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Some people are saying that after having driven a car with a turbo-charged engine, you need to let the engine run for several seconds (or even minutes) before switching off the engine. But nobody is able to explain why, which makes me wonder if it's even true at all.

Is there any kind of evidence, or technical explanation that it is actually beneficial to respect such a cooling down period?

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I'm not sure this is about skepticism. –  DJClayworth Dec 3 '12 at 16:02
    
I'm not sure a turbo engine needs such a "cooling down" period, but I have no references. –  Carlo_R. Dec 3 '12 at 16:47
    
Engines cool down fastest when you stop combusting gasoline/diesel in them. Cars are designed to be operated by near idiots. My turbocharged car gets turned off by me as soon as I arrive at the destination. Since it's engine management unit is smart enough to turn the engine off at traffic lights (and restarts itself when I engage 1st gear) it would be smart enough to keep the engine running if there were any benefit to so doing. –  RedGrittyBrick Dec 3 '12 at 16:49
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@Ryathal: Yes, I agree on safety. However, when you turn off the engine of most cars made in the last decade or more, some parts associated with the cooling system keep running, for example the fan that blows air through the radiator is electric nowadays. I've owned cars where it wasn't unusual to hear that running for a while after the ignition was turned off. In other words, manufacturers have worked out how to arrange matters so that car-owners don't need to manually manage engine operation for durability. –  RedGrittyBrick Dec 3 '12 at 22:51
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@red- your statement 'most' is not indicative of my experience. Do you have stats to back this up? –  Rory Alsop Dec 4 '12 at 8:18
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3 Answers 3

Claim

Why does a turbo engine require a “cooling down” period?

It doesn't.

Assuming we're talking about the typical turbocharger fitted to many current models of fairly ordinary car.


What manufacturers Say

According to an "engineering-based company specialising in turbocharger and supercharger design, replacement turbochargers, and manufacturers of specialist machinery and turbo components used in the turbocharger repair industry." :

Should I leave my engine ticking over before it is turned off ?

Not for normal every day driving, but still worthwhile if the engine has been under load or raced before being turned off. e.g. Towing a caravan or after climbing a long incline.

...

What is a water-cooled turbo ?

The central part of the turbo, housing the bearings, is surrounded by a water jacket through which the engine’s water coolant is passed. This water continues to circulate after the engine is turned off, cooling the turbo, and preventing heat soak.

According to a major manufacturer of turbochargers

Following a hot shutdown of a turbocharger, heat soak begins. This means that the heat in the head, exhaust manifold, and turbine housing finds it way to the turbo’s center housing, raising its temperature. These extreme temperatures in the center housing can result in oil coking.
To minimize the effects of heat soak-back, water-cooled center housings were introduced. These use coolant from the engine to act as a heat sink after engine shutdown, preventing the oil from coking. The water lines utilize a thermal siphon effect to reduce the peak heat soak-back temperature after key-off

Conclusion

No, there is no need to run a turbocharged engine after stopping the vehicle - unless you have an older vehicle with an early type of turbocharger and have been pushing the engine to it's limits for an extended time. Check the owner-manual for your car to see if the manufacturer recommends any special procedure, if it doesn't you can assume none is needed.

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- 1 as your conclusion is not supported by the text in your answer. Some cars have water cooled cores, but it is by no means the norm. My car instruction Manual specifically states the driver should idle the engine for a few minutes after motorway cruising. It was made in 2006. –  Rory Alsop Dec 4 '12 at 8:16
    
@Rory: In my conclusion, perhaps an 8-year old vehicle falls into the "older" category. I'll update the answer to clarify. –  RedGrittyBrick Dec 4 '12 at 9:57
    
From Ford Galaxy owner manual, p. 153: "SWITCHING OFF THE ENGINE Vehicles with a turbocharger CAUTION Do not switch the engine off when it is running at high speed. If you do, the turbocharger will continue running after the engine oil pressure has dropped to zero. This will lead to premature turbocharger bearing wear." No cool down is needed, only you should not press the gas pedal before you switch off - which is a natural thing to do anyway. –  Suma Dec 4 '12 at 11:57
    
Slightly more info regarding Ford's EcoBoost engines: "The test ran EcoBoost at maximum boost flat out for a 10-minute period. Then the engine and all cooling were abruptly shut down and the turbo was left to “bake” after this high-speed operation. If that sounds severe, imagine repeating this cycle 1,500 times without an oil change. That’s what EcoBoost’s turbos endured." Source: media.ford.com/article_display.cfm?article_id=29657 –  Kevin McCormick Jan 30 '13 at 15:02
    
@Suma that just means to not turn off the engine while it's revving at high rpm. If you've brought the car to a stop normally and responsibly that's no problem. –  jwenting May 31 '13 at 6:08
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TLDR: Yes, some turbochargers need time to decompress otherwise they could be damaged.

Perhaps my answer only applies to old turbocharged vehicles. My experience has only been with an early 90s Nissan Exa and an early 00s Subaru WRX and both of those had a "turbo timer". With the timer the car would continue to run for about 1 minute after switching off the ignition (and removing the key). My understanding was that this was normal for turbo cars.

Over Pressure

Turbos can generate a lot of pressure when it's running. That pressure needs to have somewhere to go. When the engine is running it just blows out the exhaust, but when the engine just stops there is a buildup of pressure (not enough to crack a piston head like I've been told but), enough to blow backwards through the compressor, damaging it.

Here's an excerpt from wikipedia

In this situation, the surge can raise the pressure of the air to a level that can cause damage. This is because if the pressure rises high enough, a compressor stall will occur, where the stored pressurized air decompresses backward across the impeller and out the inlet. The reverse flow back across the turbocharger causes the turbine shaft to reduce in speed more quickly than it would naturally, possibly damaging the turbocharger.

Turbo Lubrication and Excess Wear

The engine feeds lubricant to the turbo. With the engine off the turbocharger isn't being lubricated.

This from turborepair.com:

Once you turn off the engine in your car, the turbocharger will continue to spin for up to a minute or more and during this time oil is not being delivered to the turbochargers bearings, causing wear.

This from myturbodiesel.com

If you were driving hard and hot, a 1 minute idling period or a few minutes of sensible driving before shut down should be enough to let the turbo cool down and receive fresh oil. If the turbo is too hot and does not receive cooler oil upon shutdown, the oil could become burnt and "coking" may occur.

This from allpar.com

Hot shutdowns cause extensive deposits of carbon and shellac on the turbine end. As the deposits break up and flow into the oil they score and wear the bearing bore, bearing and shaft journal.

The twist

Turbos don't run all the time, they only run when they need to. Which means if you've been coasting around a car-park for a minute, the turbo isn't running and there is no pressure buildup. In this case you don't need the turbo timer, you can shut off your engine without waiting.

Sources

You can read about turbos here and here's some rev-heads talking about turbo cooldown and how important it is.

Edit

RedGrittyBrick's answer mentions 2 links:

turbotechnics.com (a manufacturer) say allowing the turbo to decompress is "worthwhile" in some instances.

turbobygarrett.com (also a manufacturer) say hotstopping can "result in oil coking". It also says water cooled systems can reduce this, but not all turbos are water cooled.

Many Garrett turbos are water-cooled for enhanced durability

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Who turns off an engine when it's cranking over at 4000 RPM (say)? Surely getting down to idle for a few dozen revs (i.e. less than a tenth of a second as you pull into your parking space) is sufficient to relieve the back pressure. –  dmckee Dec 4 '12 at 1:35
    
This is a theoretical answer. We don't care how a turbo works, or what theories are there of why it can get damaged. They are no better than the claim. We want a study that tried to turn off cars in different manners and showed a significant statistical difference. Please avoid speculative answers. –  Sklivvz Dec 4 '12 at 1:45
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@Sklivvz, The only people likely to perform such tests with statistical evidence are manufacturers who are obviously bias. The unbiased experts on this subject would be mechanics who actually see the wear (and write about it on blogs and messageboards to which I've linked). Their opinions aren't speculative, but based on eye-witnessed evidence. –  Coomie Dec 4 '12 at 2:24
    
I disagree. Why would a manufacturer lie about the proper usage of its products, or their lifetime, etc.? Why would you trust mechanics who are clearly subject to selection bias, as they obviously see broken cars, not all cars? The plural of anecdotes is not data, as they say. A statistical study can be performed by anyone... –  Sklivvz Dec 4 '12 at 7:20
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@Sklivvz Which manufacturer? If your referring to the links in RedGrittyBrick's answer, one says engine cooldown is "worthwhile" in some cases and the other says allowing cooldown can help by "preventing the oil from coking" (in models that aren't watercooled). That's 2 manufacturers that agree that (at least in some cases) hotstopping can damage the turbo. Skeptics.se doesn't accept studies performed by the community. –  Coomie Dec 4 '12 at 7:54
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Yes, a turbo receives oil from the running engine. When the engine it shut off and the turbo is still spinning at high rpm it can wear the bearings from lack of oil. My ford truck with a turbo says to allow 1-2 minutes of idle time only after highway speed drives or heavy loads. Lookup turbo timers. Some performance cars come with them. Basically you shut the ignition off and the car stays running for a set amount of time.

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Welcome to Skeptics Stack Exchange! Please provide some references to support your claims. –  Sklivvz Dec 29 '13 at 20:39
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