[shashanka]

^^^You are quite right typlo, and i was a bit surprised that the thread has hung on for so long! I felt sure that the matter had been plumbed to the depths and that there was not much else to discuss.

Going mildly OT, it brings to mind our experience with very large marine engines (upwards of 20000 HP) of a generation ago. Among the major mfrs, MAN had come up with their own solution in case of an emergency stoppage due to generator failure and the consequent time lag (usually less than 25 seconds) before the emergency power supply (the emergency generator) cut in and restored power to the ME lub. oil pump.

For this time lag (long enough to destroy the T/C bearings without lubrication) MAN had resorted to a small tank (~ 1 cub mtr or so if I recall) high over head in the engine room. This tank had an outlet pipe near the bottom which lead down to join the turbo oil inlet piping. The flow rate had been calibrated by an orifice at the outlet and the tank height provided the pr.head at the turbo inlet. The tank itself was kept full by means of a branch piping coming from the main lub. oil pump discharge. An over flow pipe led the excess oil from the tank back to the engine sump.

During the emergency shut down, oil from this overhead tank came to the rescue of the T/C by flowing down via the outlet (with calibrated orifice) and providing sufficient lubrication to the turbocharger during its winding down period till it finally stopped.

[Jeroen]

Originally Posted by typlo

Mostly, calculations involving the flow rate and standard orifice diameters. Of course my point won't stand true for the different shapes and sizes of components we have in vehicles from different makes but 30 seconds is still very conservative because 30 seconds is a long time in terms of engine or its components running without lubrication and no designer/engineer would risk that. The lubrication starts as soon as you start up your engine and oil takes merely seconds to reach your turbo. The flow rate stabilizes over the next 10 seconds or so. You're good to go. Keep in mind you're still not at operating temperature.

As you say lubrication starts instanteanously at engine start. I have done for various reasons some experiments taking oil pressure readings at various parts on car engines. It is usually seconds at best/worst. No, admittedly oil pressure is not steady flow, but still. Not sure why it would take 10 seconds to stabilise. let alone 30 seconds. Oil temperature is a factor. Or rather perhaps ambient temperatures. You leave your car overnight at -30oC and you tend to hear some very worrying sounds when you start that cold engine and cold oil. Of course, these days idling a petrol engine, led alone a diesel engine will do next to nothing for the oil temperature. In order to get the oil temperature up you need put a bit of load on the engine. i.e.d drive it carefully.

Originally Posted by typlo

The manufacturers have taken into account all of this but haven't bothered to update the manuals because most people would find it confusing. So they stick with the 1-minute rule. Taking it easy on a cold start holds much more weightage for the longevity of your power train.
.

Fact is very few people read a manual to start with. Even the esteemed petrol heads on this forum admit to that; we have a dedicated thread on it. In the real world even fewer people read manuals most likely. I wonder how many people would not even know, or care, whether they have a turbo or not. My wife, or for instance, my son in law would not have a clue. So a small wonder why the roads of the world are not littered with turbo’s missing some bearings?

I, on the other hand, do read manuals. Back to frond, several times. And I can’t remember last time I have seen this mentioned on any of our modern cars. Certainly my wife’s Fiesta and Focus don’t make any mention of it.

So, to a large extend it might be academic anyway these days.

Even so, I stick to my original advice I have dispensed about a billion times on this and other car forums. When in doubt go with the manufacturers manual, not the internet.

Originally Posted by shashanka

Going mildly OT, it brings to mind our experience with very large marine engines (upwards of 20000 HP) of a generation ago. Among the major mfrs, MAN had come up with their own solution in case of an emergency stoppage due to generator failure and the consequent time lag (usually less than 25 seconds) before the emergency power supply (the emergency generator) cut in and restored power to the ME lub. oil pump.

For this time lag (long enough to destroy the T/C bearings without lubrication) MAN had resorted to a small tank (~ 1 cub mtr or so if I recall) high over head in the engine room. This tank had an outlet pipe near the bottom which lead down to join the turbo oil inlet piping. The flow rate had been calibrated by an orifice at the outlet and the tank height provided the pr.head at the turbo inlet. The tank itself was kept full by means of a branch piping coming from the main lub. oil pump discharge. An over flow pipe led the excess oil from the tank back to the engine sump.

During the emergency shut down, oil from this overhead tank came to the rescue of the T/C by flowing down via the outlet (with calibrated orifice) and providing sufficient lubrication to the turbocharger during its winding down period till it finally stopped.

This is not a MAN specific solution. This particular solution has been around on ships for as longs as they have had turbo charged engines. We certainly had this tank with gravity feed to the turbo’s on MV Street Florida when I joined it in 1978 on my very first trip as a then apprentice marine engineer. The engine was a two stroke, B&W, 12.5000 HP. Two turbo’s

Jeroen

Jeroen

[Rahul Bhalgat]

Originally Posted by R2D2

With my NA car I follow a 1-2 minute idle after a run at highway speeds.

Interesting. What benefit can we expect from this practice? If the engine has been running at 2000-2500-3000 rpm on highway, it would be well lubricated already.

[R2D2]

Originally Posted by Rahul Bhalgat

Interesting. What benefit can we expect from this practice? If the engine has been running at 2000-2500-3000 rpm on highway, it would be well lubricated already.

I do this to eliminate hot spots within the motor after running at highway speeds - for the lower temp coolant in the radiator to make its way around the block and similarly let the engine oil continue lubing heated parts at lower pressure. A sudden cut off of lubrication and coolant circulation is in my reckoning not something I would like with my cars.

Some of my friends even open the hood/bonnet to let the engine cool down faster..which I don't practice unless the engine temp gauge showed a markedly increased temperature.

PS - it is for the same reason that I do not switch off the engine if the automatic radiator fan is on. One it goes off then I switch off the engine about 10 seconds later.

[MSAneesh]

I drive a Brezza with the national turbo diesel. What i have noticed is, when cold started in the morning, if listened carefully, the engine note is a little bit like a sine wave with a little bit of ups and downs. When idled for about 30 seconds, it becomes flat(uniform) note. That is when I move into first gear and start rolling.
Subsequent starts in the day usually gives the flat note much quicker. In cold weather, usually an evening start after office also takes 30 secs.
Has anyone observed this? Experts can explain why this is so; Oil flow, or turbo spooling or something else.
Or..am i psyched out?

[shashanka]

Originally Posted by Jeroen

....This is not a MAN specific solution. This particular solution has been around on ships for as longs as they have had turbo charged engines. We certainly had this tank with gravity feed to the turbo’s on MV Street Florida when I joined it in 1978 on my very first trip as a then apprentice marine engineer. The engine was a two stroke, B&W, 12.5000 HP. Two turbo’s Jeroen
Jeroen

Sorry old boy, beg to differ. Undoubtedly the above solution has been around for a while, but not for all T/Charged marine engines. This is a solution applicable for all T/chargers with journal/sleeve bearings (Napier based) including MAN who supplied its own T/chargers using journal bearings. These bearings run the risk of stepping into boundary lubrication conditions in case of lub. failure - just a step away from wiping away the bearing surface & resultant catastrophe.
Not so with BBC/ABB t/chargers (we had these on some of Mitsui's car career fleet where I was C/E from '98 to '08) which have low friction bearings (read ball/roller) with independent self-contained lub systems (including sump) isolated from the engine lub system. The lubrication is carried out by a pump mounted on & driven by the turbine shaft itself, thus requiring no external input. As the the shaft speed falls (with fuel cut-off), lubrication reduces till standstill conditions.

Mods, please do delete the post if you feel it is wildly OT!

[Jeroen]

Originally Posted by shashanka

Sorry old boy, beg to differ. Undoubtedly the above solution has been around for a while, but not for all T/Charged marine engines.

Sorry chum,
Never said it was. I said it was not a MAN specific solution as you suggested, and it is not. I also said it has been around for about as long as we have had turbo charged engines on ships. Never said it was the only solution out there.

Even if you go back to the very early days of turbo charging you are likely to find a gravity fed system. If not as a primary way of lubricating, it will be there as a back up system.

Even on some of the as you point out low friction bearings with self contained system you might still find the gravity fed systems as a precaution.

Initially there were some concerns whether these self contained system would be able to ensure sufficient lubrication when the engine was switched off. Due to over pressure in engine rooms, the turbo’s tended to spin, slowly but surely, on differential pressure between the engine room and the outside.

So they added the gravity system which is very simple. It still requires a pump to push the oil up to the gravity tank. And if the pump stops and the tank emptied all bets are off. On some engines I have sailed on you had to clamp down the turbo’s during prolonged engine idle time.

Lastly, contrary to your point, the gravity oil fed turbo on my first ship was a BBC Turbo charger. This ship and it’s engine were built in the mid/late 60’s in Japan under license from Burmeister & Wain

Although I have not made an exact count, I believe the gravity feeds were more often used on the slow speed engine then the medium speeds. I can not even remember what we used on the medium speeds on the various ocean tug and AHT I sailed on. Probably all simple self contained stuff.

Back to car turbo’s again

Jeroen

[vharihar]

To cool the turbo after a long, hard drive, I have read that one should either idle it for a minute, or run it *slow and easy* for the last 3-4 kms of reaching your destination, if you aren't able to idle it for a minute.

Question:
Taking it *slow and easy* means driving at slow road speed? Or low engine RPM (almost to the point of, say, lugging the engine)?

[NaXal]

Originally Posted by vharihar

Question:
Taking it *slow and easy* means driving at slow road speed? Or low engine RPM (almost to the point of, say, lugging the engine)?

I am not an expert here so do wait for others to comment.

But as far as I know, every Turbo Charged Engine has its own specific engine RPM speed for Turbo engagement.

Keep the engine RPM below that.

For example, Aspire 1.5 TDCi spools up the turbo after 1500RPM so I try to keep the engine around 1000/1200 RPM for that slow down period.

Vehicle is able cruise easily within that in top gear so highway cool down isn't a problem if I am aware of the stop location in advance.

Otherwise I do idle it for a couple of minutes before turning of the engine.

[Jeroen]

Originally Posted by vharihar

Taking it *slow and easy* means driving at slow road speed? Or low engine RPM (almost to the point of, say, lugging the engine)?

Originally Posted by NaXal

But as far as I know, every Turbo Charged Engine has its own specific engine RPM speed for Turbo engagement.

There is no specific RPM at which the turbo starts spinning. It spins all the time as long as the engine is running. So even at idle the turbo still spins.

The speed of the turbo is determined by two parameters. The amount of exhaust gas, (i.e. volume per time unit) and the temperature of the exhaust gasses.

In order to cool down the turbo you want a high volume of relative cool exhaust gasses to go through it. High volume means high engine RPM. But cool exhaust gasses also mean that the engine should carry a as light load as possible.

In practice, just drive normally for a few minutes, don’t lug and make sure to keep the RPMs up a bit.

Jeroen

[vharihar]

Originally Posted by Jeroen

In order to cool down the turbo you want a high volume of relative cool exhaust gasses to go through it. High volume means high engine RPM. But cool exhaust gasses also mean that the engine should carry a as light load as possible.

In practice, just drive normally for a few minutes, don’t lug and make sure to keep the RPMs up a bit.

Jeroen

If I just drive normally albeit at slightly higher RPMs as you suggest for the cool down in the city roads leading up to my home (in the last 2-3 kms of a long, predominantly-highway trip), how does the turbo/engine differentiate bween the hard highway driving vs the cool down period driving?

[IndigoXLGrandDi]

Sorry if I sound stupid?

Originally Posted by Jeroen

In order to cool down the turbo you want a high volume of relative cool exhaust gasses to go through it. High volume means high engine RPM. But cool exhaust gasses also mean that the engine should carry a as light load as possible..

If I am not wrong this is possible only when the transmission is in Neutral and engine at idle rpm. There is as actually no load on the engine when the car is in Neutral and engine is at idle rpm (irrespective of the speed of car).

Originally Posted by Jeroen

In practice, just drive normally for a few minutes, don’t lug and make sure to keep the RPMs up a bit.

Won't keeping the rpms up a bit put load on turbo instead of relieving it?

Originally Posted by vharihar

how does the turbo/engine differentiate bween the hard highway driving vs the cool down period driving?

[Jeroen]

Originally Posted by vharihar

If I just drive normally albeit at slightly higher RPMs as you suggest for the cool down in the city roads leading up to my home (in the last 2-3 kms of a long, predominantly-highway trip), how does the turbo/engine differentiate bween the hard highway driving vs the cool down period driving?

Obviously, the car or rather the engine has not got a clue what road you are driving on. But the speed at which you are driving makes a huge difference on the engine loading and therefor on the temperature of the exhaust gasses. As you know air resistance rises sharply as speed increases. So in order to maintain say 120 km/h the engine has to produce a considerable higher output than say at 80 km/h. Engine output (i.e. horse power) has a direct correlation to the exhaust temperature.

Say you drove for an hour on the motorway at 100 km/h, get of the motorway and drive the next 2-3 km at a sedate 40-50 km/h. Effectively the engine load in those last few kilometers is greatly reduced and therefor the exhaust gas temperature is substantially reduced too. And that is the effect you are after. You want to get relatively cool air through that turbo

Originally Posted by IndigoXLGrandDi

S
If I am not wrong this is possible only when the transmission is in Neutral and engine at idle rpm. There is as actually no load on the engine when the car is in Neutral and engine is at idle rpm (irrespective of the speed of car).

Technically the engine is still loaded, but it is a small load. It still takes power to turn the engine over, move the valve train, run the cooling pump, turn the fan, turn the alternator. A couple of horsepower easily If you are running an AC add a few more horsepower. But of course, at idle it is the lowest load you can have.

Originally Posted by IndigoXLGrandDi

Won't keeping the rpms up a bit put load on turbo instead of relieving it?

In order to cool the turbo you need relative cooler exhaust gas and plenty of it. As I said a combination of exhaust temperature and volume.

To maintain a steady speed you need a certain loading of the engine. By slowing down from Motorway Speed/loading to sedate cruising speed for the last few kilometres you have already reduced the engine loading considerably. Therefor you will have also reduced the exhaust gas temperature. By dropping a gear, you hardly change the loading of the engine, but you do put a lot more air through the turbo. It is a bit theoretical, the biggest gain is of course reducing the speed and therefor reducing the loading of the engine and subsequently the exhaust gas temperature.

It is all relative. Whether you cruise at 80 km/h or at 130 km/h makes a huge difference for your engine loading and therefor exhaust gas temperature! So any cool down period required (if at all) is less when you were cruising at a slower speed on the motorway to start with.

Truth is, in normal driving we rarely load up the engine anywhere near it’s maximum output. You really have to nail it constantly. Pedal to the metal. Most of us do that rarely, or maybe for the more enthusiastic drivers amongst us, have an opportunity to do so. So it is probably fair to say that most turbo’s on cars rarely reach anywhere near their maximum operating temperature. Make no mistake, they can handle it easily, hour after hour. My wife’s little Ford Fiesta will happily cruise at 140-150 km/h hour after hour on the German Autobahn. Nothing will break, nothing will wear down.

But you can imagine that it makes quite a difference of the turbo cooling down after such a drive, then we she does her weekly shopping run and drives at 60-70 km/h for 15 minutes to get to the supermarket.

Car manufacturers will be very prudent and careful in their recommendations, so they base their cooling down periods on the worst case scenario.

When in doubt always follow whatever the manufacturer tells you.

Enjoy those turbo’s, and unless you drive like the proverbial bat out of hell constantly, the changes of damaging your turbo are remote at best/worse.

Think of it like this, most drivers are probably totally unaware they have a turbo fitted to their cars/engine. Even fewer people actually read a manual. So in practice, apart from the petrol heads on forums like these, most drivers haven’t got a clue about turbo’s and or cooling down period. Although I do not think anybody has measured it, I doubt the percentage damaged turbo’s differs between those that cool down and those that do not.

But you can not go wrong with cooling down a turbo. Whether you drive the last few miles a bit more sedately, or let the engine idle for a bit afterwards. I am an engineer at heart and there are certain things I do or just won’t do to an engine. Even thought I know it does not make that big a difference if any at all. It just makes me feel better. Cool down periods on modern car engine is something I do not worry about.

Jeroen

[vharihar]

Thanks Jeroen.

I am no auto/mech engr, but my gut feel told me all these days that the engine rpm is a *proxy* for the "loading", by means of the transmission. In other words, whether you end up revving the engine at 2500 rpm by driving in the 5th gear at 90 kmph, or at the same 2500 rpm by driving in the 3rd gear at 40 kmph, I thought that the engine loading was the same in both cases (since the rpm is the same).

But you seem to say that the loading is higher in the 90 kmph case.

I didn't know that.

The specific recommended manner of driving about "easy driving for the last few kms" is still unclear to me. I guess net net, we can quit worrying about this issue. Idle for a minute at halt if you can, else don't lose sleep over it.

[Rahul Rao]

@vharihar.
Load does matter. Actually with low load engine is running at lower compression and generating lesser torque.
Compare a steady 3rd gear 2500 rpm at 90 and 5th gear 2000 rpm at 90 on the same road stretch, both conditions will generate almost the same power, but 3rd gear 2500 Rpm will generate lesser torque yet you have the potential of much higher torque and acceleration if you floor it, add to that the torque multiplication as the engine is at higher speed in lower gear, you have the potential to get much higher acceleration too.
Check your instantaneous efficiency on the MID. A highway stretch that shows you 25km/lit at 80, will show you something like 18km/lit at 120. Now in the first case you are burning 4 lit/hr while in the second 6.7

lit/ Hr, but naturally generating more heat and exhaust.



Rahul