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Diesels ignite the mixture by compression, no spark there, just the shear heat of compressed air. When it's really cold outside, injection timing is of utmost importance in order to get an easy cold start by injecting the fuel at exactly the right time. In older TDIs like mine, the timing when starting is fixed at the rotary fuel pump and not controlled by the ECU yet. A hard to start TDI when it's cold probably have its basic timing off from the optimal position. In common rail diesels that's not a problem anymore as timing is always controlled by the ECU.
My TDI always started in the winter and, from a FE standpoint I'm a bit ashamed to say, I never plugged it unless it was below -30 C.
I've never had my D2 or ULSD gel even by -40 C. They must use a whole lot of anti-gel additives up here.
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