Ideally, diesel engines should be 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.
Problems of running at low loads include :-
- Internal glazing and carbon build
- Low cylinder pressures and consequent poor piston ring sealing
This poor combustion leads to soot formation and un-burnt fuel residues which clogs and gums piston rings, which causes 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 smooth 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.
Un-burnt 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 and neighbours. The carbon out put from light load running is increased
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 detected in the 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 progressed to the stage where the piston rings have seized into their grooves, this will not have any effect.
The on site available load is not usually enough for load testing and again engine damage will result if this is used as the weekly or monthly load test. This situation can be dealt with by hiring in a load bank for regular testing, or installing a permanent load bank. Both these options cost money but are better than the alternative of under loading the engine.
Workspace Technology recommends that a load bank test is carried out on an annual basis.
The load bank test should consist of the following activities:-
- Impact loading the generator to 50% to check the recovery time.
- Load test to 100% for up to 2-4 hours depending on size of generator.
- Load test at 110% for one hour. (No more than 1 hour in every 12 hours)