Q: Light Load Running means what?
A: Light load running is considered to be anything less than 25% of full load for naturally aspirated diesel engines and anything less than 33% of full load for turbocharged diesel engines.
Engines running continuously for 30 minutes or more at light loads, can develop serious problems, which lead to increased maintenance and often major repair work.
Lightly loaded engines often fail to reach their designed operating temperatures. Running clearances in piston assembles, valve gear and other bearings may be excessive when designed operating temperatures are not reached. Commonly, the pistons and rings are not expanded enough to form the designed seal with the cylinder or liner wall. Lower than designed operating temperatures may result in excessive unburnt fuel inside the combustion areas.
Light load running often causes the following undesirable effects ..
1) Excessive gas “blow-by”, which causes crankcase pressurisation. This may force oil out the crankcase breather, or cause other oil leaks.
2) If the dew point temperature is not exceeded for long enough, there is a risk of condensation causing rust and other corrosion within the engine.
3) Condensation and gas blowing past the piston rings causes the engine lubrication oil to become sludgy with carbon, plus the oil may become acidic (corrosive).
4) Unburnt fuel will wash lubrication oil off the cylinder walls, causing excessive cylinder or liner wear. Unburnt fuel then dilutes the lubrication oil. Fuel in the lubrication oil increases the risk of a crankcase explosion!
5) Poor contact via the designed oil film between the piston rings and cylinder or liner, may cause distortion due to poor heat transfer.
6) Increased need to frequently change the lubrication oil due to 3) & 4).
7) Piston rings commonly stick with heavy carbon deposits, because oil from the cylinder wall, or from down the valve stems is not burnt or vapourised by exhaust gases.
8) Sulphuric acid vapour (H2SO4), formed from sulphur in the fuel, causes cold corrosion of injector nozzles.
9) At low rpm, there is often a large amount of valve overlap. In this case, exhaust pressure can overcome air intake pressure resulting in a back flow of exhaust gases through the inlet manifold. If this occurs, carbon can build on the inlet valves, ports, manifold and turbocharger impellar. Then inlet valves may not shut properly causing loss of compression and the turbo will become unbalanced.
To minimise the effects of light load running, do all 3 of the following ..
1) Constantly keep lubricating oil and engine coolant temperatures as high as possible, within makers recommendations, by adjusting thermostats and coolant valves if possible.
2) Immediately before light load running, run the engine on as high a load as possible and ensure designed operating temperatures are reached.
3) On completion of light load running, run the engine on at least 50% full load for at least 30 minutes.