What happens to an engine during a cold start in sub-zero temperatures?

Back in the 80’s a friend and I each had VW Rabit Diesels. Those damn things would never start after setting out all day in temperatures less than 20F. The cars had block heaters but the place we worked at did not have provisions to plug them in. We worked second shift and more than one night we would just let the cars idle in the parking lot all night. IIRC 8 hours of idling and the fuel gauge hardly moved. Was nice getting into a warm car at the end of the shift. My buddy tried the trick of firing a small charcoal grill under the engine a couple of hours prior to the end of the shift. No good, I ended up giving him a ride home.