 This keeps the plug's filament glowing hot, and allows it to ignite the next charge.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.