Piston Engines - Ignition — Page 72, Lesson 89

I want to walk you through the ignition system of a piston engine, and I want to start with the big picture: the dual ignition system. Every aero piston engine is fitted with dual ignition. That means two electrically independent ignition systems. Each engine cylinder has two sparking plugs, and those two plugs are fed by two separate magnetos. So you have two completely separate paths to fire each cylinder. Why do we do this? Two reasons. First, it reduces the risk of engine failure caused by faulty ignition. If one system fails, you still have the other to keep the engine running. Second, it increases the power output of the engine. By igniting the cylinder charge at two points, you reduce the combustion time. The charge burns faster, which is more efficient and produces more power. Now let's look at the heart of each system: the magneto. A magneto is a self-contained, engine-driven electrical generator. It produces a series of extra high tension — EHT — electrical sparks at the sparking plugs, in the correct firing sequence, to ignite the petrol and air mixture. "Extra high tension" just means very high voltage. The magneto combines two principles: the permanent magnet generator, or PMG, and the step-up transformer. Together, they generate the EHT voltage necessary to break down the gap between the sparking plug electrodes. That "break down" is the spark jumping across the plug gap. Here's how it works, step by step. Inside the magneto you have a primary coil — a few hundred turns of thick wire. A small magnetic field in that primary coil is made to collapse at regulated intervals. That collapse is caused by the opening of a pair of cam-operated contact breaker points. When the primary magnetic field collapses, the lines of magnetic force cut through thousands of turns of very thin wire that make up the secondary coil. This induces an EHT voltage in the secondary coil. This is electromagnetic induction — a changing magnetic field inducing voltage in a coil. That induced EHT voltage is then taken to a rotary switch called the distributor. The distributor distributes the voltage to the sparking plugs in the correct firing sequence. Now, here's a critical timing detail. The cam-operated contact breaker points and the distributor rotor are geared together. That way, the spark appears at the sparking plug just as the contact breaker points open. And both the contact breaker cam and the distributor rotor rotate at half engine speed. Let me show you this circuit. Now, there's one more critical component: the capacitor, also called the condenser. The prime function of the capacitor is twofold. First, it prevents burning or arcing across the contact breaker points. Second, it assists in creating the extra high voltage in the secondary coil by causing a rapid change of flux — that is, a rapid change of magnetic field — in the primary coil. This increases the efficiency of the magneto. The capacitor is fitted in parallel with the contact breaker points and the magneto control switch. Let me explain why it's needed. The magneto relies on the rapid collapse of flux in the primary coil. That collapse is caused by the contact breaker points interrupting the current flow through that coil. But without a capacitor, when the points open, the voltage would try to arc across them — that would burn the points and slow down the flux collapse. With the capacitor across the points, here's what happens. The voltage that appears as the points open charges up the capacitor. Only a small, weak spark appears at the breaker points. And critically, current in the primary coil ceases to flow, which allows a very rapid collapse in primary flux. So the capacitor does two things: it stops arcing at the contact breaker points, and it allows a rapid collapse of primary flux. That rapid collapse is what induces the high voltage in the secondary coil — so the capacitor is essential for the magneto to work efficiently. To sum up the whole chain: the cam opens the contact breaker points, the primary flux collapses rapidly thanks to the capacitor, that collapsing flux induces EHT in the secondary coil, the distributor routes that EHT to the correct sparking plug, and the plug fires. And you have two of these systems running independently for every cylinder.