How long to idle in the cold?

[DPKetchum]

Pre turbo is the only place to install a egt gauge. I never heard of post turbo until this forum. I had a 1st generation (93 W250)and NO ONE even discussed the issue of post turbo. If it was brought up by a NEWBIE it was like don't even bother with installing one then.

 

[Jefff929]

My pre-turbo temps run 500 to 600 at 65 mph. Hard pull up a steep hill, unloaded will get to 1500 if I don't let up on it. It idles around 300 degrees.

Same for me. haven't hooked the 5er on since the gauges were installed. Now I'm thinking of hooking it up and going for a ride just to see the EGT's :-laf

 

[rbattelle]

If it was brought up by a NEWBIE it was like don't even bother with installing one then.

Ouch! I like my post-turbo gauge, thank you very much! After I get an ATF manifold, then I'll put in a pre-turbo gauge too.



-Ryan

 

[DPKetchum]

You may like it but its not good info your reading from it. Having to read it and it says 800 degress but let me think. Is that really 1200 degrees cyl. temp or 1300 or humm. By the time that you get that sorta its toast. Its a easy install I'm sure post turbo. Fact is thats not were its suppose to go to be any real benifit.

 

[rbattelle]

You may like it but its not good info your reading from it.

I have a different opinion, but I certainly respect your opinion on the matter. We should "agree to disagree" on this one.



-Ryan

 

[Kennywould]

Hey Kennywould - I'm a Steelers fan too, doesn't that mean all my opinions are automatically correct?



-Ryan

HeHe... a wolf in sheeps clothing? Hard for me to believe, unless your a transplant! :-laf :-laf :-laf

 

[Kennywould]

You may like it but its not good info your reading from it. Having to read it and it says 800 degress but let me think. Is that really 1200 degrees cyl. temp or 1300 or humm. By the time that you get that sorta its toast. Its a easy install I'm sure post turbo. Fact is thats not were its suppose to go to be any real benifit.

I have to agree, it would be confusing but I run pyro's in a boat motor in the collectors, you train your brain to think 1350 is the limit, over that your toasting pistons real quick! It's personal preference pre or post but in a gasser you can pull a plug and check the color, so you know without a doubt 1350 is the number. I prefer pre on mine... .

 

[DPKetchum]

I was messing with diesels some before I knew about TDR and it was never even suggested to put pyros post turbo. Then I found TDR I think 4 or 5 years ago or so and never did post turbo "ever" come up(once or twice as I stated) until the 3rd generation forum and I held my piece thinking DO WHAT! So now I've said it.

 

[Dean Upson]

As far as the cool-down goes, I have a pre-turbo probe and have been surprised at how fast the egt goes down (or up). When I pull into a fuel stop, by the time I am up to the pump and get my log book out, I am cooled down to 300* or less. I shut the ebrake off when I enter the stop. My temps usually run around 600* running flat on the interstate and can go to 1100* or so when towing heavy going up a long grade.



Dean

 

[CJEliassen]

I respectfully disagree because this assumes no heat transfer between the turbine and the exhaust stream. Given the very low specific heat of air I expect the exhaust gas temperature to approximate the turbine temperature within just a few degrees.

Steel does not cool down that fast. The turbo can be several hundred degrees hotter than the EGT. As a helicopter pilot and mechanic, I can tell you that we let the turbines cool down for several minutes before shutdown. We don't look at any temps, because they are wortless. They will read roughly the same prior to cool down as they will after. EGT's do not give you any indication of the temperature of the metal.



If what you said is true, we would never have a problem with coking, and there would be no need for all the cool down timers for sale.

 

[rbattelle]

EGT's do not give you any indication of the temperature of the metal.

I very respectfully disagree.



On edit: I'm surprised you don't watch the EGT gauge before shutting down your helicopter. Is it because the engine manufacturer dictates a minimum idle time? Of course you want to idle for a bit even after the EGTs reach a safe level on a turbine engine so that the case, and rotor temperatures can settle down and (perhaps most importantly) to account for the possibilities of hot zones outside the "view" of the thermocouples. With my truck, even when the EGT gauge indicates a temperature well below 300 I idle it for 10-20 seconds just to be sure the bearing compartment for the turbo (through which no gas passes) has time to cool a bit to prevent coking.



If the EGT gives no indication of the metal temperature, then why would anyone want to look at any EGT gauge at all? You're implying the EGT could be 1100 and the metal could be 300, if there really is no connection between EGTs and metal temperature. And if there is no connection as you imply, where does the metal heat come from? Radiation alone?



On further edit: you must idle your turbine engine because you need to be sure the bearing temperatures fall below the coking temperature of the oil. There is no convenient way to measure bearing temperature using a thermocouple in the exhaust stream. I expect the turbine blades, and vanes to be right about the same temperature as the EGT gauge indicates.



-Ryan

 

[CJEliassen]

I very respectfully disagree.



On edit: I'm surprised you don't watch the EGT gauge before shutting down your helicopter. Is it because the engine manufacturer dictates a minimum idle time? Of course you want to idle for a bit even after the EGTs reach a safe level on a turbine engine so that the case, and rotor temperatures can settle down and (perhaps most importantly) to account for the possibilities of hot zones outside the "view" of the thermocouples. With my truck, even when the EGT gauge indicates a temperature well below 300 I idle it for 10-20 seconds just to be sure the bearing compartment for the turbo (through which no gas passes) has time to cool a bit to prevent coking.



If the EGT gives no indication of the metal temperature, then why would anyone want to look at any EGT gauge at all? You're implying the EGT could be 1100 and the metal could be 300, if there really is no connection between EGTs and metal temperature. And if there is no connection as you imply, where does the metal heat come from? Radiation alone?



On further edit: you must idle your turbine engine because you need to be sure the bearing temperatures fall below the coking temperature of the oil. There is no convenient way to measure bearing temperature using a thermocouple in the exhaust stream. I expect the turbine blades, and vanes to be right about the same temperature as the EGT gauge indicates.



-Ryan

Actually, yes. That is exactly what I am implying. If you are cruising along at an EGT of 300 degrees and you floor it and let the EGT's climb to 1100 degrees, then the turbine takes time to heat up to 1100 degrees. This is why bursts of EGT to 1500 or 1600 degrees won't cause any damage. However, if the temps were sustained, the metal would eventually get to this point.



And you contradict yourself with your statement of "hot spots" outside the view of the termocouple. If all the EGT gases are the same temp, then why would you have hot spots. Well, the reason is that you may have hotspots because the metal does not cool down nearly as fast as the gases around it.



I have ran turbo charged engines with EGT's at 1700F and the turbo is glowing red. When the power is reduced, it can take 30 seconds or even a minute for the turbo to stop glowing.



Are you saying that everyone here who purchased a count down timer wasted their money?

 

[InThinAir]

I think that these conversations about pre or post are funny. There are advantages to both. Too say that post is of no value, you would have to argue with a lot of OTR mechanics because that is where most of theirs are installed. Most E-brakes come with a place for the probe built in. So to say that you don't know what you are doing when installing it post, is pretty short sighted. Post is the only location that you can safely say that the turbo has reached a proper shut down temp. If you using a pre to determine shut down temp, to quote" don't even bother installing one". The pre turbo temp is now 300 so that is like 400 post or is it like 250.



It all depends on what you have the guage for. I am more concerned with longevity so I want to know what the turbo temp is before I shut down. My trucks are mostly stock and I am not too worried about a meltdown. My trucks have seen temps that would make most poeple cry out like a school girl. And I have seen no ill effects, so I drive it with an eye towards the guage, but use it most for cool down.



---Doug

 

[opjohnny]

You guys are going to have to learn how to play nice. Thank you for all the answers. The temp of that bearing is a complex problem. I figure the 5 min to 1 min in the manual covers a hot day. Just feel kinda silly when it's 15deg F idling the truck getting the roll eyes from the family. Might have to dig out the engineering books and look. Now if I can only find that thinking cap.

 

[LightmanE300]

OPjohnny, I think you'll spend more collective minutes wasting time looking it up than just letting your truck idle 15 seconds before shutdown - all the time . .

 

[opjohnny]

I had all that extra time since MNF is over for the season! Maybe I should just get some gauges. Do otr trucks come with egt gauges as standard equipt?

 

[rbattelle]

I have ran turbo charged engines with EGT's at 1700F and the turbo is glowing red. When the power is reduced, it can take 30 seconds or even a minute for the turbo to stop glowing.

And what was the EGT when the turbine was red hot? I'd bet you all the cash in my wallet it was above the luminosity temperature of the materials in the turbo.



Countdown timers are certainly useful - especially for those without an EGT gauge. It's a way of automatically idling for the suggested times in the owners manual.



I think the big thing is the bearing compartments, which don't get any air flow through them. Certainly these cannot be conveniently measured by EGT thermocouples and so even if the EGT is low it's good to let things idle for a bit to be sure the bearings are cooled below 300 degrees (approximate coking temperature for the oil).



You seem to suggest that the difference in specific heat of air and steel is of no bearing on the problem. You also imply that there is no heat transfer between the metal and the gas (as you admit to implying). If there is no heat transfer, then where is the heat from the turbo dissipating? For that matter, how does a heating coil work? By your logic, if I blow air across a resistance heater that's been heated to 1000 degrees the air that passes across it will simply remain at ambient. Get my drift? You seem to be ignoring heat transfer here, but I'm not that brilliant so maybe there's a part of the problem I'm not quite seeing.



This is an interesting discussion... I'm not trying to argue with you per se, I'm just curious why your view of things is so greatly different from mine.



-Ryan

 

[opjohnny]

Going to have to find a SAE paper on this. I think the fresh oil is doing some of the cooling at idle on the bearing. If 300 deg F. is the magic number for the oil to stay below what is the sump temp of the engine oil? Is it near water temp? higher or lower? I don't think I have ever had an oil temp gauge on anything I drove.

 

[rbattelle]

Going to have to find a SAE paper on this.

Amen!



CJEliassen, I respect your opinions on the matter. Being a gas turbine mechanic you must know a thing or two about turbomachinery and heat transfer. As I said earlier in this thread, the things I've posted are purely my opinion. I strongly urge each reader of this thread (and all my posts for that matter) to do your own independent research and come to your own individual conclusions about things. I only offer my opinion... judge it as you see fit.



Now, on to more opinions...



opjohnny - I have an oil temperature gauge, but I'm not sure it reads very accurately due to its position atop the oil filter mount. It's calibrated for 140-320 degrees F. Driving down the road the needle remains pegged at 140. I suspect this is due to the cooling effect of air flow across the filter and probe when the vehicle is moving. When I stop and idle the temperature gradually rises to as high as 170 on a hot day. That's just a theory. The reason I believe the gauge is inaccurate is I've read elsewhere that the oil temperature pretty much tracks the water temperature, so I would expect an oil temp closer to 200 degrees.



I'd like to move my oil temp probe, but I don't know where else there's a convenient port for it...



-Ryan

Disclaimer - I mean absolutely no ill-will toward anyone in this thread. My "tone" is not one of anger or confrontation. Please don't anyone think I'm being mean-spirited in any way.

 

[CJEliassen]

And what was the EGT when the turbine was red hot? I'd bet you all the cash in my wallet it was above the luminosity temperature of the materials in the turbo.



Countdown timers are certainly useful - especially for those without an EGT gauge. It's a way of automatically idling for the suggested times in the owners manual.



I think the big thing is the bearing compartments, which don't get any air flow through them. Certainly these cannot be conveniently measured by EGT thermocouples and so even if the EGT is low it's good to let things idle for a bit to be sure the bearings are cooled below 300 degrees (approximate coking temperature for the oil).



You seem to suggest that the difference in specific heat of air and steel is of no bearing on the problem. You also imply that there is no heat transfer between the metal and the gas (as you admit to implying). If there is no heat transfer, then where is the heat from the turbo dissipating? For that matter, how does a heating coil work? By your logic, if I blow air across a resistance heater that's been heated to 1000 degrees the air that passes across it will simply remain at ambient. Get my drift? You seem to be ignoring heat transfer here, but I'm not that brilliant so maybe there's a part of the problem I'm not quite seeing.



This is an interesting discussion... I'm not trying to argue with you per se, I'm just curious why your view of things is so greatly different from mine.



-Ryan

Ryan,



The EGT's drop immediately to about 400-500F while the metal is still glowing red.



I never said that there is no transfer of heat between the metal and the air. I am saying that the heat transfer is considerbly slower than then drop in EGT. If you have a hair dryer and you put your hand in front of it, the air is much much cooler than the coils heating the air. Those coils can light paper on fire. The air is nowhere near hot enough to do the same.



If, and only if, you have a post turbo EGT probe, you will see a slightly higher EGT and a very slight drop as it cools, but it gives you very little indication of the temperature of the turbine itself unless you were to perform many tests as the turbine cooled and see how it relates to EGT's. If you have a pre-turbo EGT, there is absolutely no indication of turbine temps.



The temperature differential dictates the rate of heat transfer. A turbo at 1100 degrees with EGT's at 300 is going to cool much quicker, but as it drops down to 600 degrees, the rate of cooling slows, and it continues to do so.



Since metal and liquid conduction is a better conductor of heat than air conduction, a significant amount of the heat in the metal will be dissipated through the shaft and into the circulating oil.