Hydraulic Governors

Although hydraulic governors have more moving parts and are generally more expensive than mechanical governors, they are used in many applications because they are more sensitive, have greater power to move the fuel control mechanism of the engine, and can be timed for identical speed for all loads.

In hydraulic governors (fig. 5-7), the power which moves the engine throttle does NOT come from the speed-measuring device, but instead comes from a hydraulic power piston, or servomotor. This is a piston that is acted upon by fluid pressure, generally oil under the pressure of a pump. By using appropriate piston size and oil pressure, the power of the governor at its output shaft (work capacity) can be made sufficient to operate the fuel-changing mechanism of the largest engines.

The speed-measuring device, through its speeder rod, is attached to a small cylindrical valve, called a pilot valve. The pilot valve slides up and down in a bushing, which contains ports that control the oil, flow to and from the servomotor. The force needed to slide the pilot valve is very little; a small ball head is able to control a large amount of power at the servomotor.

The basic principle of a hydraulic governor (fig. 5-7) is very simple. When the governor is operating at control speed or state of balance, the pilot valve closes the port and there is no oil flow.

When the governor speed falls due to an increase in engine load, the flyweights move in and the pilot valve moves down. This opens the port to the power piston and connects the oil supply of oil under pressure. This oil pressure acts on the power piston, forcing it upward to increase the fuel.

When the governor speed rises due to a decrease of engine load, the flyweights move out and the pilot valve moves up. This opens the port from the power piston to the drain into the sump. The spring above the power piston forces the power piston down, thus decreasing the speed.

Unfortunately, the simple hydraulic governor has a serious defect, which prevents its practical use. It is inherently unstable; that is, it keeps moving continually, making unnecessary corrective actions. In other words it hunts. The cause of this hunting is the unavoidable time lag between the moment the governor acts and the moment the engine responds. The engine cannot come back to the speed called for by the governor.

Most hydraulic governors use a speed droop to obtain stability. Speed droop gives stability because the engine throttle can take only one position for any speed.

Therefore, when a load change causes a speed change, the resulting governor action ceases at a particular point that gives the amount of fuel needed for a new load. In this way speed droop prevents unnecessary governor movement and overcorrection (hunting).