The "glow plug" was kind of shaped like a spark plug, but instead of a gap for high voltage arcs, it had a small loop of platinum wire. To start the engine, yo

The "glow plug" was kind of shaped like a spark plug, but instead of a gap for high voltage arcs, it had a small loop of platinum wire. To start the engine, you had to pre-heat the plug. This entailed attaching a battery, clipping one terminal to the top of the glow plug and the other to the engine, to send a current through the platinum wire. You'd use a 'D' cell, or maybe one of those big old No.6 types. A mere 1.5V, but you needed enough current to heat that platinum wire. This electrical apparatus had to be disconnected from the engine before you could fly the plane. Not sure if you could detach it before starting or not. That little wisp of wire would cool pretty darned fast with the current removed. I know with the larger RC planes the practice was to leave the glow plug heater attached until after the engine had started. Anyway, once the fuel mixture hits that hot wire, there's this catalytic reaction where the fuel at the platinum surface burns extra hot. This imparts enough heat to the glow plug wire to keep it hot enough for the next stroke. Idk what magic keeps the fuel from detonating before the piston reaches top of stroke, but I can attest these engines are fussy little things. They didn't idle very well; throttle them down too much and they just conk out. You can see other examples of this catalyzed combustion, and get a sense for what goes on inside the glow engine. These butane cigarette lighters that burn blue have a little bit of catalytic wire in them, you can often just see it glowing white hot just inside the flame opening. There also used to be these propane powered space heaters that had a gauze mesh of the stuff to boost the combustion efficiency.