Newer diesels have strategies to warm up the engine within 10 or 15 minutes of start-up. This is because they are technically required to meet tier 4 emissions within 10 or 15 minutes. I forget if its 10 or 15. A cold engine will not meet emissions, the hotter it is, the cleaner it burns. (Except NOx) That is why most new trucks idle for about a minute then go to high idle and apply backpressure to the engine to create boost. More boost means more fuel. More fuel means more heat. Simple really.
Manufacturers only did this and recommend this since the advent of Tier 4 though? Why do you think that is? They are required to, that's why.
Caterpillar had a chart that had idle times for their engines. I'm not about to find it but it used to have minimum idle times for ambient temperatures. I am ballparking these numbers, so don't hold me to it, but I am very close. At +5 they wanted 3-5 minutes of idling. At -10C it was 15ish minutes. At -20C all they said was to idle as necessary.
Cold weather can cause piston skirts to collapse. (If Cabertosser researched what shape a piston is at -30C he will now know why piston skirts collapse and then score cylinders)
Heavier oils used in diesels require more time to warm up to provide proper lubrication. This prevents things like valves from sticking open. Which also requires enough ambient temperature of the valve guides so as not to "freeze" the oil entering it. Having your lubricants able to meet their requirements, as a lubricant, also requires them to be in certain temperatures too. That I consider quite important before you spool up your turbocharger to 100,000 RPM. This also applies to the proper film strength of the oil. Your crankshaft will thank you for providing proper film for heavy load purposes.
If you never bring your diesel to proper temp its susceptible to freezing off its breather and pumping the oil out.
Depending on the temp, your transmission and power steering benefit from the engine warming up.
I have seen oil pumps shear off their drive trying to move heavy oil at higher RPMs
I disagree with the article HunterDave posted. I can shoot videos of equipment starting up and monitor the engine temp in real time on ET. In extreme cold, 7 minutes would likely not have frost off the block yet, let alone at temp. Many times I have not brought a piece of iron off low idle for 20 minutes to just start warming it up, but that is extreme conditions. The article is correct that it may not rise in temp anymore but what it isn't saying is that your entire cooling system and engine will heat-soak to say 120F (just a random number I'm using) with extended idle times. With only 7 minutes the only thing that is 120F is the water jacket on the block and the cylinders. Then the thermostat opens and thermal shocks that engine core down to 50F. Go ahead, I make a very good living due to cold weather. I've never experienced a major engine failure, but lots of people pay me to fix theirs. Many change their operating style, and oils after they talk to me about the failure they just experienced.
Block heaters, and webastoes are great but idle time is required. Even at +5 you should be idling it for 5ish minutes if you went by manufacturers recommendations. Idling for 10-20 is probably about right for most situations.
I would also say that applies to gas engines as well.
Idling with straight pipes doesn't matter if its a diesel or gas job, its just loud. That is irrelevant. That has nothing to do with whether an engine should be idled.
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