Last month we mentioned that TBN depletion is the primary reason modern diesel oils are “used up” or depleted in service. This month, let’s talk about the secondary reason: soot.

Soot can be a really bad actor in diesel engines because it can cause oil thickening, which can result in lubricant starvation in critical components. That’s one of the reasons why it is important to run viscosity tests on used oil samples.

In the past, oil thickening occurred more frequently than it does today. Oil formulators learned that adding ashless dispersants can dramatically slow oil thickening due to soot content. Today’s oils contain twice as much ashless dispersant as oils did just a few years ago. And it works very well.

But long before the oil starts to thicken, soot can accelerate wear of critical engine components. If engine components aren’t flushed often with new oil, soot can build, even though the soot concentration in the bulk oil remains at acceptable levels. When that happens, the soot can act a little bit like a valve grinding compound, increasing wear significantly in those components.

One of the reasons overhead clearances were frequently checked and adjusted was due to accelerated wear caused by soot. The problem was so serious that a few years ago several engine manufacturers made mechanical changes in order to better flush certain valve train components. Soot-related wear had been decreasing over the last few years. But then the Environmental Protection Agency upset the balance with its 2002 diesel emission regulations.

To meet the “pull-ahead” 2004 emissions requirements (which took effect in October 2002), most engine manufacturers resorted to retarded injection timing and exhaust gas recirculation to minimize NOx levels. When you retard timing, it reduces NOx by reducing peak combustion temperatures. But one negative side effect of retarded timing is incomplete combustion. In a diesel engine, the result is increased soot levels, both in the exhaust and in the oil. EGR, of course, recirculates the soot-laden exhaust through the engine.

Engines redesigned early on to respond to the 2004 emissions requirements had significantly higher levels of soot in their oil than previous designs. As a result, the engine manufacturers once again went to work to reduce soot levels. Nothing was sacred. One manufacturer even changed injector spray angles and the combustion bowl in the piston to obtain more complete combustion. As a result, most 2004 through 2006 engines are significantly cleaner than their predecessors.

So why am I ranting about soot? In 2007, to further reduce NOx levels, all engine manufacturers will be forced to either use EGR or to increase the amount of exhaust gas being recirculated. Now here’s the quiz (there are no penalties for wrong answers):

What do you think will happen to soot levels then?

Let me emphasize these two points:

1. Engine design will be changing constantly between now and at least 2010.

2. Oils and fuels will change significantly in 2007.

My hope is that by 2010, engine manufacturers will be allowed to utilize selective catalyst reduction to reduce exhaust emissions. The engines of today, with their retarded timing, come with a built-in fuel economy penalty. (Do I need to mention that the cost of diesel fuel is increasing? We all would benefit if the fuel cost of operating a truck were to decrease.)

How can you be certain oil change intervals aren’t over extended? I can see only two ways, which are:

1. Utilize manufacturers recommended oil change intervals.

2. Conduct constant field testing and used oil analysis on new units in your fleet.

Of course, I prefer the second method. It’s not that difficult, and you can save operating expense if you do it properly. I’ll continue this topic next month.