Greetings all- I just got back from our annual trip visiting family. Hauled our 27ft Travel Trailer from San Diego CA to Dubach Louisiana and back approx. 3200 miles. What a road trip (half of the trip seemed like was crossing Texas). Our truck pulled flawlessly. Something that I pondered while driving the long miles is the cool down period when stopping. I know that it is recommended to let the motor idle for a while before turning it off especially after pulling at highway speeds for several miles. Since I don't have any gauges I could not tell how hot every thing is/was. I wondered how long I should let it idle when coming in for re-fuel. I wanted to post this and get yalls opinions/experiences.
Thanks in advance for your time and response.

 

if youre just stopping to gas up i would just leave it running. by the time you would wait the 6-8 minutes to cool it down you could be filled up already. i dont ever turn mine off just to fuel up. but if you want to turn it off then i suppose 6-8 minutes is a decent time to let the idle cool stuff off. if you had gauges then you could really see how much cooler stuff gets just from idleing it......

 

Bill I would have to agree. Being able to watch the temps on gauges, you would be surprised how quick it actually does cool down. Once my EGT's get down to around 300* I'll shut it off.

 

Gauges are really the key since it depends on how hot you were when you stopped. Sometimes it doesn't take but 30-60 secs and sometimes in the summer I have waited 4-5 min. If gauges aren't in the future than Bill lined it out for you.

 

when towing or driving agressively I am more concerned with actual turbo temp more then just egt. your egt will cool off fast compared to the carterage of the turbo, which uses oil to cool it off. I would say a rule of thumb is to let the turbo wind down, egt to go to 300, and then wait an additional 1 min if you were just hot rodding it a little bit. If I just got off the hyway, Ill let the truck "cool" for about 3 min, because there is a lot of heat built up in the turbo and such from reving 2k all that time.
Its the bearings of the turbo that you need to worry about, not the egt. If the turbo is hot still, then the oil will coke in the center section and cause a lubrication failure. Letting the center section of the turbo cool by running the engine oil through it, will let it last longer, and not "ruin" your oil as fast by burning it.
I have dealt with a lot of gasser turbos that have been "driven hard and put away wet" and had to replace a number of them with oil just burned into the centersections and failed because of lube issues. I reccomend letting the truck cool for around 3 min or so after getting off the hyway, or after pulling heavy loads.

 

Interesting JR, thanks for the little lesson. Makes sense...

 

Thanks for the replies...UNTAMND you bring up the point of what I have read and cautioned about, I just didn't know how to articulate it. Would there be any out ward signs of trouble, or is it just cumulative over time and not known until a breakdown?

 

I'd say 3 min is a minimum time especially during the summer and if you've been towing at altitude then I'd say 6 min is a minimum time because everything will be hotter to begin with and the cool down will be slower due to the thin air ... and opening the hood definitely helps both the turbo and engine cool down quicker. Just for fun I'll see how well my EGT equation predicts EGT cool down...

EGT={(1.2)(MAT)+(19,000)/(AFR)+(100)} *F ...where MAT=Manifold Air Temperature *F and AFR=Air Fuel Ratio which is defined by AFR=MAF/MFF where MAF=Mass Air Flow lbm/min and MFF=Mass Fuel Flow lbm/min.

Say you've been towing for awhile and then you immediately park and let the engine idle as you watch the EGT gauge. Say the Atmospheric Air Temperature is 80 *F and the initial MAT is 138 *F. The MAT will slowly decrease to about 90 *F as the intercooler and the air intake plumbing cools off. So for VE=0.80, RPM=700, AAP=14.7 psia, BP=0 psig, and MAT=138* F the initial MAF is... MAF={(VE)(RPM)(AAP+BP)}/{(MAT+460)(2.882)}={(0.80)(700)(14.7+0)}/{(138+460)(2.882)}=4.8 lbm/min.

At idle the Volume Fuel Flow is 0.5 gal/hr and this corresponds to a MFF=0.058 lbm/min and using the above idle MAF=4.8 lbm/min gives an initial idle AFR of... AFR={(MAF)/(MFF)}={(4.8)/(0.058)}=83 ...and this gives an initial EGT of... EGT={(1.2)(MAT)+(19,000)/(AFR)+(100)}={(1.2)(138)+(19,000)/(83)+(100)}=166+229+100=495 *F.

After the MAT cools to 90 *F the MAF increases to... MAF={(VE)(RPM)(AAP+BP)}/{(MAT+460)(2.882)}={(0.80)(700)(14.7+0)}/{(90+460)(2.882)}=5.9 lbm/min ...and the AFR increases to... AFR={(MAF)/(MFF)}={(5.9)/(0.058)}=100 ...and this predicts an idle EGT of... EGT={(1.2)(MAT)+(19,000)/(AFR)+(100)}={(1.2)(90)+(19,000)/(100)+(100)}=108+190+100=398 *F!

However the "100" constant value in the EGT equation accounts for the convective heat that's picked up by the intake airflow due to its contact with the hot combustion chamber and cylinder walls and the "100" value is for an engine that's under load ...so at idle this "100" value eventually becomes a "50" value as the hot combustion chamber and cylinder walls cool down and then the final idle EGT becomes... 398-50=348 *F ...and going from an initial idle EGT=495 *F just after parking to a final EGT=348 *F is exactly what I experienced during my 8+ years of towing with my 7.3L!

The "1.2" constant in the EGT equation accounts for the intake airflow picking up more heat as it's compressed than it releases as it expands... and the "19,000" constant in the EGT equation combines many factors including the Btu heating value of diesel, the specific heat capacity of the combustion gas, the exhaust valve timing, the injection timing and pressure, etc...