[Barryng]

In 1985 I bought a Nissan 300ZX Turbo. In those days there was much concern for destructive oil coking in the center bearing after shutting down the engine. There was a ton of discussion about this, turbo failures were not uncommon, there was a plethora of clever aftermarket gadgets to provide some minimal oil flow through the bearing when the engine stopped, and I would religiously let the engine idle for about 30 seconds before turning it off.



Now, turbos are ubiquitous. I have two late model turbos including a BMW X3 and my new Boxster and the concern for the center bearing cooking in its own heat after engine shutdown just does not appear to come up. What changed and what now protects the oil from breaking down in the center bearing after oil flow stops when the engine stops?

 

[johan]

In 1985 I bought a Nissan 300ZX Turbo. In those days there was much concern for destructive oil coking in the center bearing after shutting down the engine. There was a ton of discussion about this, turbo failures were not uncommon, there was a plethora of clever aftermarket gadgets to provide some minimal oil flow through the bearing when the engine stopped, and I would religiously let the engine idle for about 30 seconds before turning it off.



Now, turbos are ubiquitous. I have two late model turbos including a BMW X3 and my new Boxster and the concern for the center bearing cooking in its own heat after engine shutdown just does not appear to come up. What changed and what now protects the oil from breaking down in the center bearing after oil flow stops when the engine stops?

There is an electric pump for that in our cars

 

[718S]

I’m guessing synthetic oil has helped?

 

[Barryng]

I’m guessing synthetic oil has helped?

I am sure it does but I used Mobile 1 going back to about 1979 or so. When I had the 1985 300ZX turbo, it was understood synthetic was much better but even it did not prevent the center bearing from eventually failing. I did not perceive the turbo itself, due to the center bearing, had a 100,000 durability even when doing all reasonable due diligence to minimize an issue.


I noticed a fan runs for a short time after I stop the engine. At times it runs at an obviously higher speed than other times. Is this fan directly associated with the pump referenced above that I assume continues to move oil through the bearing until the turbo cools sufficiently to preventany abrasive oil degradation?

 

[718S]

There is an electric pump for that in our cars

I thought they have mechanical oil pumps? Are you saying there is a supplemental electric pump specifically for this post shut down for turbo?

 

[johan]

I thought they have mechanical oil pumps? Are you saying there is a supplemental electric pump specifically for this post shut down for turbo?

I might be wrong, coolant sounds like water.

Electric coolant run-on pump for turbocharger

The electric coolant run-on pump for the turbocharger is located at the front right in
direction of travel on cylinder bank 1 in the 718 Boxster/S MY17. It is switched on by
the DME control unit depending on the operating conditions. It can also be activated
as required after the hot engine is switched off in order to cool the turbocharger by
circulating the coolant.

 

[718S]

I might be wrong, coolant sounds like water.

Not fair! You added that quote about the coolant... I didn't know that.

 

[DriveInHouston]

I used to have a Mercedes C200 Kompressor (a supercharger) and I always tried to be near idle for at least 30 sec before shutting the engine off. This was because the supercharger is always on and under load. A turbo under city conditions, very seldom comes under load, and at least in my city, I can't come in in the driveway like Hamilton coming in to change tires...

I'd be hard pressed to think that I ever parked the car with the turbo under load... EVER! So, I pull in at the parking spot and shut down the engine without thinking twice about it.

 

[Cinnamon]

I would say the turbo is essentially oil-cooled, so in this case the "coolant" is oil. In past decades I was concerned about cooking the turbo and would make sure to let the motor idle a little while before shut down. But then it seemed that upon approach to my destination whether home or elsewhere the engine was under very light load for a while so the turbo should not be particularly hot and my concern went away. Having fluid circulating through the 718's turbo bearings for a bit after shut down is reassuring however!

 

[Barryng]

I would say the turbo is essentially oil-cooled

I am sure that is correct and the forced flow of oil is the primary coolant for the bearing. However, when the engine stops, unless something else is done, the flow of oil suddenly comes to a stop, the heat energy remaining in the exhaust turbine continues to conduct into the bearing, causing the oil, even synthetic, to degrade into an abrasive products. There are many strategies used to mitigate this problem. One is to continue to maintain oil flow after the engine stops with an electric pump or some sort of stored energy device. Another is to wrap the bearing (and contained lubricating oil) in a water jacket and continue some flow of water at least for a short time with an aux cooling pump. Nevertheless, my original post really was just looking for a response from someone that has factual knowledge of what Porsche does in our cars. I am certain if there was no specific engineered solution to mitigate this problem, turbos now would still be just as unreliable as they were 35 years ago. "johan", in a previous post quoted something about an electric coolant run-on pump for the turbocharger but I was not sure from his post if this indeed is the solution in our cars and if the fan that runs on after the engine is stopped is associated with this.

 

[ZXTT95]

There is a link somewhere on this forum (I think) to a 2017 Boxster technical intro. I think that's what Johan was quoting. It includes pictures, and another section on thermal management that all together suggests that coolant (in this case referring to coolant and not oil) is pumped through the turbo, including after the engine is shut off.

 

[Barryng]

There is a link somewhere on this forum (I think) to a 2017 Boxster technical intro.

Thanks, I have that manual but obviously I was not smart enough to remember it.

Page 52 has a diagram of the cooling system. Paying attention to the little arrows (I assume represent check valves), a flow loop can be seen with the "electric run-on pump for turbocharger" driving flow to the turbocharger then to the radiators and ultimately back to the suction of the run-on pump. This appears consistent with why the radiator fans fan run for a short time after the engine is shut down.

Although not shown in the flow path diagram (and it is titled "Function overview"), I have to assume there is a flow path around the run-on pump to allow good flow during normal operation when the run-on pump is probably not running. OR, does the run-on pump run all the time and assures at least a minimum coolant flow to the turbocharger? Additionally, the flow path from the radiators to the run-on pump goes through the "switched coolant pump". Again, because most pumps when they are not running can represent significant flow resistance, how does the run-on pump flow easily go through it?

I understand my questions probably cannot be answered by anyone here but, just in case, I am throwing them out there.