Diesel Timing, Duramax Diesel, Diesel Cooling System

Written by Michael Patton Monday, 02 November 2009 12:57

Diesel Timing Your Cooling System?

diesel-timing With a powerplant that only operates at 35% efficiency, losing three percent to diesel timing issues – a common occurrence – represents nearly 10% of available output... that is BIG. By itself, that three percent can reduce the speed of your truck on an extended grade enough so that the cooling system no longer receives sufficient air to fulfill the heat load rejection requirements of the cooling stack. On the other hand, gaining that three percent with optimum diesel timing can not only mean maintaining the power and speed that your truck needs to keep the cool air flowing... it can also mean that your engine actually sends less heat into the cooling system.

I have a confession to make: I am not a big fan of dynos. Torque curves on paper don’t excite me that much. Instead, I like to see a truck’s work performance in the real world. A vehicle that has been tested with a 15,000-pound trailer was not meant to be confined to a gerbil cage flexing its quadrupeds. If you were to extend that five-second WOT power level for two minutes, those curves would erode: a 300 HP truck optimized in a cool garage for those five seconds will often lose 25 to 75 HP on an sustained desert grade.

As you may have guessed already, this article focuses on work trucks and the work scenarios for which the DMax was built. A Corvette can produce 300 HP just like a Duramax, just not for very long. If it were to produce that level of power for more than a minute or so, at legal speeds, its relatively low grade cooling system would be unable to off load the tremendous heat created by that sort of power. In comparison, the Duramax is equipped with a robust cooling system that will keep it cool during extended high-heat-generating periods such as those experienced during towing or hauling. Of course, I am not trying to compare the DMax and the Corvette to say that one is better than the other: each is superior in its own role. The Corvette’s cooling system, while certainly less robust, is also far less parasitic than the Duramax system. The Vette produces power to accelerate quickly and then it does not need that sort of power – or cooling – anymore. For the Duramax to put out 300 HP for extended periods, it was equipped with a powerful, but more parasitic, cooling system that can shed the heat created by the engine. This ability to keep the engine from melting comes at a cost...

The Cooling System: A Necessary Evil

The bad news – but news that we have learned to live with in the world of internal combustion – is that if an engine has a final rated output of 300 HP, it sheds nearly the same amount of power (an additional 200 to 300 HP!) through the cylinder walls and heads. This is the dilemma faced by all internal combustions engines: the cooling system must be present to handle the waste heat produced by the engine, but it does so by removing power in the form of heat energy, power that could be driving down the piston, creating torque and, ultimately, turning the wheels. This fact is part of why all internal combustion diesel engines have such terrible overall efficiency – rarely higher than 35%. As we move forward in this article and discuss diesel timing issues and their affect on power production and on the cooling system, do your best to view the cooling system as a necessary evil: the less power that we end up feeding the cooling system through the engine, the more power left to produce actual work. Such positive adjustments can be realized through injection timing... as we shall see.

One more note before moving on. In the Spring 2009 issue of maxxTORQUE, I wrote more broadly on “Timing the Diesel”. If you have not read that article, it won’t hurt to do so. This current article narrows the application of diesel timing discussed in that article to high load scenarios.

A Look Inside the Duramax Engine

Finding optimum diesel timing under high load conditions can be boiled down to maximizing the temperature difference between (A) the highest combustion temperatures (~4,000ºF) and (B) the exhaust gas temperature. The greater the difference in these temperatures, the more work is being performed on the pistons – work that ultimately translates to horsepower at the wheels. (Photo Credit: © GM Corp)