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Diesel Defense

2011 October 3

Running your engine at proper rpms ensures a clean diesel

If diesel engine abuse was a criminal offense, I would have been convicted and jailed long ago. You see, I was a "wet stacker" and "crankcase diluter" without even realizing that these are diesel offenses; ignorance though is no excuse for the havoc I wreaked on my engines. Experience with hundreds of sailors though convinces me of one fact: my cell block at the penitentiary would be full of other sailors doing hard time just like me.

We understand the necessity of fuel conservation, especially when undertaking extended passages. The simple truth is that motorsailing at low rpms is the necessary evil we undertake to make headway in light wind conditions using as little fuel as possible to extend fuel range. What we don't understand: diesels perform most efficiently in the 70% to 80% range of rated output. Running the engine at slow speeds and loads below 40% of designed output leads to the crimes cited above along with decreased efficiency and engine damage.

Fuel and air are combined in diesels at precise mixture ratios and then injected into the cylinders as a very fine mist. The mixture combusts at maximum efficiency when compression and engine temperature are within designed specifications. Running with insufficient load leads to low operating temperatures and the engine fails to burn the fuel completely.

The unburned portion forms a sludge that fouls fuel injectors, valves, and turbochargers and leads to an accumulation in the exhaust system. A telltale sign of the problem is an oily, nasty, sticky residue on the exhaust pipe, or stack­-thus the term "wet stacking."
Some of the unburned fuel also leaches lubricating oil within the combustion chamber; inefficient lubrication damages piston rings and the cylinder walls. Having taken a toll here, the oil/fuel then passes out of the cylinder to the oil pan and dilutes the oil reserves there. This is known as "crankcase dilution," and is closely related to the wet stacking offense.

Happily, the effects of inefficient fuel burn are not permanent, and we convicts can make reparations. Industrial diesels used to power enormous generators are commonly beset with the same problems. The solution is to provide supplemental load to the system by incorporating a resistive load bank. This apparatus determines optimum running temperatures kilowatt hour production and provides supplemental resistance to increase load that ensures maximum fuel burn and reduces engine wear.

Sailors needn't turn to these complicated systems; we can obviate the damage by periodically running the engine at high rpm to bring operating temperatures up and burn off the accumulated material. Loading can be calculated as the percentage of engine rpm while in use divided by the manufacturer's maximum steady state recommended rpm:

Determine your loads incurred while motoring at conservative levels, which are certainly under the 70% to 80% range suggested to maximize fuel burn. Now determine the rpms needed to achieve that figure, and run the engine for a couple of hours at that level every so often to remove any sludge that may have built up on engine components.

I like to operate at about 1,800 to 2,000 rpm when facing long hours (or days) of light winds. Having served my time and being rehabilitated, I've learned the benefits of throttling up to 2,500 on occasion, and am happy to say I've now been clean for years.