IDLING=COOL DOWN?

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

OK, someone is going to have to explain this to me. I understand the "need" for a turbo cool down after long hi way driving, etc. What I don't understand is how idling produces said cooling. Heat is removed from the engine via radiational cooling (i.e. cool air passing over the fins of the radiator). Since idling means a hell of alot LESS air moving over the coils than say driving at 75mph, how does this idle cooling work?

I ask this as yesterday I drove about 200 mi in the very hot weather and got off the hiway to off load some equipment into another vehicle. I left the engine running (as required) for the turbo cool down. However, only AFTER idling a few minutes did I hear the electric fan come on. This means that the engine was actually getting hotter, not colder as it idled as the fan is tripped by temp.

So, explain to me how idling means cooling?

Oh, and FYI at no time, even during long climbs in mountains (well, what passes for mountains here in the east) did the temp ever move above "11:58" on the gauge.

BCool · **Posted:** Tue Jul 18, 2006 11:24 am

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.

Ripster · **Posted:** Tue Jul 18, 2006 11:29 am

When you idle the heat created by the exhaust and esp the turbo temp is reduced. With an EGT gauge
you can see it happen, a must in my book. Turbo is probably 800 going down the highway or more, once
you coast and stop the temperature of the exhaust gas will slowly go down to around 400 or less if you
set there. We are talking about exhaust gas temp, not water temperature in this case.

alljeep · **Posted:** Tue Jul 18, 2006 11:38 am

Yes, the cool down is for the TURBO. EGT with a stock muffler on a hot day could approach 1,000 defrees F under the right conditions. If you shut if off quick you can cook it as the engine oil curculating through it stops.

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

So, if I understand this right. The cool down is for the exhaust temp/turbo , not the engine temp.

Since the electric fan came on only after a few minutes idling this means that the engine temp was actually rising while the exhuast temp was falling? I'm neither an engineer nor physisist (thank god), but what you guys are saying seems like there would come a point where the engine temp rise would quickly outpace the exhaust temp fall and idling would be counterproductive for cooling.

oldnavy · **Posted:** Tue Jul 18, 2006 1:05 pm

vtdog wrote:

So, if I understand this right. The cool down is for the exhaust temp/turbo , not the engine temp.

Since the electric fan came on only after a few minutes idling this means that the engine temp was actually rising while the exhuast temp was falling? I'm neither an engineer nor physisist (thank god), but what you guys are saying seems like there would come a point where the engine temp rise would quickly outpace the exhaust temp fall and idling would be counterproductive for cooling.

The fan is kicking on to move more air through the heat exchanger for the a/c and not to cool the engine coolant. You will notice that a/c always runs cooler when you are moving down the road then sitting still.

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

Ok Old Navy, I believe you, but sure seems like black magic to me

oldnavy · **Posted:** Tue Jul 18, 2006 1:17 pm

vtdog wrote:

Ok Old Navy, I believe you, but sure seems like black magic to me

Does doesn't it. :wink:

swoughen · **Posted:** Tue Jul 18, 2006 4:15 pm

The cooling is needed for the pressure and temperature buildup on the intake side of the fuel injectors when the turbo is engaged... ?

The fuel temperature and pressure is much reduced when the engine is idling and the turbo is not engaged - what exactly happens to the turbine fins, do they melt, warp or become ceased if the cooling does not take place... ?

DZL_LOU · **Posted:** Tue Jul 18, 2006 4:47 pm

The idling is just for the oil to lubricate and cool the TURBO well. It has no bearing on engine temp.
The turbo bearings need to be cooled (i.e. lubricated since oil is a coolant for these turbo's)

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

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

cerich · **Posted:** Tue Jul 18, 2006 7:22 pm

cool down may make you think it means cold, it doesn't. What it means is that the temp of the turbo is dropped enough that when the oil stops flowing on shut down it doesn't do exactly what the grease from your burger does on the BBQ grill, turn into black sticky crap.

If you want to see this in action change from synthetic to dino oil now when the temps are high, do an hour on the highway, drive into a air conditioned parking garage and shut here down suddenly with no cool down. When the turbo is replaced the damage wll be easy to see.

oldnavy · **Posted:** Tue Jul 18, 2006 7:25 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

Actually the diesel is so thermal efficient that it will cool down while at idle and get cool enough that fuel doesn't properly burn. Sounds like you need to do some research on google.

Jeger · **Posted:** Tue Jul 18, 2006 7:29 pm

Even though your coolant may get hotter at Idle it is still nowhere near the temperature of the turbo. So even if your coolant got up to something really hot like 300 degrees :shock:

It would still cool down an 800 degree turbo.

If you want your entire system to be at its coolest before shutdown the best thing you could do would be to coast down a long incline in neutral.

oldnavy · **Posted:** Tue Jul 18, 2006 7:38 pm

By the way your normal coolant temp is 173*F for the CRD, and it has a viscious heater for the engine coolant to help it warm up faster and help hold coolant temps up at idle.

Jeger · **Posted:** Tue Jul 18, 2006 7:45 pm

oldnavy wrote:

By the way your normal coolant temp is 173*F for the CRD, and it has a viscious heater for the engine coolant to help it warm up faster and help hold coolant temps up at idle.

Learn something new everyday

Thanks.

BTW our chihuahua is viscious too.

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

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?

oldnavy · **Posted:** Tue Jul 18, 2006 8:29 pm

Jeger wrote:

oldnavy wrote:

By the way your normal coolant temp is 173*F for the CRD, and it has a viscious heater for the engine coolant to help it warm up faster and help hold coolant temps up at idle.

Learn something new everyday

Thanks.

BTW our chihuahua is viscious too.

Dang laptop can't spell for crap. :roll:

oldnavy · **Posted:** Tue Jul 18, 2006 8:34 pm

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?

Many gas engine tech's have problems learning diesel because they are so different then gasser.

Do a search and read up on EGT's, turbo's, cylinder wash, thermal properties compared to gasoline and all will be made right in your world. I say this again not being a SA or AH of a person. You will be able read and understand it better there then I can state everything here while I try to watch TV and type.

AZ CRD · **Posted:** Tue Jul 18, 2006 8:55 pm

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.

IDLING=COOL DOWN?

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