punch through the coil housing in search of an easier path. Toyota and some other manufacturers put a coil directly over three of their spark plugs and fed the companion cylinder with a single spark plug wire fed from that coil. Ford replaced their 1989 vintage TFI-driven DIS system with a smarter Electronic Distributorless Ignition System (EDIS) in 1991, equipping Explorers and Escorts, and eventually most of their vehicles with coil packs or Coil-On Plug systems, and with these systems came the Variable Reluctance Sensor (VRS) that generated its signal at the crank pulley using a missing tooth for reference. The new EDIS system included repetitive spark at idle, but unlike the Multiple Spark Discharge (MSD) systems so popular with hot rodders, the factory-designed repetitive spark evaporates as soon as the engine breaks above an idle. In the midst of all these changes, ignition modules began to disappear, drawing on a Chrysler trick from the mid-1970’s Lean Burn days; the Engine Controller (PCM) took over the job of firing the coils, eliminating some hardware and wiring and making the whole process of sparking and controlling ignition timing vastly more efficient.Today, Coil-On-Plug (or Coil-Near Plug) systems have given the PCM total control of each and every spark event, and repetitive spark remains the order of the day on many, if not most COP-equipped cars. OEM COP coils, even though potted, are particularly susceptible to moisture and can be somewhat prone to cause Radio Frequency Interference (RFI) issues with other vehicular electronic systems, even in cases where the coil isn’t misfiring. CNP coils generally contain their own little transistor for primary circuit control. R.W.M.
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Last Updated ( Saturday, 15 December 2007 )
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