Diesel engine problems

Diesel engines can suffer damage as a result of misapplication or misuse, such as internal glazing (occasionally referred to as bore glazing or piling) and carbon build-up. Ideally, diesel engines should be run at least 60% to 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. Another potential problem is cylinder cavitation and erosion.

Internal glazing and carbon build-up

Internal glazing and carbon build-up is due to prolonged periods of running at low speeds or low loads. Such conditions may occur when an engine is left idling as a 'standby' generating unit, ready to run up when needed, (misuse); if the engine powering the set is over-powered (misapplication) for the load applied to it, causing the diesel unit to be under-loaded, or as is very often the case, when sets are started and run off load as a test (misuse).

Running an engine under low loads causes low cylinder pressures and consequent poor piston ring sealing since this relies on the gas pressure to force them against the oil film on the bores to form the seal. Low cylinder pressures causes poor combustion and resultant low combustion pressures and temperatures.

This poor combustion leads to soot formation and unburnt fuel residues which clogs and gums piston rings, causing a further drop in sealing efficiency and exacerbates the initial low pressure. 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 which smooths the bore and removes the effect of the intricate pattern of honing marks machined into the bore surface which are there to hold oil and return it to the crankcase via the scraper ring.

Hard carbon also forms from poor combustion and this is highly abrasive and scrapes the honing marks on the bores leading to bore polishing, which then leads to increased oil consumption (blue smoking) and yet further loss of pressure, since the oil film trapped in the honing marks is intended to maintain the piston seal and pressures.

Unburnt fuel then leaks past the piston rings and contaminates the lubricating oil. Poor combustion causes the injectors to become clogged with soot, causing further deterioration in combustion and black smoking.

The problem is increased further with the formation of acids in the engine oil caused by condensed water and combustion by-products which would normally boil off at higher temperatures. This acidic build-up in the lubricating oil causes slow but ultimately damaging wear to bearing surfaces.

This cycle of degradation means that the engine soon becomes irreversibly damaged and may not start at all and will no longer be able to reach full power when required.

Under-loaded running inevitably causes not only white smoke from unburnt fuel but over time will be joined by blue smoke of burnt lubricating oil leaking past the damaged piston rings, and black smoke caused by damaged injectors. This pollution is unacceptable to the authorities as well as to neighbors.

Once glazing or carbon build up has occurred, it can only be cured by stripping down the engine and re-boring the cylinder bores, machining new honing marks and stripping, cleaning and de-coking combustion chambers, fuel injector nozzles and valves. If the problem is detected in its early stages, running an engine at maximum load to raise the internal pressures and temperatures allows the piston rings to scrape glaze off the bores and allows carbon build-up to be burnt off. However, if glazing has been allowed to progress to the stage where the piston rings have seized into their grooves, this will not have any effect.

The situation can be prevented by carefully selecting the generator set in accordance with the printed guidelines provided by the manufacturers. Additionally, the use of fine (<3 microns) oil filters can prevent the buildup of carbon particulate that contributes to the varnishing.

For emergency-only generator sets, it may be impractical to use the supported load for testing. A temporary or permanent load bank can be used testing. Sometimes the switchgear can be designed to allow the set to feed power into the grid for load testing.^[1][2][3]

Cylinder cavitation and erosion damage

One phenomenon that can affect water-cooled diesel engines is cylinder cavitation and erosion. This is due to a phenomenon in high-compression engines where the ignition of the fuel in the cylinder causes a high-frequency vibration that causes bubbles to form in the coolant in contact with the cylinder. When these tiny bubbles collapse, coolant impacts the cylinder wall, over time causing small holes to form in the cylinder wall.^[4] This damage is mitigated in some engines with coatings, or with a coolant additive specifically designed to prevent cavitation and erosion damage. Engines damaged in this way will require the affected cylinder to be repaired (where possible) or will be rendered unusable.

See also

• Diesel engine runaway

References

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