Since the ignition timing is not controlled electrically, as in a spark ignition engine or by fuel injection, as in an ordinary diesel, it must be adjusted by the richness of the mixture, the ratio of nitromethane to methanol, the compression ratio, the cooling of the cylinder head, the type of glow plug, etc. A richer mixture will tend to cool the filament and so retard ignition, slowing the engine, and a rich mixture also eases starting. After starting the engine can easily be leaned (by adjusting a needle valve in the spraybar) to obtain maximum power. Glowplug engines are also known as nitro engines. Nitro engines require a 1.5 volt ignitor to light the glow plug in the heat sink. Once primed, pulling the starter with the ignitor in will start the engine.