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Regency Generators Knowledge Base

Effects of Low Load on Fixed Speed Diesel Engines

What is low load?

Low load varies between engine manufacturers and even different models of the the same engine. Typically low load can be defined somewhere between 30-50% of the prime rating.

How long do I need to run at low load to cause a problem?

Again - that varies. The time taken for some of the low load symptoms to show up varies by engine manufacturer and even engine model. No fixed speed diesel engine is immune to the the effects of low load.

What common problems are associated with low load?

Low load manifests itself in several ways. Common symptoms of low load include:

  1. Soot deposits in the cylinders.
  2. Wet stacking of soot deposits in the Turbo.
  3. Carbon deposits on the liner, piston and injector nozzles.
  4. High Crankcase Pressure (Front Real Oil Seal Leaks).
  5. Oil leakage from the manifold / exhaust.

The most common problems we have experienced are oil leakage from the manifold / exhaust and failure of oil seals due to high crankcase pressure. In both cases the issue is caused by poor sealing of the piston rings which require cylinder pressure in order to seal correctly to the cylinder (or liner). The lack of pressure in the cylinder allows oil and un-burnt fuel to seep past the ring, as well as exhaust gas.

The fuel / oil will leak from the exhaust manifold (especially on engines that do not have a full sealed manifold) and the high crankcase pressure will try and escape via the easiest route. This is normally via the crank front and rear seals. Usually as the rear seal is considerably larger than the front, the rear seal take the brunt and tends to leak first.

Bore Glazing is a more sever effect of low load and you can read more about that here.


Running at low load is also inefficient. Low load can change the calculation as to is a diesel generator is more or less expensive than mains power.