Proper Loading of Marine Diesel Generator Sets

Among the most common issues people report with their marine generator sets are dirty exhaust or “wet stacking”. Most frequently, this condition is caused by cylinder glazing and carbon build-up. The root cause is simple: underloading the genset.

Diesel engines in marine gensets should typically run at least 60-75% of their maximum rated load. Short periods of low load running are permissible providing the set is brought up to full load, or close to full load on a regular basis.

Running a diesel engine under low load conditions causes low cylinder pressures, and consequent poor piston ring sealing. The ring-to-liner seal relies on combustion gas pressure to force the piston rings against the oil film on the cylinder bore. Low cylinder pressures result in low temperatures and poor combustion, leading to soot formation and unburned fuel residues and ultimately clogging of the ring grooves. Advanced cases of this condition can even cause the rings to stick in the grooves, further damaging the cylinder bore.

Cylinder glazing occurs when hot combustion gases blow past the now poorly sealing piston rings, causing the lubricating oil on the cylinder walls to flash burn, creating an enamel-like glaze. This smooths the bore and minimizes the effect of the honing marks machined into the bore surface.

Hard carbon is highly abrasive, and is also formed from poor combustion. It wears the honing marks on the bores, leading to increased oil consumption, evidenced by blue smoke. Poor combustion also clogs injector tips, resulting in black exhaust smoke.

The problem is further exacerbated by the formation of acids in the engine oil from condensed water and combustion by-products which would normally boil off at higher temperatures. Acidic buildup in the lubricating oil causes damaging wear to bearing surfaces.

Running your generator set lightly loaded will inevitably shorten the service life of the engine, and results in pollution that is unacceptable to you, to port authorities and to neighboring vessels. Prevent costly repairs and downtime by maintaining adequate engine load on your genset.

If you encounter any of the above problems, or just want more information, contact your nearest Caribbean dealer, visit or call Parts and Power at 284-494-2830.