Fuel Consumption at High Engine Speed
This is an engine running at a higher speed than needed doing the same job while burning more fuel. As speed rises, friction losses and pumping losses grow; noise and wear increase in the same direction. Choosing the lowest suitable speed the job requires is the most direct way to save.
Part of the fuel an engine consumes goes into useful work, and part into its own internal losses. These internal losses grow with speed: piston-ring friction, bearing losses and the pumping work spent drawing air into and expelling it from the cylinder. If you can obtain the same pump flow at a lower speed, you gain the difference directly.
The drilling literature offers a concrete reference for the scale involved: on mechanical drilling rigs the diesel consumption of the pump engine at idle has been reported as an average of 45.8 litres/hour. When consumption is at that level even with no load, the cost of time spent at unnecessarily high speed becomes self-evident.
It would not be correct to give the gain as a percentage; the result depends on the load, the engine and the work profile. The mechanism, however, is clear: use the stepless potentiometer to select exactly the speed the job requires, drop to idle during pauses, and stop the engine during long waits.
The context in which this term is used: Electronic speed and throttle control on a diesel engine