Glow plugs are normally used for diesel engines when starting from cold. They then glow for a short while just before the starting moment to heat up the combustion chamber and then go out when the engine is running. But when you use them to work in a direct injection internal combustion engine for hydrogen fuel, it's different. Here, the glow plug must be on all the time and a temperature of roughly 900-1100 degrees Celsius is required. But even this method shows both pros and cons, if you compare it to the Diesel engine, for example, the hydrogen engine takes more fuel, but an advantage seen in the studies was that it emitted significantly less NOx than the Diesel engine. Another problem with this type is that today you don't have any glow plugs that are able to stay on and glow all the time, but maybe this problem can be solved in the future.[1]


[1] Ibid. s. 9