Turbo cool down

Posted by NC Wayne on March 21, 2006 at 21:31:01 from (64.12.116.74):

Hey guys, I see posts on here all the time talking about letting a vehicle idle 'to allow the turbo to cool down' before shutting off the engine. I've worked on heavy equipment and turbo charged diesels, for a living, most of my life and I've never heard of letting the engine idle so the turbo can 'cool' down on a typical vehicle. A turbos 'hot side' runs hot whether the engine is idling or running full throttle. True you don't want to overheat one, but overheating typically happens when an engine has been "turned up" and there is raw fuel getting dumped into the exhaust causing the exhaust gasses to actually be burning in the turbo and exhaust instead of being simply the by product of a burn in the combustion chamber. That's why you see engines that have been "turned up" having pyrometers installed to let the driver know before he screws up and melts something down with exhaust gasses actually burning in the exhaust system instead of the combustion chamber. Fact is the hotter you can keep the hot side of the turbo, within certain parameters, the more efficient it runs. It's a basic law of nature, heat is energy, so if your loosing heat through the turbo housing your taking away some of the power of the hot exhaust gasses. Basically the more the exhaust gasses cool while going through the turbo the more power your loosing through that heat loss. In fact it's not uncommon to see a turbos hot housing enclosed in a thermal blanket not just to keep heat out of the engine bay but also to keep the heat in the turbo. Now the reverse is true on the intake side, the colder you can keep it and the air the better. Basically the colder and more dense the air the more power the engine can produce hense inter and after coolers, etc.
Most of the engines I've worked on over the years have a typical exhaust temp in the 500-600 degree range. So, think about this logically, you have a mass of iron and steel setting there comprised of the exhaust manifold, turbo, and associated piping at say a median temp of 550 degrees. By itself it's gonna take that mass A LONG time to cool down even with the engine shut off. Now take into account that the engine is still running and putting hot exhaust gas into it through the exhaust system.....Do you honestly think letting the engine idle for several minutes is gonna allow that mass to 'cool down' a significant amount? I've measured exhaust temps on a manifold and a turbo with the engine at an idle as well as at wide open throttle and there isn't that much difference in temp from low end to high end in a typical engine. Now one like I mentioned above that has been turned up is gonna show a difference but not your typical, factory installed, truck or tractor engine.
The main concern, the main the reason you let the engine idle is to let the turbo SPOOL down. In other words even if the engine is turning at a typical idle speed of 500-700 RPM the turbo is turning in excess of 10,000 RPM. So if you just turned in your drive and the engine was turning at 2400 RPM just a minute or so before then the turbo is likely still turning well in excess of 25,000 RPM. The speeds in most cases are greater than that I know. I was just using round numbers for example, and the speeds will vary with the turbo, but when you hear a turbo whine just remember it's because the impeller is turning so fast the tips of the blades are exceeding the speed of sound. This said when you shut the engine down, even from an idle speed, the oil flow throughout the engine stops. This includes oil flow to the turbo. So, the turbo is setting there spinning for several minutes with no oil flow which, as you know, is not good.... I recently timed one on a customers engine that is configured to shut down from 1250RPM due to the application. The turbo spun for nearly 5 minutes after the engine was shut down, even after setting at the 1250 idle speed for several minutes after coming down from a full load speed of 2400.---So, guys, don't worry about letting the turbo "cool" down, do some temp measuring on your own and you'll see what I mean. Just be dang sure to let them "spool" down or it's gonna sieze as a best case or wear to the point the impeller draga snd it disentegrates and feeds metal particles to your engine.

Replies:

• Re: Turbo cool down buickanddeere 14:22:41 03/23/06 (0)
• Re: Turbo cool down NC Wayne 22:45:41 03/22/06 (5)
  • Re: Turbo cool down Can't even use my name 14:11:11 03/24/06 (0)
  • Re: Turbo cool down RodInNS 15:04:37 03/23/06 (2)
    • Re: Turbo cool down NC Wayne 19:20:38 03/23/06 (1)
      • Re: Turbo cool down RodInNS 20:04:54 03/23/06 (0)
  • Re: Turbo cool down Burnie 02:30:32 03/23/06 (0)
• Re: Turbo cool down RodInNS 14:32:09 03/22/06 (0)
• Re: Turbo cool down ricedaddy 12:30:53 03/22/06 (0)
• Re: Turbo cool down RN 11:13:11 03/22/06 (0)
• Re: Turbo cool down-good idea! Can't even use my name 09:39:49 03/22/06 (0)
• Re: Turbo cool down the tractor vet 08:47:15 03/22/06 (0)
• Re: Turbo cool down bubba2 07:59:52 03/22/06 (0)
• Re: Turbo cool down Billy NY 06:04:07 03/22/06 (0)
• Re: Turbo cool down massey333 05:35:31 03/22/06 (0)
• Re: Turbo cool down Jon Hagen 22:07:17 03/21/06 (3)
  • Re: Turbo cool down Old Pokey 06:27:39 03/22/06 (0)
  • Re: Turbo cool down Bobcat 04:57:19 03/22/06 (1)
    • Re: Turbo cool down Harley 13:01:49 03/22/06 (0)

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