IDLING=COOL DOWN?

Ripster · **Posted:** Tue Jul 18, 2006 8:58 pm

This unit cools down fast on a coast and in gear. I have about a 3/4mile coast down into town, and I can be 800 at the top of the hill, coast down the hill and turn in a parking lot and be at 400 idle in gear to a parking spot and it will start to climb up a bit, get an EGT gauge, they pay for themselves.

oldnavy · **Posted:** Tue Jul 18, 2006 9:02 pm

AZ CRD wrote:

vtdog wrote:

Quote:

Sounds like you need to do some research on google

.

I don't know about research on google, but I can state a few physical principals that apply which demonstrate why I have questions.

1. Diesel engines are NOT adiabatic.
2. The engines must have some external cooling system (normally heat radiation via liquid into cooling unit)
3. The rate of heat loss (transfer) is DIRECTLY proportional to the surface area of the heat transfer device (radiator), the effeciency of heat transfer of the liquid cooling fluid, the temperature differential between the heated liquid and the ambient air (or liquid in liquid/liqid transfer as in marine diesel), and the flow of either air, or liquid across the radiation fins to allow the heat to be removed.

Given that "oil" is the cooling medium of the turbos, the physical principals of heat transfer still apply. Therefore, the reduction in heat transfer rate due to both loss of internal liquid movement (oil) inside the cooler and the reduction of air flow across the radiator fins must reduce the cooling capacity at idle.

Therefore, the crux of the matter relates to how much the reduced spinning of the turbo contributes to the coolling of the turbo unit vs the loss of cooling capacity. I personally don't know wheteher the extreme temps are due to exhaust gas and/or friction of the turbine.

I don't claim to have the answer, but knowing a little about heat loss in metals (and i really do mean a little) it would seem that the recommended cooling times are way too short to bring the temps down from 800-1000F. I don't know the flash and/or decomposition temp of the synthetic oil (which I assume is higher than dino oil) but it would seem to me that the cooling requirments for the engine and/or turbo MUST be sufficient to keep the oil from cooking off.

So, again I ask the quesdtion, if the cooling capacity is reduced because of idle speed, how the hell does that oil get so cool so fast?

Excellent points. I've been thinking the same thing. But, I couldn't have stated it as clearly.

Because the oil temp also drops at idle and besides as someone else stated the oil is much cooler then the turbo which has had very hot exhaust blowing through it at speed up to at least 25,000 rpm's and the 6.4 qts of oil flowing through a turbo at idle will bring it down to 300*/400* real fast. That is why really smart folks use the high temp capable synthetic oil in turbo's.

Ripster · **Posted:** Tue Jul 18, 2006 9:05 pm

Buy an EGT guage and you can watch it in action, a picture is worth a 1000 words.

Jeger · **Posted:** Tue Jul 18, 2006 9:06 pm

Turbo's get hot mainly because of exhaust gas. Exhaust gas is obviously cooler when there is no load or when idling.

It may be that the cooling RATE is slower while sitting still at an idle ,but the fact is that it is still cooling the turbo down with oil flow.

Does anyone know if our turbos are only oil cooled? Or do they also have water cooling.

oldnavy · **Posted:** Tue Jul 18, 2006 9:09 pm

Jeger wrote:

Turbo's get hot mainly because of exhaust gas. Exhaust gas is obviously cooler when there is no load or when idling.

It may be that the cooling RATE is slower while sitting still at an idle ,but the fact is that it is still cooling the turbo down with oil flow.

Does anyone know if our turbos are only oil cooled? Or do they also have water cooling.

No, water boil at too low at temp for street use.

Ripster · **Posted:** Tue Jul 18, 2006 9:11 pm

2003 GT Turbo Cruiser was oil and water cooled, temps on freeway were 1200 pretty hot,
but never had any issues, tough as nails, excellent design.

Jeger · **Posted:** Tue Jul 18, 2006 9:20 pm

I guess to answer the main question of how it gets from 800-1000 down to around 400 so quickly you need to realize that at idle not only is the oil still doing its thing but you are no longer pushing hot exhaust gas through the turbo. So I suppose it is the combination of the two things..cooling...and reduced temp input that allows the fast cool down times. Plus the entire contents of your oil never get to the 1000 degree mark that temperature is the temp of exhaust gasses.

vtdog · **Joined:** Fri Sep 30, 2005 11:27 am **Posts:** 640

Quote:

I guess to answer the main question of how it gets from 800-1000 down to around 400 so quickly you need to realize that at idle not only is the oil still doing its thing but you are no longer pushing hot exhaust gas through the turbo

Ok, then that is the answer: at idle the turbo is not spinning in hot gas and cooling can occur

One more minor question: what happens to the exhaust at idle. Is is shunted past the tubo vanes directly out the tail pipe?

RFCRD · **Posted:** Tue Jul 18, 2006 11:30 pm

vtdog wrote:

So, again I ask the quesdtion, if the cooling capacity is reduced because of idle speed, how the hell does that oil get so cool so fast?

Short answer: No torque load. A few things to note about newer electronic diesels:
1). They idle relatively cool. It would be interesting to see a real temp gauge on this engine. Most modern diesels can hardly maintain 160-170 deg water temps when idling on a 70 deg day. The ECM will lean the fuel delivery to a bare minimum at an idle without a load causing it to run cold. Detroit Diesel even warns against long idling because combustion temps are too cold leading to fuel contamination of the motor oil.
2). Flow of fluids (oil and coolant) are very important in reducing not only the turbo temp but also drawing heat out of the head and block.
3). The exhaust side if turbo casting can get scorching hot (almost glowing red) under extreme load. Even a 400 deg exhaust gas flow will cool this down considerably.
4). The center shaft of the turbo is oil lubed and cooled. Keeping the oil flow moving is important to draw excess heat from the center shaft and housing.

As a genreal rule of thumb: If you ever get a moderate overheat and you have adaquate coolant and oil in the engine, you are usually better off letting the engine idle rather than just shutting it down hot. Most Detroit Diesel products are actually programmed to throw a CEL and go into a progressive "power down" mode when overheating occurs to protect the hardware. Don't know if the CRD it set-up this way.

Just do yourself a big favor. When starting cold, let it idle a few minutes before driving to get some heat in the block. Drive moderatly until fully warmed. Cool it down to draw the heat out of not only the turbo but also the head and block (which may include some slower driving followed by a couple minutes of idling) before shutting down. Your diesel will thank you and stay happy for a long time.

kjfishman · **Posted:** Tue Jul 18, 2006 11:35 pm

BCool wrote:

huh, thats good, mine gets to about 3:00 in hot weather down here on steep grades. I wonder if mine is broken. It seems to get hotter with bio.

There was a TSB for that issue awhile back. You may not be overheating the PCM was not calibrated correctly on some units, simple flash to fix.

alljeep · **Posted:** Wed Jul 19, 2006 7:37 am

oldnavy wrote:

Because the oil temp also drops at idle and besides as someone else stated the oil is much cooler then the turbo which has had very hot exhaust blowing through it at speed up to at least 25,000 rpm's and the 6.4 qts of oil flowing through a turbo at idle will bring it down to 300*/400* real fast. That is why really smart folks use the high temp capable synthetic oil in turbo's.

Tried to educate myself last night and found this good basic primer on turbos - RPMs up to 150,000 !!!

http://www.turbo-kits.com/how_turbos_work.html

Here is another one that states some applications go up to 300,000 RPMS :shock:

with some good cutaway pictures. Also mentions oil cooled and water cooled turbo applications as well as fluid bearing and combination ball bearing with oil fluid cooling.

http://www.turbomustangs.com/turbotech/main.htm

Both of them dance around the cool down issue. I think one states that temperatures must normalize before shut down in order to prolong the life of the turbo.

Cutaway of a TO4:

onthehunt · **Posted:** Wed Jul 19, 2006 7:58 pm

OK- my turn!! Turbo's get hotter the more you use them. More demand for power=more fuel burned=more heat. The whole cool down thing is really not that important UNLESS you are really using the engine. Say like towing or cruising at 75mph for awhile. Just driving around town isn't going to build the kinda heat to actually need a cool down period. Chances are by the time you pull into your parking spot the turbo is already cool enough. Turbo cooling at idle occurs because there is no load on the vehicle. Fuel injection is enough to keep engine running and no more. So when you pull off the road with your heavy trailer the turbo is pushing exhaust gas that was 900f but is now getting cooler and cooler because you are no longer dumping fuel into engine. This cooler exhaust(400f!) brings the turbo temp down. Also more importantly, the oil is still circulating and this also helps bring the temp down and provides lube for the very hot bearings in the turbo. Also your assumption that oil is the cooling medium is wrong. It is a combo of the exhaust gasses, oil , and air flow over the turbo. The oil is primarily there to lube the bearings. Remember that the same oil is also circulating through the engine. This is where the oil is cooled off. I think is this confusing to many because when we refer to cooling the oil we mean cooling at or below engine operating temp. Turbo's are at 300-400f normal and 900f hot. Engine's hot are 210f. Hence you get oil cool down even when your engine is hot. BTW the last oil sample told me my oil flashpoint was 415f(shell rotella-t). Imagine shutting off your jeep and the exhaust temp is 900f There is no oil made that would stand up to this.

Ripster · **Posted:** Wed Jul 19, 2006 9:02 pm

Can't be too many questions after that one!

Goglio704 · **Posted:** Wed Jul 19, 2006 10:13 pm

Somebody may have made this point, but I didn't see it. When you are idling for cool down the turbo is producing little or no boost. Boost is air compression. Air compression always produces heat. Just one more aspect of the equation. In other words, a turbo that is working hard is generating heat itself. It would get hot when generating 20 PSI even if it were being spun by room temperature air. This is not to contradict anything others have said. Just one more (smaller) piece of the puzzle.

retmil46 · **Posted:** Thu Jul 20, 2006 12:32 am

And in some cases it may be more than the turbo you're protecting.

In the search that eventually led me to my second diesel, the '87 MB, I was often told the particular diesel in this model year had a rep for getting cracked heads, and how to check for this.

Couple days ago, I got the Bentley Owners Bible for the W124 MB's. Under the engine section, it made specific mention of this as a potential problem, and gave a simple method to avoid it.

If you'd been running the car hard enough to make good use of turbo boost, prior to shutting down it recommended giving the engine at least a 30 second or more cooldown period to avoid getting a cracked head.

redcrd · **Posted:** Thu Jul 20, 2006 8:45 am

That is correct turbos dont need cool down unless heavy accel, towing, long higher speed drives have taken place

RFCRD · **Posted:** Thu Jul 20, 2006 1:55 pm

retmil46 wrote:

And in some cases it may be more than the turbo you're protecting.

In the search that eventually led me to my second diesel, the '87 MB, I was often told the particular diesel in this model year had a rep for getting cracked heads, and how to check for this.

Couple days ago, I got the Bentley Owners Bible for the W124 MB's. Under the engine section, it made specific mention of this as a potential problem, and gave a simple method to avoid it.

If you'd been running the car hard enough to make good use of turbo boost, prior to shutting down it recommended giving the engine at least a 30 second or more cooldown period to avoid getting a cracked head.

Yep...and crack a wet liner, you won't be a happy camper either. Do yourself a favor, warm it up slowly and cool it down when you're done using it. There is more heat energy than you all think stored in that piece of iron.

cerich · **Posted:** Thu Jul 20, 2006 3:30 pm

RFCRD wrote:

retmil46 wrote:

And in some cases it may be more than the turbo you're protecting.

In the search that eventually led me to my second diesel, the '87 MB, I was often told the particular diesel in this model year had a rep for getting cracked heads, and how to check for this.

Couple days ago, I got the Bentley Owners Bible for the W124 MB's. Under the engine section, it made specific mention of this as a potential problem, and gave a simple method to avoid it.

If you'd been running the car hard enough to make good use of turbo boost, prior to shutting down it recommended giving the engine at least a 30 second or more cooldown period to avoid getting a cracked head.

Yep...and crack a wet liner, you won't be a happy camper either. Do yourself a favor, warm it up slowly and cool it down when you're done using it. There is more heat energy than you all think stored in that piece of iron.

Warm it up slowly??? How?, it has a viscious heater hat does the job in like 200 yards to operating temp. By the time I hit the stop sign on my residential cul d sac the libby is warm.

RFCRD · **Posted:** Thu Jul 20, 2006 6:50 pm

cerich wrote:

RFCRD wrote:

retmil46 wrote:

And in some cases it may be more than the turbo you're protecting.

In the search that eventually led me to my second diesel, the '87 MB, I was often told the particular diesel in this model year had a rep for getting cracked heads, and how to check for this.

Couple days ago, I got the Bentley Owners Bible for the W124 MB's. Under the engine section, it made specific mention of this as a potential problem, and gave a simple method to avoid it.

If you'd been running the car hard enough to make good use of turbo boost, prior to shutting down it recommended giving the engine at least a 30 second or more cooldown period to avoid getting a cracked head.

Yep...and crack a wet liner, you won't be a happy camper either. Do yourself a favor, warm it up slowly and cool it down when you're done using it. There is more heat energy than you all think stored in that piece of iron.

Warm it up slowly??? How?, it has a viscious heater hat does the job in like 200 yards to operating temp. By the time I hit the stop sign on my residential cul d sac the libby is warm.

True that the viscious heater is a big help but getting warm air out of the interior vents doesn't mean that the metal mass of the engine and transmission are warmed to operating temperature. Just look how long it takes to get the transmission to upshift to 5th on a cold morning (several miles). This is done from a temperature sensor and computer logic, an intellegent move on DC's part to protect an expensive component.

CATCRD · **Posted:** Thu Jul 20, 2006 6:52 pm

vtdog wrote:

Wait, stop right there

Quote:

The turbo bearings need to be cooled (i.e. lubricated since oil is a coolant for these turbo's)

Ok, so what is cooling the oil? Again, it has to be radiational cooling either through the radiatior, or oil cooler. If the vehicle is at rest and idling, then less air is passing over the coils, which means less heat loss, etc. So, idling actually reduces the colling capacity of the truck. I understand that the turbo may be spinning slower, etc BUT the loss of heat removal via reduced (or eliminated) air flow has to effect the turbo cool down possibility. In addition, the engine itself is actually getting warmer on idle and that would further reduce the chances for oil cooler heat transfer due to temp differential being reduced as heat output from the hot engine would actually be increased

Whew, I thought I had problems understanding how a propane refrigerator works, but this is even more of a mess

It's not the air flowing over the outside of the turbo that's cooling it. It's the exhaust gases flowing inside. They can reach oveer 1000F on a hard pull and even 800 just driving down the highway. At idle the temp of your exhaust gases will drop and drop down to like 300 to 400 degrees because the engines not burning much fuel. This is cool enough not to cook the oil in the turbo bearings, like others have said. It has nothing to do with fans or coolant temp.

IDLING=COOL DOWN?

Who is online