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Determining Maintenance Intervals
Let's face it, maintenance is a pain. You need to either take your truck to the lube center at a truckstop, or schedule time at your dealer or independent repair shop.

If you choose to have your PM (preventive maintenance) done at a truckstop while you shower and eat, you may find long waiting lines (especially at meal times). Your choices are to wait and lose time, go elsewhere when the lines are not as long and lose additional time, or schedule an appointment at a shop and lose as much as a half-day while you get there, wait and get back on route.

The ideal choice would be to schedule your truck's downtime to fit with your days off, but who wants to waste precious personal time taking care of their truck? But without PMs, reliability goes out the window. The whole idea of a PM schedule is to prevent breakdowns. The worst scheduled downtime is infinitely better than any unscheduled downtime (which is also called a breakdown).

Therefore, we need to determine how often we actually need to do our PMs. The longer we can extend the interval, the fewer instances of downtime we'll have. The shorter the interval, the greater the reliability that your rig will not break down. This is one of many trade-offs we deal with in trucking. How do we make the determination?

To start this two-part series, let's take a quick look at how manufacturers' recommended service intervals have changed over the years. While PM involves many things, such as chassis lubrication, filter replacement and a general check of hoses, belts and wiring, it is thought of primarily as oil change time. Other tasks may need to be performed at other times, but when oil is drained, everything else gets done, too. At a meeting of The Maintenance Council a few years ago one of the fleet maintenance managers observed that if the oil is not changed, mechanics don't take preventive maintenance seriously. When it is, they pay attention to everything.

Oil serves several functions in an engine. Of course it lubricates, greatly reducing friction between the relative motion of rubbing or turning parts. Oil cools the engine, moving one-third or more of combustion heat to the crankcase. It seals, keeping combustion gases from blowing by the piston rings and up the valve guides. Oil cleans the engine dissolving gum and varnish, carrying metal particles, soot and acids into the crankcase and to the filters. It protects the engine, by neutralizing acids, suspending particles and preventing wear, especially under temperature and pressure extremes.

As it does its jobs, oil changes physically and chemically. Some of the chemical change is accelerated by soot and acid in the oil. We need to change oil periodically in order to:

  • Compensate for viscosity changes

  • Remove suspended solids

  • Remove suspended chemicals

  • Replace depleted additives

How often oil needs to be replaced depends on how long it can be kept clean, or at least at some acceptable level of contamination. That, in turn, depends on several factors. How cleanly does the engine operate? Newer engines, designed to produce fewer exhaust emissions, burn far cleaner than older engines. Since the 1980s, injection pressures have increased about tenfold. Unbelievably precise electronic controls, redesigned combustion chambers, optimized turbo boost and cleaner, reformulated fuel, all combine to cut pollutant production during combustion. It has decreased more than 95 percent, with greater reductions on the way. But soot and acids are still being produced. To keep them out of the air, engines have been redesigned to put that soot and contaminants into the oil. To meet the latest EPA standards, the newest engines will create much finer soot particles than ever before, but there will be much more soot in the oil. While old CD grade oils of the 1980s could safely retain up to 1-percent soot, the latest CH-4+ oils can manage up to 7.5 percent and still retain other properties. Because of the expected heavy soot loading, especially with exhaust gas re-circulation (EGR) there are several strategies available to help keep oil cleaner longer, with both new and older engines:

  • Stabilize your fuel temperature and remove entrained air, to make combustion more complete.

  • Use fuel additives to keep injectors clean and lubricated, so deposits don't cause injectors to have inefficient spray patterns.

  • Use auxiliary or depth-type bypass filtration, or a centrifuge, to get soot out of the oil before it can damage the engine or degrade the oil. Cellulose media are better than synthetics because they absorb water- based acids, coolant and any free water.

  • An alternative to bypass filters is the Cummins Centinel. It pulls oil from the crankcase, mixing it with fuel to be burned in the engine. The used oil that was removed is replaced with clean, new oil, a few drops at a time. This helps maintain oil at an acceptable level of contamination. No regular oil changes are required for hundreds of thousands of miles.

Operations play a major roll in degrading oil and determining oil drain life:

  • On- or off-highway Off-highway operations put additional stress on your engine and your oil. Some dust is bound to get into the intake. It will affect combustion, causing additional soot to form and it will contaminate the oil.
  • Idling Excessive idling (more than five minutes) starts to cool the engine, leading to inefficient operation. In the winter, it draws in cold outside air while burning only enough fuel to keep the engine running. Without a load, engine temperatures will drop below 165O F. According to a new Argonne National Laboratories study, when a truck's engine is idled for heat only, its efficiency drops to 11 percent. By contrast, auxiliary generators or fuel- fired heaters operate in the mid 80 percent range. When an engine operates below 185O F, fuel does not burn completely. It forms soot and sludge, and contaminates oil. That, of course, shortens drain intervals.
  • Duty cycle The ideal condition for any engine and its oil is to operate at a steady speed and load. Anything that differs from this, to the extent of the difference, shortens oil life. Such things as start-and-stop city driving, pickup & delivery operations and varying loads all affect intervals.
  • Terrain Routes through hilly terrain may shorten oil life compared to runs on flat terrain.

If you have made an oil drain interval determination and any of these factors (on/off highway time, idling, duty cycle, terrain and temperature change drastically, you should probably re-evaluate your PM interval).

Next issue, we'll examine the best ways to get oil analysis done, and how to track results and interpret them. Then we'll use the results to determine an optimal PM interval.

Paul Abelson is Land Line's technical editor and freelances from his office in Lisle, IL.

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