Another reason on older earth-moving equipment, is that shutting them down takes about ten minutes... you have to cool the turbos so the oil doesn't coke in the bearings. Then to start them, you use a small gasoline engine (Pony motor) to first heat the main engine, then turn it over slowly to circulate the oil... then a little faster without compression, then add compression and hope you don't stall the pony motor out. Repeat 'till the big motor fires up. Takes at least 20 minutes. Not really applicable to the jack wagons doing this in thier ford winstar, but none the less.