Duramax LLY Oil Cooling, Diesel Engine Cooling
Written by Michael Patton Tuesday, 15 January 2008 14:19
Duramax Oil Cooling, Part 1
The fish are in the freezer, and with that, it is time for my annual reel cleaning and vehicle servicing regimen; an oil change is always part of it. I am reminded of what that oil went through on all those fishing trips.
It all started with a realization that my thermo-viscous fan should not be such a common occurrence. I seemed to fly effortlessly up steep mountain grades with my big camper load, on a 103ºF Arizona day, in air conditioned comfort, and in complete complacent silence, dreaming of bass boils. Then I awoke, eyes wide and startled, to the sound of a 747 landing on top of me… THAT FAN!
I wanted to silence that fan, and all heat issues its aural signature represented. I remember the first time I heard it, going 70 MPH on slight, rising terrain with NO load. On a 114ºF sunny day, it doesn’t seem to shut off. Adding insult to injury I know I am losing two to three miles per gallon whenever it is in full spin. I have heard the statement, “that fan is normal, be glad you have it,” a hundred times. I am glad. Glad I don’t have to believe that any more. What if I said that you are an oil overheater, and you have never seen it? Crazy am I? Do you have an oil temperature gauge? If you knew what your oil temp was when you towed through my back yard, you would own one. The oil overheats on every summer camping trip. Presently, oil is cooled indirectly at the stock engine mounted oil-water cooler. Oil heat must be conveyed to the coolant, then to the radiator, then to the atmosphere, making it indirectly cooled.
I have heard the statement, “that fan is normal, be glad you have it,” a hundred times. I am glad. Glad I don’t have to believe that any more.
Below, you see the Stacked Plate Duramax Coolant-to-Oil Heat Exchanger. If it looks unfamiliar, you are looking at the side that fastens to the motor. The coolant passes over a series of plates which contain the flowing hot motor oil. On the far side (left), the filter thread is visible. This heat exchanger mounts to the driver’s side of the motor and houses the filter element. The oil inlet and outlet are the parallel ports on the bottom. The coolant comes straight from the water pump and enters on the upper right (not visible), and emerges at the large plate opening at the left.
Duramax LLY Engine Oil Heat Exchanger
Simplified, it looks like this:
Simplified Diagram of the Duramax LLY Engine Oil Heat Exchanger
Heat Transfer for Dummies
We should look at what is expected of the oil. But before we do – since oil interfaces with the coolant in the heat exchanger – it will be helpful to understand the cooling system first.
The cooling system either rejects the heat the motor produces or it doesn’t. If coolant rises 15ºF in the motor, it must be cooled 15ºF in the radiator. When this does not happen, engine coolant temperature (ECT) rises over time in direct correlation with the cooling shortage, until, once again, the heat off-loaded equals the cooling system heat generated (see ECT Increase Chart on page 20). To demonstrate, consider the motor as a single mass of 1,000 pounds, with an average heat capacity of 0.20 BTU per pound-foot. If ECT rises at a rate of 5ºF per minute to the overheat stage, then the shortage can be easily calculated. The simple formula is:
Mdot*Cp*deltaT = Qdot
M is the flow rate of the coolant, usually expressed in pounds per hour.
Cp is the fluid’s capacity for holding heat energy, specific heat.
DeltaT is the temperature change of the coolant or oil.
Q is the heat in BTU per hour
1000 pounds * 0.20 (BTU/Pound-Foot) * 5 (ºF/min) * 60 (min/hour) = 60,000 BTU/hour
If ECT rises doubly 10ºF per minute, instead of five, then the radiator heat rejection deficit is also double: 120,000 BTU/hour.
Because of limited airflow, an aerodynamic reality, the rejection ability for the OEM radiator on a hot day is around 400,000 BTU/hour (Qdot); add 30,000 if the AC is off. At times of extended high loads, however, we need more like 500,000. Just take 80 GPM of coolant that weighs 8.3 pounds/gallon, and a heat capacity of 0.8 BTU/pound-foot, and some quick math, solve for deltaT in the formula above and we get 16ºF. All the radiator has to do is cool the coolant 16 degrees. Can that be so challenging? Considering that a drop of coolant spends exactly 0.75 seconds in the radiator; sure, that can be a challenge. Even worse, that highway-cruise 60 MPH air parcel is slowed down 82% with all the obstacles to air flow. A 65 MPH vehicle speed produces, at best, 11 to 12 MPH kinetic airflow through the radiator. This reduced quantity of cooling air is heated significantly, before ever reaching the radiator and in cascade fashion, by:
- Transmission Cooler (4ºF)
- AC Condenser (12ºF)
- Charge Air Cooler (48ºF with stock boost, 95ºF with augmented boost).
Added up, they total 64ºF. On a 100-degree day, the little air that even makes it to the radiator is at a nicely pre-heated 164ºF! Understanding this should make the challenge more clear.
Yet the only difference between the vehicle that overheats and the one that does not is often only two degrees coolant temp exiting the radiator. Two degrees is 60,000 BTU/hour and that’s all we have to come up with to make a noticeable difference.
As a simplified model, this is easy to understand. Since it is not likely that we can get the radiator to do much better, we are left with adding capacity to the shortage, right? Well, yes, but how shall we do this? Which coolant shall we build capacity upon?
Yes he has been drinking anti-freeze again, there is only one ‘coolant’, albeit green, yellow, orange-red… not amber!
Actually, in today’s high performance motors there are two coolants, both equally important in the transport of combustion heat away from the motor: water and oil. The water half is fairly well understood. Oil, on the other hand, is underrepresented, relegated to simple lubrication duties; yet it is now much more. (I am using the term “water” here, since a 50/50 coolant mixture takes on most of water’s thermal properties). The term “coolant” will now apply to BOTH water AND oil in this article...
In this article...
Oil, The Other Coolant
The RPM Connection
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