Speed Control Accuracy

The basic parameter for a governor's accuracy in controlling engine speed is the so-called speed droop. This is the percentage of engine-speed increase that occurs when loading is removed from a Diesel engine with no change in control-lever (accelerator) position. Within the control range, this speed increase may not exceed a certain value. The controlled upper speed limit is the absolute maximum value and occurs when the engine, starting at its maximum full-load speed, is relieved of all loading. Engine-speed increase is proportional to the change in load; the greater the load change, the greater the increase in engine speed.
The desirable speed droop depends upon engine application. For an engine powering an electrical generator, for instance, a small droop is required so that load changes result in minimal speed change. For motor vehicles, a large speed-droop value is better because it results in more stable speed control when small load changes take place (such as on acceleration or deceleration) and therefore better driveability. A small speed droop would give jerky operation during load changes.