alt Hacker NewsWell.. as so many things in engineering: it’s a tradeoff. Mechanically, it’s also very important to let the engine warm up and cool down before and after heavy load. Primary reasons for warmup are that the oil is more fluid on higher temperatures and metal is also less fragile when 50 or so degrees above freezing. Cooldown is especially important on turbo engines as the turbo on hundreds of thousands revs per minute and high temperature needs oil to be pumped through the bearings. If you turn it off after heavy load on engines that do not keep the oil running after motor shutdown, there is a chance that oil burns and gets stuck in the bearings, degrading performance or worst case breaking the turbo.
Also for the warmup case, tolerances inside the engine are designed assuming a hot engine. At lower temperatures, the metal hasn’t expanded yet causing also more stress on various parts such as the piston (seals).
And for the turbo I forget to mention that if you leave a turbo engine idle for a few minutes that this allows both the temperature and speed of the turbo to reduce. Many trucks have a sticker in the dashboard saying “do idle engine x minutes before shutdown to avoid turbo damage”. (But also do not idle for extended periods which I’m not 100% what’s the reason. I guess it’s due to diesel engines in some cases getting too cold while idling.)