Stories about marine diesel engine longevity are legion and it seems to be that the harder they are worked, the longer they last. While that may sound counterintuitive, its a proven fact. In my experience in the marine industry as a mechanic, boatyard manager, and now consultant, I cant ever recall seeing a marine diesel engine that died of over-use. That is, they rarely if ever wear out. Instead, they die of several other causes, which include accessory failure (heat exchanger, exhaust manifold, and raw-water issues), poor maintenance (infrequent oil and coolant changes among others), and under-loading (running lightly loaded, which leads to carbon and varnish formation). Related to the latter is the insidious poor or improper break-in of engines, particularly generators.
Its tough to properly size a generator. It seems that no matter what the designer or builder does, he or she chooses a generator that is either too big or too small (often the former). The reason for this selection tension is that gensets must be capable of running intermittent heavy loads, the most common of which are air conditioning compressors. In order to carry, and more importantly, start this load (start-up loads for motors and air conditioning compressors can be three times their running load), the genset has to have enough ampere guts. The rest of the time, however, when the air conditioning is off or when its not starting up, the generator is often grossly under-loaded. Running chronically under-loaded, as has been discussed by this author many times, is simply undesirable for any engine, particularly a diesel.
The challenge generators face, however, is even more vexing. In order to live long, productive lives they should, ideally, be loaded between 50 percent and 75 percent most or all of the time. I know, thats unrealistic but remember, I said, ideally. Theres only one thing thats more important than running properly loaded and thats obtaining proper break-in.
Break-in is a process whereby the engines piston rings properly seat or wear into the cylinder walls. Break-in typically occurs or does not occur without the operators knowledge, so theres no easy way to be sure its occurred, particularly if the generator has been operated before youve purchased the boat.
The cylinder walls inside an internal combustion engine are not, contrary to popular belief, smooth and shiny. In fact, they are intentionally scratched or scored in a fine cross hatch pattern, which creates a series of mini-peaks and valleys. The piston rings face, the portion of the ring that rides along and over the cross hatched cylinder wall, is narrow so only a small portion is actually making contact with this cross hatch surface. Each groove in the cross hatch acts as an oil reservoir, which holds oil, allowing a film to be created between the ring and the cylinder wall. During proper break-in, as the piston and rings move up and down in the cylinder, they wear against and into each other, establishing a custom fir of sorts (this is one of the reasons oil must be changed early on in a new engines life, the wearing-in creates metal contamination within the lubricant). The tops of the peaks are worn flat, while the valleys remain intact to hold oil. The ring face wears too, which increases its surface area, supporting the oil film. When this occurs properly, just the right amount of oil film is maintained between the flattened ring face and the flattened peaks of the cross hatch.
When an engine is operating, a phenomenon known as break mean effective pressure (BMEP) is created within the combustion chamber. This is the pressure created by the controlled burning and expansion of the fuel air mixture. The higher the power setting of the engine, i.e., the greater the load, the higher the BMEP, which is critical to the break-in process because its what ensures that the piston rings are pushed against the cylinder walls with sufficient force to achieve proper break-in. If insufficient BMEP is present during break-in, i.e., the generator is lightly loaded for its first 50100 hours, the cylinder walls will glaze or develop a polished appearance. When this happens, theres no going back, once the cylinder walls are glazed the only solution is to remove the cylinder head and hone the surface to reestablish the cross hatch.
The proof as to whether the break-in occurred may not overtly reveal itself for a thousand or more hours. If glazing occurs then compression is compromised and combustion gasses travel around the piston rings, often referred to as blow by, and into the crankcase. These gasses are hot and they carry with them combustion byproducts like soot and water into the crankcase, which leads to excessive heat, oxidation, and oil contamination. Its also more likely that oil will slip by the rings into the combustion chamber during the intake stroke when glazing is present, which means oil is consumed and carbon is created within the combustion chamber.
When blow by is excessive, these gases are often visible or apparent at the outlet of the crankcase ventilation hose. This is a hose that travels from the valve cover of the generators engine either down the side of the block where it vents or it may be routed to the engines air intake manifold or filter, where normal crankcase gasses are consumed and burned by the engine. Its normal for a small amount of gas and water vapor to exit from this hose, however, if what looks like steam, water, oil, or worse, milky froth, are constantly dripping or running from this hose, its possible the engine is suffering from excessive blow by, which is nearly always a result of worn or improperly seated piston rings (its unlikely that the rings would be worn out in less than 10,000 hours if broken in and operated properly thereafter). A more objective assessment of an engines potential blow by involves a compression test, a leak down test, or a crankcase pressure test. Any or all of these will reveal whether blow by is excessive.
Where generator break-in is concerned the first 50 to100 hours is critical. Its important that the generator be placed at 75 percent load, achieving sufficient BMEP, for this period of time. Many generator manufacturers specifically call for this procedure, spelling out the details in the installation and owners manuals. For instance, Alaska Diesel Electric, manufacturer of Northern Lights generators says
The first 100 hours on a new or reconditioned engine are critical to its life and performance Maintain at least a 75 percent load on your generator set for the first 100 hours. Varying the load will help seat the rings.
If you are having a new generator installed, or if you are taking delivery on a new vessel, insist that the manufacturers break-in procedure be followed for the generator. If the builder or installer is not willing to do this, you can do it yourself; however, its important that the generator not be run for longer than necessary to test the installation prior to break-in. In order to achieve the proper load, preferably 75 percent, it may be necessary to add one or more heaters (I prefer the radiant, oil-filled variety for this process, they are safe and utilize no exposed heating elements) in addition to your air conditioning units until your amp meter indicates approximately 75 percent load. This should be varied periodically, which normally happens as air conditioning or heating units switch on and off thermostatically. Finally, check your owners manual. Most generator manufacturers call for other procedures in conjunction with or upon the completion of break-in. These include oil and filter change, adjustment of belt tension, valve adjustment, and torquing of cylinder head bolts to name a few.
If you are among the unfortunate vessel owners whose generator is suffering from glaze-induced blow by, all is not lost. After confirming this diagnosis using one of the above-mentioned tests, the cylinders can be honed and then proper break-in carried out. Regrettably, beyond honing or wholesale replacement there are no silver bullets for this scenario, and thus its often best to cut your losses rather than struggle on with a balky generator thats suffering from blow byit wont get any better. With proper break-in and maintenance, on the other hand, you can expect a generator to operate reliably for many thousands of hours.