Factors determining the success or failure of combustion initiation using a glow plug have been investigated through experimental work on a single cylinder, common rail diesel engine with a geometric compression ratio of 15.5, and a quiescent combustion bomb with optical access. A glow plug was required to avoid engine misfires when bulk gas temperature at the start of injection was less than 413°C. The distance between the glow plug and the spray edge, glow plug temperature and bulk gas temperature were important factors in meeting two requirements for successful ignition: a minimum local temperature of 413°C, and a minimum air/fuel vapour equivalence ratio of 0.15-0.35.