We’ve already covered the impulse coupling and the shower of sparks — Page 72, Lesson 93

Let’s pick up right where we left off — we were in the middle of the starting aids for the magneto. I want to make sure you’ve got the full picture of the three systems that help get the engine turning over, because they all do slightly different jobs. We’ve already covered the impulse coupling and the shower of sparks. Now I want to finish the shower of sparks properly, because there’s a second part to it — the Low Tension Booster Coil. The shower of sparks system, as we said, feeds high voltage directly to the distributor rotor’s trailing, starting, or retarded brush — that’s the electrode that fires late in the cycle to give you a retarded spark. That retarded spark is what avoids kick-back during the starting cycle. The system is switched ON for starting and OFF after start-up. Now, the Low Tension Booster Coil — and note the name carefully, Low Tension means low voltage — supplies a low voltage to the magneto primary during the starting sequence. This augmentation of the primary — that is, boosting the primary current — permits normal operation of the magneto. So instead of feeding high voltage straight to the distributor, this one works through the magneto’s own primary circuit. This system requires a battery supply, and it’s connected to the primary magneto, which is typically the left one. When you switch it on and engage the starter, the booster coil feeds a high voltage directly to the distributor rotor’s trailing arm, again giving you that retarded spark to avoid kick-back. And like the shower of sparks, it’s switched ON for starting and OFF after start-up. Then we have the impulse coupling, which we touched on. That’s a mechanical device that uses a spring to temporarily increase the speed of rotation of the magneto, giving a large retarded spark during the starting cycle. And the key point there — no action by the pilot is necessary. It’s automatic. Now let’s move on to a completely different topic — magneto and distributor venting. This is about moisture, and it’s a real operational concern. Since magneto and distributor assemblies are subjected to sudden changes in temperature, the problems of condensation and moisture are considered in the design of these units. Here’s the physics: moisture in any form is a good conductor of electricity. If that moisture is absorbed by the nonconducting material in the magneto — such as the distributor blocks, rotor arms, or coil cases — it can create a stray electrical conducting path. Let me explain what that means in practice. The high-voltage current that normally arcs across the air gaps of the distributor can flash across a wet insulating surface to ground. Or, the high-voltage current can be misdirected to some spark plug other than the one that should be firing. This condition is called flashover, and it usually results in cylinder misfiring. So you’ve got a spark going where it shouldn’t, and the cylinder that should have fired doesn’t. Now, how do we deal with this? The answer is waxing. For this reason, coils, condensers, distributors, and distributor rotors are waxed so that moisture on such units will stand in separate beads and not form a complete circuit for flashover. Think of it like water beading on a waxed car — the droplets stay separate instead of spreading into a continuous film that could conduct. But there’s a further complication. Flashover can lead to carbon tracking, which appears as a fine pencil-like line on the unit across which flashover occurs. Here’s the mechanism: the carbon trail results from the electric spark burning dirt particles which contain hydrocarbon materials. The water in the hydrocarbon material is evaporated during flashover, leaving carbon to form a conducting path for current. And here’s the critical part — when moisture is no longer present, the spark will continue to follow the track to the ground. So even after the moisture dries out, that carbon track remains conductive, and the problem persists. Now, you might ask — why not just seal the magneto completely? The answer is that magnetos cannot be hermetically sealed to prevent moisture from entering a unit, because the magneto is subject to pressure and temperature changes in altitude. So the design has to accommodate venting rather than sealing. Finally, let’s switch to a completely different type of engine — the diesel. Unlike the conventional spark-ignition engine, the diesel does not require an ignition system at all, thus saving on complexity and weight. The diesel is classified as a compression-ignition engine, where ignition of the fuel/air mixture is a function of the rise of temperature of the air due to compression. Much higher compression ratios occur in the diesel — ratios of 25:1 are not uncommon. At these compression ratios, the fuel self-ignites, thereby eliminating the need for a spark-generating system. And for cold starting, diesel engines usually employ a system of glow-plugs — and that’s where the excerpt cuts off, so we’ll pick up with the glow-plug system next. Let me just recap the key terms you need to hold onto: flashover, carbon tracking, waxing, compression-ignition, and the 25:1 compression ratio. Those are the professional terms you’ll be tested on.