Running Without Load

What happens to an engine when it is exercised without a load, or is allowed to exceed the manufacturer’s recommended delay for a cool-down after shedding its load? Detailing the undesirable conditions that develop provide a better understanding about why "running without a load" is a bad idea.

Wet stacking

The 2013 edition of NFPA 110, Emergency and Standby Power Systems, A.8.4.2, states "Light loading creates a condition termed wet stacking, indicating the presence of unburned fuel or carbon, or both, in the exhaust system. Its presence is readily indicated by the presence of continuous black smoke during engine-run operation. The testing requirements of 8.4.2 are intended to reduce the possibility of wet stacking. If equivalent loads are used for exercising, it is suggested that all essential loads be energized first, with the equivalent load used only to supplement the test. If the normal power were to fail during the exercise period, it would negate the urgency to automatically remove the equivalent load as described in”

Cummins Engine Co. defines wet stacking as a condition that can occur when running an engine at a no-load or less than 30% of the unit’s standby power rating, that manifests itself "in the accumulation of carbon particles, unburned fuel and condensed water and acids in the exhaust system due to incomplete combustion caused by low combustion temperatures."


Carbon particles are deposited on top of the piston and in the injectors when fuel is not burned completely. Depending on the amount of time that the engine is run on low load, the engine may not be able to perform to its maximum rated load until these deposits have been burned off by operating the engine at higher loads.

Fuel/oil dilution

Piston rings are designed for optimum sealing under elevated combustion pressures. When these pressures are not reached due to the application of low loads, the fuel injected into the combustion chamber tends to get between the cylinder wall and the piston rings, causing dilution of the lubricating oil with fuel, with subsequent formation of acids and loss of lubricity.

Water contamination

Because the lubricating oil does not reach the desirable operating temperature, condensation of water will likely form in the engine oil pan. According to Cummins, exercising the engine "is required to maintain a coating of lubricating oil around engine bearings and to maintain corrosion inhibitor throughout the cooling system, therefore extending the generator package life."

The bottom line

An engine needs to be exercised at monthly intervals, and preferably weekly intervals, to maintain its healthy condition. This should be done at the manufacturer’s recommended operating temperature — which can only be achieved by running it under recommended load (usually 30% to 50% of designed capacity). If this is not possible, and if additional building loads cannot be periodically transferred to each Level 1 and Level 2 Emergency Power Supply or generator, a load bank procedure is probably the only option.

Note: If loading the generator is not possible each week, or month, then the generator should be run for a period of time just long enough for the engine water temperature to stabilize. This should take no more than 5 minutes in most cases if thermostats are operating properly.

Chisholm, president of MGI Consulting, Orlando, Fla., has provided emergency power supply systems (EPSS) consulting services and education to more than 1,500 health care facilities. He serves as a member of the National Fire Protection Association's Technical Committee responsible for NFPA 110, Emergency and Standby Power Systems and the Electrical Section of NFPA 99, Health Care Facilities. He also serves as a primary emergency power consultant to the U.S. Army Medical Department (AMEDD) and the Department of Defense.

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