TURBO - Myth or Truth

C

CWD

Guest
Hello

When I bought my 1.4TDI Sport almost a year ago now, the dealer said that when coming to halt, before turning the engine off I should wait a few seconds.

He claimed that because the turbo was working all the time turning the ignition off immediately you stopped could cause potential turbo problems in the long run.

I have scrupulously followed this advice ever since but just wondered whether other members do the same.

PS. I also wait until the OK goes out on the DIS before starting up the engine. Mind you, that's a touch of OCD.
 
I think this is standard advice, especially after a drive when you have been driving the car 'enthusiastically'. Allowing the turbo a few minutes to idle before switching off is claimed to ensure that its bearings do not run dry and risking premature failure. This advice is stressed for the Smart ForTwo with its tiny and very much overworked turbo.

Although I have been driving my A2 well within its limits since I bought it in September, after a long session on the motorway I always let the turbo idle for a minute of so before switching the ignition off.
 
Hi CWD
That's sound advice from your dealer.
Its particularly important after the engine has been working hard. Flying down the motorway then pulling in for fuel is a good example. The turbo can literally be red hot and still spinning at high revs so imediately switching off the engine, which also stops the oil supply to the turbo bearings, is not good news. Letting the engine tick over for a few seconds allows the turbo slow down and everything cool off is definitely worth doing. Its also worth taking things gently when first starting the engine as this allows the oil supply to reach the bearings before the turbo spins up to 150,000+ rpm when it under load

Cheers Spike
 
Hi


As a Turbo is governed by exhaust and inlet pressure it stays permanently in a highly pressurised environment. If you choose to switch the car off while under full throttle or motorway driving then you are mad. If you pull off the motorway into a petrol station, the few seconds of slow to idle speed is enough to slow it completely. And you don’t need to idle. Years ago when my Renault 5 GT Turbo and Uno turbo where new, we all felt it was a necessary requirement, but I understand it is a complete myth and not required. Impellar blade RPM at idle is governed by the pressure from the incoming air, as the wastgate has allowed the air to bypass the exhaust blades giving no further resistance. Therefore The Turbo impeller return to idle RPM almost immediately and don’t require 10, 20 or 30 second idle time. (Additional) This is because your impeller blades can not spin independently of the idle pressure forced into the cylinders by the normal idle of the engine. In other words because the turbo is “inline” to the air intake system, It can’t freely spin at high RPM while the inlets on the engine don’t require it. Therefore the pressure or resistance slows the turbo impeller almost immediately and doesn’t allow them to freely spin to a stop some 10,20 or 30 seconds later.

If you do it and don’t mind doing it, then it won’t do any harm. But I was adamant that it was required for years until the last 5 years or so.




Emm puts on his hard hat :)
 
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I don’t think the level of temp cooling is relevant over the short 20 sec idle, what would it be 1egree? 5 degrees? The problem is that oil will not evaporate that much as our little 3 pot doesn’t run at high temp or pressure.

Your generator may need a “cool down” period but I imagine it is completely different to normal car engines with wastgates, throttle bodies, gearchanges etc. and can not be used as a good example of a car turbo guide. I imagine it runs flat out most of the time or at least the turbo will be to generate the pressure, but a car works completely different, with a wastegate you lift of and it opens fully allowing the exhaust pressure to miss the blades. This means the turbo will only spin at inlet pressure for a moment. Then you change gear, the throttle is open and the wastegate closes spinning the turbo back to exhaust pressure speeds and forcing the air in. sometime turbo stall would occur, requireing dump valves either closed loop or to atmosphere. I fully admit that I don’t understand how a generator works with your hospital but I’m sure it doesn’t work in the same way a turbo car does. I don't think the temp or the spinning are important enough to require 20 sec idle but if you have always done it, it doesn't matter. But that’s just my opinion
 
Thanks to everyone that replied. You've certainly dispelled any myth. So, I shall continue to rigorously maintain my idleing regime.
 
I second EMM, it is no harm doing it, but I don't think is is much of an issue if you don't. Just imagine how many typical polo, golf, a3, a4 and a2 tdi owners don't do that and the turbos are running just fine.

I've always thought my friend's turbo timer on his STI is just a gadget and the car has got a dump valve as well. The temperature argument is somewhat reasonable. But then when your car is at idle and the engine is running, there is minimum flow of air to carry dead heat so I'm not sure it is as effective as slowing down at low rev for a small distance.

my 2 pence
 
Aparently it's said that turning off the car means the turbo's shaft will be sat in oil and will most likely boil this oil reducing oil life and coking up the shaft, damage seals etc etc, however since oil quality has improved since turbos were first fitted to cars I really cant see the advantages these days.
I personally dont think its worth doing, and if you are doing it then I wouldnt do it for too long as said above theres no airflow over rads, IC's etc..
Usually slowing down on the way upto the house from the m'way/dual carriage way etc will be enough slower less enthusiastic driving to cool stuff down...

/2p
 
I've heard some Jaguar models has an electric pump that keeps the turbo lubricated while the turbo is spinning down after the engine has been turned off.
On the other hand my friend who owns a Saab 900 Aero (turbocharged of course) has driven over 300 000 kms (200 000 miles) without problems. He however always lets his car idle befor turning off the engine.
 
As opinoins on this topic seem to differ wildly, I have done a bit of research and got the following advice from an engineer who works for a turbo manufacturer........
Allowing the engine to idle for a minute or so before switching off is good practice but not normally necessary 99% of the time with small car turbos. Generally the time it takes to park up is enough to allow the turbo to cool down.
At idle the turbo spins at a few thousand rpm but the bearings usually operate in an oil bath to prevent starvarion when the engine is switched off. Same applies at start-up when the oil bath prevents the bearings running dry for the initial few seconds until full oil pressure builds up. On the rare occasions you need to stop quickly after working the engine hard then let the engine idle before switching off.
The most important tip to prolong turbo life is to use the correct spec oil and change it at the correct service intervals.
Things are a bit different on big diesel engines. The significantly larger mass of metal in the impeller wheels stores a lot of heat and this can transfer down the shafts, baking the oil onto the seal and bearing areas. In these cases idling the engine before switching off can definitely prolong turbo life.

Cheers Spike
 
My previous experience of a turbo car was my Audi S2 - this had a couple of protection devices designed to cool the turbo after the engine had been switched off - the main system was an electric pump in the coolant circuit that activated after you've turned off, which pushed coolant through the turbo housing for 5-10mins after a run giving all parts of the turbo (housing, bearings, impellor) a chance to cool down gradually.

If the car was really hot, an additional system would then cut in after the water pump which started the main rad fan as well, allowing additional heat soak to be removed.

I consider it to be good practice to allow your engine a chance to take a bit of a "warm down" before you turn off, either through slower driving for the last mile or so, or to let the engine idle for 10-30secs before switch off.

Does the A2 Tdi have an after-run pump backup for the turbo?
 
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