A charcoal fire under the engine may have worked 90 years ago but there is a risk of melting wiring harnesses on modern vehicles.
Brought back memories of my youth. Car broke down in middle of winter and had to ride motorcycle back and forth to work for two weeks. Bike wouldnāt start in sub zero mornings without a charcoal fire in a hibachi grill underneath it. Just getting grill going was tough. Those were some brisk ridesā¦
Diesels do not use much gas idling, As another thought.
I donāt think they use any gas, just diesel from what Iāve heard. 
Sorry @Barkydog, just couldnāt pass that one up. 
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coworker had to take battery in house the last few days as it was -25f in mn. no garage of course. he is poor. that would ensure battery is good for truck but motor is still cold