Direct Oil Cooling and the Turbo Diesel Direct Oil Cooler (TD-DOC)

Direct Oil Cooling, Part 2

Written by Joel Paynton Monday, 14 April 2008 19:00

In Direct Oil Cooling – Part One, we discussed indirect and direct cooling methods and we examined oil’s new role as a coolant, rather than a mere lubricant, with new technologies such as under-piston oil squirting shifting more – up to 50% more – of the turbo diesel’s total cooling burden to the oil. We observed worst-case oil temperatures exceeding 360ºF, causing oil pressure to plummet and rendering oil, as a lubricant, virtually useless. In Part Two, we’ll look at the science behind indirect and direct oil cooling and discover why direct cooling, properly engineered, offers the only solution that can control:

Oil temperature,
Viscosity, and
Flow rate

to design specification limits under those same high load/high RPM conditions; and, substantially expand the overall capacity of the cooling system. In doing so, we will discover a host of benefits that can be realized only by directly cooling your oil.

Fact: As we saw in Part One of Direct Oil Cooling, GM has chosen to design and implement an indirect oil-cooling system for the Duramax engine. One result – other than living with scorching oil temperatures – of this OEM system that is not able to keep oil temperature under control during times of high combined work loads and high RPM is a host of after market solutions that promise to atone for the cooling system’s shortcoming. Unfortunately, many of them simply attempt to beef up the existing system with a bigger radiator or another fan instead of providing a solution based on a sound engineering approach that does not create undue burden on the overall system. That is, they continue to try, piggyback style, to cool the oil indirectly. While these would-be solutions may succeed in reducing oil temperature, for example, from 360ºF to (only) 330ºF, they do so at a great burden to the Duramax electrical system as well as taking a toll on precious fuel economy.

It’s Too Much, Darling

The ability to directly control oil within specified limits comes at a co$t generally considered unacceptable by auto manufacturers. When GM produced its second generation Corvette, for example, it chose to factory-fill the known 280ºF lubrication system with synthetic oil. This decision was NOT based on a ploy to entice enthusiasts – a great marketing tool – but because, looking at the bottom line, GM decided it was smarter to spend $5 on more tolerant oil than to provide the sports car with a more robust oil cooling system. They even had the foresight to market this decision as a high-performance, no-compromise benefit of buying the car. Now, I am an engineer and not an accountant and so I do not know – or want to know – all the data that go into the decisions of what gets included and what gets tossed when designing and building my truck; but, what is good for stock dividends is not necessarily good for my engine’s oil – or, in the long run, for the engine itself. Rather than the direct oil cooling system on the Duramax that I would have liked to see, we find the DMax coolant-to-oil heat exchanger that was introduced in Part One. It places the burden of cooling the oil on the radiator fluid (that I’ll refer to as “water” for the rest of this article.) The water, tasked with cooling the engine, must also cool the oil that, as we have already mentioned, can exceed 360ºF – under stress conditions – when using the indirect cooling method.

More of the Same

In an effort to provide a solution, some companies have offered upgrades to the indirect cooling system in the form of electric fans or larger radiators to enable the system to cool more water than it had been able to previously. The idea is that cooler water is the best way to arrive at cooler oil. As we will see, it is not a well though out idea. It goes down something like this:
You drive to your local truck shop on a sunny day and someone, maybe even someone you trust, walks up to you and says,
“Did you know that water holds 40% more heat than oil?”

This someone then concludes for you that water is obviously a superior cooling agent than oil; and therefore, increasing the capacity of the existing, indirect cooling system is, of course, the way to go. This “salesman” will likely follow up that seemingly inarguable fact by quoting the flow rate advantages of water over oil to further build your trust.

To show the shallow logic employed in the above statements that argue in favor of the indirect cooling method will take some time: first, I will address the statement that water is a superior cooling agent and then we will compare “flow rate advantages” and what they actually mean inside our DMax engine.

duramax-oil-cooler-oil-temp-increase-over-time

Water, the Superior Cooling Agent (and Why It Doesn’t Matter)

In the world of heat exchange, it is common knowledge that a pound of water holds 40% more heat than a pound of oil. This is important because the more heat a substance can hold, the greater potential it possesses as a coolant that is able to carry away heat from the engine. Theoretically, a pound of water can, when circulated through my engine, remove a full BTU of heat; a pound of oil, on the other hand, only removes 0.6 BTU. The truth is, while water, theoretically, is the best cooling agent found under the hood of your truck, it just doesn’t matter. There are at least two straightforward reasons for this...