system
Given the following post, respond as if you're commenting on Reddit:
human
What's the reason diesel engines are left to idle for long periods of time instead of shutting them down?

Heavy diesel engines (buses, construction equipment, transport trucks, etc) particularly tend to be left idling for long periods of time between uses. What's the rationale behind this practice?  
Is this practice still applicable to modern engine design?
gpt
Turbocharged diesel engines need some time to cool the turbocharger down, especially if it's been driven at high stress.

The turbocharger spins at much, much higher speeds than the engine, so when you drive for a prolonged amount of time under boost from the turbocharger and you cut the engine power, the blades in the turbo keep spinning, but the oil does not keep on circulating. This makes the temperature in the turbocharger rise, which can overheat the bearings of the turbo, which burns any residual oil. The byproduct of burning oil is thick gunk which can block the bearings and cost a small fortune to fix.