The components, in the order the spark passes them:
Rotating magnet — driven by the engine, it sweeps its field past the windings and induces a current in the primary.
Primary winding and contact-breaker points — the primary is a coil of few, thick turns carrying the induced current. The points are a cam-operated switch in that circuit: at the firing moment the cam snaps them open, the primary current collapses, and the collapsing magnetic field is what generates the spark voltage.
Capacitor (condenser) — wired across the points, it absorbs the current's last surge so the points do not arc and burn, and by killing the current faster it makes the field collapse harder and the spark stronger.
Secondary winding — many thousands of fine turns sharing the primary's core: the fast field collapse induces in it the very high voltage, tens of thousands of volts, that can jump a spark gap under compression.
Distributor and HT cables — the distributor's rotor hands the secondary's high-tension (HT) output to each cylinder's cable in firing order, and the heavily insulated HT cables carry it to the plugs.
Spark plugs — the gap in the combustion chamber where the voltage jumps and the burn begins.
Ignition switches — wired backwards from intuition: the switch earths the primary winding to STOP the magneto. Switch OFF means primary earthed and no spark possible; switch ON simply removes the earth. A broken switch wire therefore leaves the magneto live with the switch apparently off, which is why a propeller is always treated as live.
Two systems, two plugs
Every certified spark-ignition (petrol) aero engine carries two electrically independent ignition systems: two magnetos, each firing its own plug in every cylinder through its own wiring. The first reason is the obvious one, safety: one complete system can fail and every cylinder still fires. The second is performance: two plugs light the charge at two points, the two flame fronts cross the chamber in less time, and the quicker, more complete burn gives measurably more power, which is why running on one magneto costs a little rpm even with nothing wrong.
Helping the start
At cranking speed the magnet turns too slowly to induce a healthy spark, and the normal spark timing is too early for an engine barely moving. Two spark augmentation methods solve both problems at once:
Starter vibrator (booster coil) — battery current, rapidly interrupted by a buzzing contact, is fed into the ignition circuit during cranking, producing a continuous shower of retarded sparks until the engine fires and the magneto takes over.
Impulse-start coupling — a spring mechanism in the magneto drive: during cranking it winds up, holds the magnet back past its normal firing point, then releases it in a sudden snap, spinning the magnet fast enough for a fat spark, delivered late (retarded). Above starting rpm the coupling locks out and drives normally.
The magneto check
The run-up's magneto check exists to prove each system alone before flight, because in normal running the two magnetos mask each other's faults. At run-up power the pilot selects each magneto in turn and watches the rpm: a small drop, within the type's limit and roughly equal on each is the healthy signature, the single flame front's slower burn made visible. An excessive drop or rough running points at a fouled plug or a faulty magneto; no drop at all is also a failure: it means the selected-off magneto never actually stopped, an earth wire broken somewhere, and the switch cannot be trusted to kill the engine.
Diesel ignition
The diesel engine initiates combustion with no ignition system at all: its high compression, the 15:1 to 25:1 of the fundamentals lesson, heats the cylinder's air beyond the fuel's self-ignition temperature, and combustion begins when fuel is injected into that air near the top of the compression stroke, after the short ignition delay the injected spray needs to heat and vaporise. For cold starts, glow plugs, small electric heating elements in the cylinders, warm the air until the engine's own compression heat suffices; they are starting aids, not an ignition system, and everything in this lesson's magneto half simply does not exist on a diesel.
The mixture, defined
The mixture vocabulary, each term a definition the exam asks for:
Mixture — the ratio of fuel to air entering the cylinders, by mass.
Chemically correct (stoichiometric) ratio — the mixture in which every fuel molecule finds exactly the oxygen it needs: about 1:15 (one part fuel to fifteen parts air, by mass). It burns hottest, which makes it the exhaust gas temperature's peak.
Best power ratio — the mixture giving maximum power, slightly richer than stoichiometric at about 1:12.5: the extra fuel keeps the charge a little cooler and the burn fastest.
Lean (weak) mixture — less fuel than stoichiometric. A mixture is described as on the lean side or the rich side of the EGT peak, since the peak marks stoichiometric.
Rich mixture — more fuel than stoichiometric, the surplus evaporating unburned and cooling the charge.
The burnable range is roughly 1:8 at the rich limit to 1:20 at the lean limit; outside it, the charge will not light at all.
Rich against lean
Each side of stoichiometric trades something:
Rich mixtures — cooler running (the surplus fuel's evaporation absorbs heat), a margin against detonation, and maximum power near the best-power ratio. The costs: fuel wasted out of the exhaust, higher specific fuel consumption, carbon-fouled spark plugs, and carbon monoxide in the exhaust.
Lean (weak) mixtures — economical: specific fuel consumption falls as the mixture is leaned toward the ratio where the fuel burns most completely, a little lean of stoichiometric. The costs: hotter running near the peak, a slower flame, less power, and, at high power settings, the detonation invitation the fundamentals lesson described.
That is the whole relation between specific fuel consumption and mixture in one sentence: SFC is worst when rich, improves steadily as the mixture is leaned, reaches its minimum a little lean of stoichiometric (best economy), and worsens again if leaned further still, as the slowing burn wastes the stroke.
Leaning on the EGT gauge
The pilot cannot see the ratio, but the exhaust gas temperature (EGT) makes it visible: because EGT peaks at stoichiometric, the peak is a fixed landmark on the mixture scale, found from the cockpit by leaning slowly until the EGT rises, tops, and just begins to fall. Settings are then made relative to that landmark, enriching by a set margin from peak for cruise power, or leaning slightly past it where the engine's handbook allows best-economy running. The absolute EGT value means little and varies day to day; the peak's position is the reference.