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Dangers of Running a Generator on Low Load or No Load

Published by Jason Koffler on 29 April 2019

Widely used in industrial settings that require uninterrupted and high volumes of power, they also are regularly found on construction sites, at festivals, camping sites, sporting arenas and hotels. Many businesses rely on them, and an increasing number of homes now do as well. Specialists in building generators such as SDMO Industries provide generator solutions for an incredibly wide range of applications – from handheld portable units to enormous industrial generator sets.

It is important to realise, however, that certain considerations must be made in order to keep them running in proper order. Beyond the obvious, such as keeping the engines well-oiled and maintained, one of the most important is being aware of the ‘load’ (the amount of power being consumed by connected elements) of the generator. One might think that being aware of the load is crucial to avoid overloading the generator. However, once a generator with sufficient capacity has been chosen for the application, most diesel generators can comfortably run at 100% load for some time. In fact, it is running a generator at low or no load that poses a larger danger.

How Generators Work.

Diesel generators (or generator sets aka ‘gensets’, as they are commonly known in professional circles) consist of a diesel engine – which essentially works the same way as a diesel engine in heavy vehicles – and an electrical generator. People are often surprised to learn that in these generator sets – which are designed to provide power to sites not connected to the power grid, supply emergency power when the grid fails, or to supplement power during peak usage – the electrical generators do not actually produce energy. Instead, they convert the mechanical energy produced by the diesel engines to electrical energy. As a result, generators follow a rule of internal combustion engines – they must have a certain load attached to them in order to operate properly. Running generators on low or no load can have a range of results that can lead to problems, from inefficient running to serious damage or even complete failure.

The Results Of Low Load Or No Load Operation

The abilities and requirements of generator sets will vary from generator to generator, but there are some widely accepted guidelines. It is mostly agreed that generators are to be run at a minimum load of 30% of maximum capacity. This is an absolute minimum and far from ideal – generally, a load of 60-75% of maximum capacity is considered preferable. No load operation, other than for short diagnostic runs such as checking for proper idle, should be avoided at all costs. The consequences of low or no load operation include:

Low Cylinder Pressure

Diesel engines work by compressing hot air to an extreme extent (far more than gasoline engines) so that when fuel is introduced into the cylinder, it ignites without the need for a spark plug. As a result, high cylinder pressure is integral to the proper working of the engine. When a generator is being run at low load, low cylinder pressure results in poor combustion, decreasing the efficiency of the engine. The poor combustion causes a cyclical issue – soot and unburned fuel residue clogs the already poorly-sealing piston rings, making the low-pressure problem even worse.

Low Temperature

At low load, engines cool and run at a temperature insufficient to create proper combustion. This also causes fuel to be only partially burned. In addition to deposits, this can lead to a lot of exhaust emissions. Exhaust is the familiar white smoke seen in poorly operating diesel engines – smoke which is dangerously high in hydrocarbon emissions.


This is a phenomenon that is often a major culprit behind diesel engine damage. Hot combustion gases escape past the piston rings and instantly burn the oil lubricating the cylinder walls. The result is a smooth, enamel-like glaze along the cylinder walls, which covers the grooves meant to hold the cylinder-lubricating oil and transport it to and from the crankcase. This results in increased wear and tear due to under-lubrication and increased oil consumption. However, it is not the only low load side effect that causes this problem.

Lower Oil Performance, Higher Oil Consumption

Low load operation wreaks havoc on the engine’s oil distribution system in a number of ways. Hard carbon deposits formed as a result of poor combustion cause bore polishing, destroying the honing marks (grooves) for the oil. The oil burns and oil consumption increases. Due to the poorly sealing piston rings, unburned fuel contaminates the lubricating oil, as does condensed water and residue, which cause a destructive acid build up.

Increased Pollution

The white smoke caused by unburned fuel due to low temperatures has already been mentioned. However, this is not the only increase in pollution caused by low load operation. Oil leaking past the poorly-sealing piston rings into the combustion chamber is burned and causes distinctive blue smoke, while black smoke is the result of damaged injectors.

There are a host of additional issues caused by low or no load operation, including and the specifics of each case or low or no load will determine the severity of each issue:

> Increased pressure in the crankcase.
> Excessive wear and oil leaks in the turbocharger (if one is present).
> Carbon deposits on numerous surfaces including valves, pistons and the exhaust manifold.
> Engine exhaust slobber – black oily liquid leaking from the exhaust manifold.
> Engineer call out required.

Effect of Low Load Operation on Generators

The above damaging occurrences have a cumulative effect on generator sets. First, users are likely to observe unexplained power losses and intermittent poor performance. This is due to the inefficient operation as well as highly accelerated wear on components. Soon, components will begin to fail, resulting in unscheduled maintenance and increased downtime. At a certain point, glazing and carbon buildups become so extreme that completely striping the engine, reboring the cylinders, and machining new honing marks is the only solution. Regularly running generators at low or no load will, without a doubt, eventually result in total generator failure.

How to Prevent Low Load Operation Damage

At some point, generators will undoubtedly need to be run at less-than-optimal load. When it is not a regular and prolonged occurrence, this does not have to cause any damage whatsoever to your generator.

No load operation should never exceed 15 minutes. For low load operation, you should contact your manufacturer or your account manager at Critical Power Supplies Ltd, who will be able to advise you on safe low load operation values and durations. After low load operation, generators should be operated with an increased load for a short period to raise temperature and pressure. Once a year, a ‘load bank’ test should be run, where the generator is run for several hours in order to eliminate all deposits.

Properly run and maintained, generators can ensure that you will never be left without power regardless of power availability from the grid. Combining generators and UPS systems, meanwhile, provides both uninterrupted power and protection of sensitive equipment from power surges and fluctuations. Experts like the highly experienced professionals at Critical Power Supplies can help you design an affordable solution customised for your needs.