I think you need to start at the beginning. A diesel engine is a compression ignition engine. Unlike a petrol there is no ignition system. A diesel engine draws air only into the cylinder and compresses it to a ratio of around 18:1, or even 20:1. Much higher than a petrol engine. This highly compressed and turbulant air gets very hot, and fuel is injected onto it in a very fine high pressure spray. Fuel in this condition instantly ignites. Direct inject (DI) means fuel is injected directly on top of the piston rather than into a seperate swirl chamber as in indirect injection engines. DI's do not rely on glow plugs for cold starting.

If you have a misfire the first thing to do is a compression check. If one cylinder is low then this will be the reason. Also an injector nozzle could be badly worn resulting in a jet of fuel rather than a spray. In this case the fuel would not correctly atomise and the engine would misfire, knock and smoke. Check the pump timing too. Unfortunately none of this is a simple DIY job. Diesel fuel pump timing is critical and very sensitive, and needs to be done by someone who knows what he's doing. And checking injectors needs specialist equipment.