[SS-Traveller]

Originally Posted by SKavuri

...coking in lube oil, not thermal shock.
...the lube system has a cooler and a temperature control system.
That was why my question - does a car have a cooler for lube oil? If so, there is no need for idling to avoid coking.
...if the oil can take the temperature of the engine cylinders without coking, it should be able to face the exhaust temperature without coking. Perhaps coking is not the reason for the idling.
When the temperature of TC raises or falls suddenly, the casing and impeller have different thermal expansions and may lead to seizure (the clearances are very small). This could be the reason for idling - not oil coking.
Idling during starting also helps to circulate the lube oil to all engine lubricated areas before the engine is loaded.

Interesting line of thought. From what I understand:

1. The turbo is a 'hotspot', so even with oil cooler (the sump underneath the engine acts as one), the static oil inside the turbo can coke once the engine is switched off (no more flow since oil pump turns off with engine).

2. Heat soak at the turbo once engine is turned off actually raises the turbo temperature - less heat soak happens at the cylinders, but coking is known to happen at the rings due to many reasons. This causes the oil to 'fry' and deposit out as carbon. At the piston ring, the detonation at power stroke removes the coke and blows is out of the exhaust valve to some extent - does not happen at the turbo. Think of 2-stroke engines and the coke buildup there.

3. I like to drive off slowly, with no A-pedal input, 30-60 sec after starting up a cold engine, esp. a petrol. The ECM's anti-stall & fuel enrichment programming pushes up engine rpm when cold, so as soon as oil has circulated, if you load the engine, the rpm drops - less piston flutter that way, plus quicker warmup. If my engine were a turbocharged one, the drop in rpm would ensure that the rpm of the turbo also drops till it warms up thoroughly.

[mankamble]

This turly is a great artilce for those who have their first turbo charged cars, i have a swift LDI and i installed a K&N Apolo kit replacing my stock airfilte. My mechanic says that this will trouble my turbo and may lead to its failure, the performance of my Car is great at lower RPM, i cant seem to make the difference when the car throttles to 2500 RPM (believe thats when the Turbo kicks in). Please can you clear my doubt?

[dhanushmenon]

I have a couple of queries

1) I start the car early morning, idle it/ drive slowly out through the neighbourhood and reach a place approx 1 km away where I stop over for some time. Given that the engine has not warmed up to the levels, do I still have to idle for the duration as advised?

2) if I am driving into my car lot with the last 30-45 seconds idling in neutral , do I still have to idle it after the car comes to a halt?

Sorry if the query is too dumb. Better be stupid than being sorry

[vikram_d]

Originally Posted by dhanushmenon

I have a couple of queries

1) I start the car early morning, idle it/ drive slowly out through the neighbourhood and reach a place approx 1 km away where I stop over for some time. Given that the engine has not warmed up to the levels, do I still have to idle for the duration as advised?

2) if I am driving into my car lot with the last 30-45 seconds idling in neutral , do I still have to idle it after the car comes to a halt?

Sorry if the query is too dumb. Better be stupid than being sorry

1) It is always advisable to get the engine to optimum temperature and avoid short drives where the engine does not heat up. To avoid such a situation I have taken the long way round to a destination.

2) If you have coasted to your parking area in neutral with the engine idling then you don't need to follow this procedure.

[su1978]

Thanks GTO for this wonderful article. This is something that is always at the back of my mind since i also belong to 1.3mjd family. Even though all my colleagues and friends laugh at me for this, but i know what i am doing. To them i seem to be a fool who does this and wastes time and Vitamin D. But as goes the wise saying that "one should never try to give wisdom to some fool". I simply laugh them off...

[Sweet Chariot]

after reading many review in team bhp, i have started to follow the idle at start and stop procedure, i try to engage neutral as much as possible while my street approaches and also make sure idle in the parking lot and pick all the bags and groceries from the boot and close the gate and then turn off engine. thanks for the great article GTO.

[Jeroen]

Originally Posted by Sweet Chariot

, i try to engage neutral as much as possible while my street approaches .

Not to be nitpicking but, just so you're aware: On any injection type of engine, both petrol and diesel, as soon as you come of the throttle no injection of fuel will take place. Hence no combustion. If you leave the car in gear or D the engine will still turn over, and air will pass through the engine and turbo without combustion having taken place. Ie maxium cooling effect of the turbo!

By engaging neutral, you will actually override the above and a small amount of fuel will still be injected to keep the engine idling. Admittedly just a small amount, but combustion will take place and therefore the exhaust gas hitting the turbo is actually hotter then in the above scenario.

Also, engaging neutral and coasting your car down the road is not a particular good driving habbit, but that is a different story. Actually a different thread somewhere on this forum on that one.

Jeroen

[vikram_d]

Originally Posted by Jeroen

Not to be nitpicking but, just so you're aware: On any injection type of engine, both petrol and diesel, as soon as you come of the throttle no injection of fuel will take place. Hence no combustion. If you leave the car in gear or D the engine will still turn over, and air will pass through the engine and turbo without combustion having taken place. Ie maxium cooling effect of the turbo!

Not true. We had a long argument about this sometime back. You can find all our arguments in the thread below. The second link gives you the different types of fuel cut off that happens in any vehicles.

http://www.team-bhp.com/forum/techni...save-fuel.html

http://www.team-bhp.com/forum/techni...ml#post2392914

According to your reason, if there is only air flow then there will be rapid cool down of the turbocharger which is the biggest cause of coking. We need gradual cooling to prevent coking. To cool down turbochargers, it is better to coast in neutral so that some amount of fuel is till being burnt and there is not rapid cool down.

[Jeroen]

Originally Posted by vikram_d

Not true. We had a long argument about this sometime back. You can find all our arguments in the thread below. The second link gives you the different types of fuel cut off that happens in any vehicles.

http://www.team-bhp.com/forum/techni...save-fuel.html

http://www.team-bhp.com/forum/techni...ml#post2392914

According to your reason, if there is only air flow then there will be rapid cool down of the turbocharger which is the biggest cause of coking. We need gradual cooling to prevent coking. To cool down turbochargers, it is better to coast in neutral so that some amount of fuel is till being burnt and there is not rapid cool down.

I think it is true under normal operating conditions. That decel graph comes into play when the engine hasn't reached its normal operating temperature. Under those conditions (and a few other parameters) there could still be some fuel injection under these conditions.

To be honest I don't think its a big deal if you want to coast down the road the last few hundred yards or keep it in gear.

Bear in mind that what I decribed, letting go of the throttle, fuel shutting down and air being forced trhough the engine/turbo is indeed a perfectly normal scenario. It happens all the time when you drive your car every single time you use it!

So I think doing it one more extra time whilst you get near your home is not going to make any difference whatsoever. If it was a real problem for the engine/turbo it would have packed up long ago in normal traffic conditions where you come of the throtle all the time. For extended periods even when you go down hill and use engine braking (as you should!).

I really do like this topic, very interesting. At the same time I do wonder how many real problem with turbo's (due to not warming up or cooling down) exist in the real world. I'm active of 5-6 car forums all over the world and it's very very rare to actually have somebody reporting or asking advice about a turbo repair. (especially one due to this topics).

Maybe it's a bit similar to the endless "lubrication oil" threads on car forums. Endless, often very heated debate, but very few real problems reported of repairs due to poor oil. I've owned various turbo charged cars and never any problems. That doesn't say anything of course. Still, my current car has a supercharger! None of this coking business as it has an independent oil supply and of course, it gets driven by a belt, rather than hot exhaust gasses and a turbine wheel. And it makes a great noise!

Jeroen

[thoma]

Originally Posted by vikram_d

According to your reason, if there is only air flow then there will be rapid cool down of the turbocharger which is the biggest cause of coking. We need gradual cooling to prevent coking.

Doesn't coking occur when the oil gets burned by being stationary at the hot areas? While coasting in gear, the oil is still being circulated, only thing that it is cooled down faster. Does rapid cooling down lead to coking?

[vikram_d]

Originally Posted by Jeroen

I think it is true under normal operating conditions. That decel graph comes into play when the engine hasn't reached its normal operating temperature. Under those conditions (and a few other parameters) there could still be some fuel injection under these conditions.

I'm hoping that you read through the links that I posted. Even at optimal operating temperature the ECU will inject small amounts of fuel periodically to prevent catalytic converter oxidation.

Originally Posted by Jeroen

To be honest I don't think its a big deal if you want to coast down the road the last few hundred yards or keep it in gear.

This I agree with. The only point of my post was to make you aware that even off throttle some amounts of fuel will be injected in the engine.

Originally Posted by Jeroen

Bear in mind that what I decribed, letting go of the throttle, fuel shutting down and air being forced trhough the engine/turbo is indeed a perfectly normal scenario. It happens all the time when you drive your car every single time you use it!

It is a perfectly normal scenario, the difference being that during coasting in gear or even neutral for that matter the engine is still pumping oil into the turbo and coking will not happen with flowing oil. But when you park the car the oil supply to the turbo disappears and if rapid cooling happens then there will be coking. The objective of this thread is to make people aware of cooling/spooling down the turbo before parking the car for the night (for lack of a better word)

Originally Posted by Jeroen

So I think doing it one more extra time whilst you get near your home is not going to make any difference whatsoever. If it was a real problem for the engine/turbo it would have packed up long ago in normal traffic conditions where you come of the throtle all the time. For extended periods even when you go down hill and use engine braking (as you should!).

Doing it one more time may not make a difference, but doing it doesn't hurt either.

Originally Posted by Jeroen

I really do like this topic, very interesting. At the same time I do wonder how many real problem with turbo's (due to not warming up or cooling down) exist in the real world. I'm active of 5-6 car forums all over the world and it's very very rare to actually have somebody reporting or asking advice about a turbo repair. (especially one due to this topics).

Agreed. No arguments here. How I wish we had some stats on this.

Originally Posted by Jeroen

Maybe it's a bit similar to the endless "lubrication oil" threads on car forums. Endless, often very heated debate, but very few real problems reported of repairs due to poor oil. I've owned various turbo charged cars and never any problems. That doesn't say anything of course. Still, my current car has a supercharger! None of this coking business as it has an independent oil supply and of course, it gets driven by a belt, rather than hot exhaust gasses and a turbine wheel. And it makes a great noise!

My experience also comes from driving an aftermarket turbo petrol vehicle for about 80k kms and like you I never had any problems either.

My normal habit is to spool/cool down the turbo only after a highway drive. After a city drive I never bother as there is enough traffic in Bangalore to ensure that the turbo is not even spooled up enough to produce any boost. Also just the simple act of parking the car in the garage is enough to allow turbo cool down.

Edit: @thoma - This post of mine addresses your question.

[naveenp]

Thanks for this article GTO.

I guess quite a few of us know and follow the idling rule while starting off but not at the end of the journey. I always believed if the fan isn't running, engine shut-down is the best way to cool things down.
And i had never heard things from the turbo-charger perspective! Thanks for bringing it up.

[SS-Traveller]

Originally Posted by vikram_d

...rapid cool down of the turbocharger which is the biggest cause of coking.

Originally Posted by thoma

Doesn't coking occur when the oil gets burned by being stationary at the hot areas? Does rapid cooling down lead to coking?

Nope, rapid cooling won't coke engine oil. Allowing a small quantity of oil to remain static and heating it to anything beyond 115*C (mineral) to 130*C (synthetic) will coke it. If the oil continues to circulate, it carries away the heat.

Originally Posted by Jeroen

Under those conditions (and a few other parameters) there could still be some fuel injection under these conditions.
...letting go of the throttle, fuel shutting down and air being forced trhough the engine/turbo is indeed a perfectly normal scenario.
...I do wonder how many real problem with turbo's (due to not warming up or cooling down) exist in the real world.

Originally Posted by vikram_d

...the engine is still pumping oil into the turbo and coking will not happen with flowing oil. But when you park the car the oil supply to the turbo disappears and if rapid cooling happens then there will be coking.

I don't think that exhaust gas temperature (and turbo temperature) are so straightforwardly linked to whether the fuel supply to the engine is being cut off or not. There are so many factors that would affect coking in a turbo.

For an oil-cooled turbo running on floating metal bushes (not ball bearings), the oil supply is critical to the life of the bushes - not so much in a ball-bearing design.

If the exhaust gas temp is high, stopping oil flow will coke the residual oil in the bushes.

If the oil is too hot, it will coke as soon as circulation stops. Same applies if it is too old / degenerated. Degenerated oils coke at lower temperatures, so oil change interval is more critical in turbo engines.

If the turbo is spinning over too fast, it will continue to spin when the engine oil circulation is cut off. At full boost, the speed can be 140,000 rpm - stopping from that would take a minute or two, in which time, without oil circulating, the bushes would heat up, coke (or cook) the residual oil and lose lubrication, form carbon, and self-destruct - so don't switch off the engine from full boost.

Turbo temp in VGTs will also depend on the boost pressure being generated. If turbo vanes are "feathered back" (function of throttle and ECM programming), it'll heat up less.

So, as Jeroen said, in the real world what really matters is to allow the turbo to cool down with a 60-120 sec idling, and allow it to lube up for 30-60 sec before asking it to produce boost pressure (flooring the throttle). For anything more specific (as well as for the sake of continuing a good debate ), one would need a pyrometer to check the exhaust gas temperature at turbocharger inlet, check when it falls below 130*C, and then switch off.

[sachinayak]

Originally Posted by GTO

Crawl out of your parking spot and through your neighborhood at just over idle rpms. Diesel cars move easily with zero or minimal accelerator input. Stay below 2,000 rpm, but don't lug the engine.

What exactly is lugging the engine? In slow moving traffic and in parking lots, i tend to drive my diesel car with no accelerator input. The car drives itself. Is this bad for the engine? I believe i had read, there is no negative effect of this on the engine, but wanted to double-check. Is this any different from lugging the engine?

[Sutripta]

Originally Posted by SKavuri

I am surprised that the tiny bearings in the turbo-charger are actually hydrodynamic type. .....

Originally Posted by SKavuri

From the discussion above, the reason for the idling is because of coking in lube oil, not thermal shock. We don't face this problem of oil coking in Gas Turbines because the lube system has a cooler and a temperature control system. The unit trips if the oil temp rises above the trip temp.
....
Idling during starting also helps to circulate the lube oil to all engine lubricated areas before the engine is loaded.

Some random (and possibly OT) thoughts.
For high speed rotors, esp small ones, fluid bearings are superior. And cheaper. If the traditional ball bearing with the required characteristics can be made at all.

In a large turbine, that is the centerpiece. And to keep it running, there are various subsystems. Including separately powered lubrication and cooling systems. One can't have that luxury in smaller systems, esp. where the turbine itself is a subsystem to the main engine. As an example, larger steam turbines (for electricity generation) are Hydrogen cooled.
The problem in our case is that when the engine stops, oil flow also stops. And this can happen while the spool is still very hot. A bit of coking, (or fo that matter, anything which affects the precise flow of oil) and the delicate balance between too much and too little oil is lost. Bye bye to your bearings. Differential thermal expansion is normally not the failure cause.

When oil does enter the combustion chamber, it does coke. Though most of it is blown out, that which remains can be easily seen when the engine is stripped. Other areas of the engine don't normally reach that temperature.

The piston engine reacts much faster to load changes compared to turbines. The steam generator is a particularly bad example, but the fault there actually lies with the boilers, not so much the turbines. Which is why thermal power plants are meant to provide baseline load. With other systems (notably hydro) meant to cater to the peak requirements.

Regards
Sutripta