[sandeepmohan]

I can say this for all the Fiat Linea T Jet owners. The turbo charger is liquid cooled as well. The engine coolant is used to cool the turbo charger. It will interesting to know how they have carved out the water jackets inside that small turbo charger. There is a coolant line that runs to the turbo but no visible return line back to the coolant tank. I have verified this with my service centre as well and was quite surprised they even knew about it.

The car user manual does not talk about idling the engine during a cold start or hot shut down. Drive off immediately is what they recommend even if it is a cold start as idling the engine is just wasting/consuming fuel. If you have been driving hard, they suggest you go easy on the throttle for a little before you turn off the engine so the engine can catch its breath. I do buy this theory.

I go easy on the throttle when the engine is cold. It takes about 4-5km before the temperature gauge indicates optimum temperature. This behaviour can vary from car to car.

[TaurusAl]

Originally Posted by rajni

Although I am not an expert on this I don't think your problem has something to do with this problem. I have had similar issue with my old Petrol Swift, which obviously doesn't have a Turbo an got it resolved at the Service station. The reason given by the service folks was dust particles deposited at the fan belt or clutch plates - can't correctly remember.

Driving a swift petrol, you will not understand the turbo whistle I'm talking about in DDiS engines. Whenever you floor the accelerator you hear a whistle from the engine and no, its not of the fan belt like your petrol swift.

The only point I wanted to make is, even if you don't drive your car in Turbo RPM range, the turbo may still be rotating inside and abrupt shutting-off your car may have adverse effect on the turbo.

[A350XWB]

Originally Posted by dhanushs

That is why we idle for a minute, as while idling turbo boost is not there, and the circulating oil can cool the turbo down, even if its spinning in its own inertia.

As explained below, this idling is to cool down the turbo bearings and not the entire turbocharger. As far as I know, the problem area is the turbo bearing which uses circulating oil for lubrication and as a medium for heat exchange. When the engine is switched off, the oil flow stops, but the shaft is still spinning at a very high speed. The oil stuck in the bearings heat up quickly, as it cannot carry the heat away and results in coking (which is solid mass) and the ultimate destruction of the bearing. But in case of foil-air bearings, this problem should not happen. (Foil bearing - Wikipedia, the free encyclopedia)

Originally Posted by dadu

The problem is more than the cooling of the turbo's as most turbo manufactures provide sufficient oil flow to the bearing in their designs. However, the extremely high temperatures that the turbo creates will cause the oil in the bearing loose its viscosity and lubricating qualities and cause coking in the turbo bearing housing. Over time this coking will restrict the flow of oil to the bearing leading to its failure.

As far as I know, the reason for idling is the one given above by dadu.

Originally Posted by Sutripta

Essentially cooled by the now much (relatively) cooler exhaust gases.

Are you sure about this? I mean, cooling by the cooler exhaust? As far as I know the other parts of a turbocharger, other than the bearings are much more heat tolerant. That is, they don't melt down if the engine is switched off after a spirited drive, especially with technologies like ceramic turbine blades. I know that it's always better for components (especially metal ones) to cool down gradually rather than cooled down abruptly, which can affect their strength and composition. (e.g: a really hot cylinder head (cast iron) can crack if cold water is poured on it, directly)

[Optimus7]

Originally Posted by rajni

By the way, since this thread was started a few years ago I hope things would have changed a lot. I recently bought my first diesel car - Skoda Rapid. Any ideas if the idling is applicable to this car also.

Yes Rajni, the idling is applicable to your diesel car too. In the previous posts dadu and 1self have given a great explanation on the need for idling a turbo charged engine, and that it still applicable to newer cars too.

[rajni]

Originally Posted by 1self

Now, turbos which have an oil & water cooled central revolving assembly are much better off in the cooling department simply because they have 2 sources of cooling simultaneously. These are also ball bearing turbos so they, by superior design, produce less heat as they operate.

Most diesel turbos are oil cooled sleeve bearing turbos. It is with these that the idling-after-running-hard rule comes into significance. As dadu mentioned, coking is liable to occur when a hot turbo is not sufficently cooled & the engine stopped. Coking is a process of 'burning' of lubication oil which breaks down oil into carbon & other sticky substances. This prevents the flow of oil through the narrow & fine orifices, which results in a much-shortened life of the turbo bearings.

Thank you for such a detailed explanation 1self. Now, I know I should read a lot about Diesel engines. I just bought my first Diesel car a month ago and still learning the differences. By any chance, do you know handy what all engines come with an oil and water cooled central revolving assy as you mentioned? Or else what are the parameters one should be careful about. The Rapid's engine as per the specs given by Skoda is turbocharged diesel engine, in-line, liquid cooling system, high-pressure direct injection system, 16V DOHC, transverse in front. So, where does it say if it is just oil-cooled or oil+water cooled and about the bearing.

[1self]

Rajni, the info tech specs of the turbo will only be available in the shop manuals with the dealers. But, i strongly suspect it will be the sleeve bearing type.

[Sutripta]

Originally Posted by A350XWB

Are you sure about this? I mean, cooling by the cooler exhaust? As far as I know the other parts of a turbocharger, other than the bearings are much more heat tolerant. That is, they don't melt down if the engine is switched off after a spirited drive, especially with technologies like ceramic turbine blades. I know that it's always better for components (especially metal ones) to cool down gradually rather than cooled down abruptly, which can affect their strength and composition. (e.g: a really hot cylinder head (cast iron) can crack if cold water is poured on it, directly)

Hi,
When running at full chat, both turbines and housing run red hot. Which is why metallurgy of the turbine part is critical. It has to withstand continuous full power runs. These are therefore made of special 'superalloys'. So the question is not of heat damage to those parts themselves.

The turbo housing central part (housing the bearing) can, and increasingly is, oil/ water cooled. This is not universal, though. Also this cannot cool the turbine. If suddenly stopped after a full power run, the problem is one of heat soak into the bearing area from the super hot areas. And with a stagnant oil film, which is likely to get coked. With consequent later damage to the bearings. (Slightly OT- have you noticed in older cars with Xylene based temp gauges that the temp increased )when the engine was switched off after a hard run).

Running at low power (after a high power run) cools down everything.

1self, being I think a marine engineer, can comment authoritatively on exhaust gas temperatures of large diesels! They have to log it.

Regards
Sutripta

[A350XWB]

Originally Posted by Sutripta

If suddenly stopped after a full power run, the problem is one of heat soak into the bearing area from the super hot areas. And with a stagnant oil film, which is likely to get coked. With consequent later damage to the bearings. (Slightly OT- have you noticed in older cars with Xylene based temp gauges that the temp increased )when the engine was switched off after a hard run).

Regards
Sutripta

Yes, that makes sense. And I have noticed the temperature rise you mentioned. This heat sinking affects the weakest link in the chain; ie the turbine bearing, right?

[chaunfa]

I have been sticking to idling the Storm after a nice clip around. I just adore the way it moves and don't mind the slow minute or less for the turbo. It is anyway my first 'leal tulbo' so I'm sticking to the sticker.

[Deepalimal]

I usually follow one minute idling rule before and after my spirited runs. I have one question regarding engine switch off at red lights. If I follow idling rule strictly for one minute, I am never able to switch off my car. If suppose red light has 100 secs left and I idle for 60 secs before switching off, should i switch it on again after 10 secs so as to give 30 secs warmup before moving on ? or I can drive directly after turning the car on.