Glow plugs (11) in individual cylinders of the engine (10) are switched (20) in turn to a source of current (25). Temp. signals (T1-T4) derived (30) from them at other times pass to a control unit (40) which responds to additional inputs (S) by producing control signals for the injection pump (45). The timing (21) of the switching between the normal cylinder-heating function and the temp.-sensing function depends upon the operating state of the engine including the time taken to warm up from a starting condition in which the glow plugs are heated (25).
