Exercise is good for your diesel’s health, so open it up and blow it out on occasion.
When boaters get together, sooner or later the subject turns to fuel efficiency. Usually, a couple of owners will claim they are saving fuel by reducing their rpm/speed. But what they may not realize is that operating at less-than-optimum load may be causing damage to their diesel engine(s) and the repairs could cost more than they save in fuel cost.
Fuel conservation is important, especially on extended cruises, but the simple truth is that diesels perform most efficiently at 75 percent or more of their rated output. Running at a slower speed or lighter load than the designed output causes decreased efficiency and engine damage.
Fuel and air are combined in diesels at precise mixture ratios and then injected into the cylinders as a fine mist. The mixture combusts at maximum efficiency when compression and engine temperature are within designed specifications. Running with insufficient loads leads to low operating temperatures, and the engine fails to burn the fuel completely.
The unburned fuel forms sludge and fouls fuel injectors, valves, pistons and the exhaust side of the turbocharger. It also leads to an accumulation in the exhaust system. Excessive deposits can result in loss of engine performance as gases bypass valve seats, exhaust buildup produces back pressure and deposits on the turbo blades reduce turbo efficiency. A telltale sign of such trouble is an oily, nasty, sticky residue on the exhaust pipe or stack — thus, the term “wet stacking.” The word stacking comes from the term “stack” for the exhaust pipe or chimney stack. The oily exhaust pipe is, therefore, a “wet stack.” Some of the unburned fuel also leaches lubricating oil within the combustion chamber; inefficient lubrication damages piston rings and cylinder walls. Having taken a toll here, the oil/fuel then passes out of the cylinder to the oil pan and dilutes the engine oil. This is known as “crankcase dilution” and is closely related to wet stacking.
To understand how wet stacking happens, one must have at least a cursory knowledge of the internal operation of a four-stroke diesel engine. As the piston travels downward on the intake stroke, the intake valve opens and the turbocharger delivers compressed air to the cylinder. When the piston reaches the limit of its downward travel, it reverses direction, the intake valve closes, and the piston compresses the air in the cylinder. The temperature and pressure of the air in the cylinder rise dramatically. With the piston at the top of the compression stroke, the fuel injector sprays a fine mist of fuel into the cylinder. The air in the cylinder is hot enough to vaporize and ignite the fuel. The burning fuel adds heat to the air in the cylinder, and the pressure and temperature rise further. The hot compressed gas in the cylinder pushes the piston downward. The force of the piston is transmitted to the crankshaft through the tie rod, turning the crankshaft. The exhaust valve opens, and the piston pushes the hot gas out of the cylinder, through the exhaust valve, into the exhaust system. A portion of the exhaust gas is diverted through the turbocharger to drive compression of the intake air, and the cycle begins again.
When a diesel engine runs with less than a full load, it develops only enough power to drive its accessories and overcome internal friction. It uses very little fuel and consequently develops little heat inside the cylinder. The cylinder’s internal surfaces stay considerably cooler than when running at higher load.
A diesel engine doesn’t use spark plugs. It relies on the hot compressed air in the cylinder to vaporize and ignite the fuel. With the air cooler than the designed temperature, conditions for combustion are less than ideal. The fuel ignites and burns, but it doesn’t burn completely. What remains are vaporized fuel and soot — small, hard particles of unburned carbon. Fuel condenses in the exhaust system and soot deposits on surfaces inside both the exhaust system and engine.
In the exhaust system, fuel vapors condense and mix with soot to form a dark, thick liquid that looks like engine oil. It may ooze from the turbocharger or drip from the exhaust outlets.
Inside the cylinder, soot can form hard carbon deposits on the fuel injector nozzle. The nozzle is designed to atomize the fuel, delivering a fine mist that vaporizes readily. When the injector nozzle is fouled with carbon, its ability to atomize the fuel is compromised, and it delivers larger droplets to the cylinder. The fuel consequently vaporizes less efficiently, more fuel remains unburned and more fuel passes into the exhaust system. Wet stacking is a progressive condition; it tends to lead to more wet stacking.
It’s generally believed that prolonged operation at low loads can lead to permanent engine damage, requiring a major engine overhaul. Costs of an overhaul can run so high that replacing the engine is the most economical option.
Thankfully, the effects of inefficient fuel burn will not cause permanent damage over short periods, but over longer periods, deposits will scar and erode key engine surfaces. Skippers can avoid the damage as a result of wet stacking by periodically running the engine at loads of 75 percent and more of its rating, to raise the operating and exhaust temperatures high enough to burn off the accumulated material and blow out the soot. Proper 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 75 to 80 percent range suggested to maximize fuel burn. Now determine the rpm required to obtain proper loading and run the engine a couple of hours at this level periodically to remove any sludge that may have built up on internal engine components.
Blowing the engine out for a couple of hours will cost you a bit more in fuel but far less than an engine overhaul or, even worse, a replacement.