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).

Electronic Governors

The recent introduction of electronically controlled diesel fuel injection system on several heavy-duty high-speed truck engines has allowed the speed of the diesel engine to be controlled electronically, rather than mechanically. The same type of balance condition in a mechanical governor occurs in an electronic governor. The major difference is that in the electronic governor, electric currents (amperes) and voltages (pressure) are used together instead of mechanical weight and spring forces. This is possible through the use of magnetic pickup sensor (MPS), which is, in effect, a permanent-magnet single-pole device. This magnetic pickup concept is being used on all existing electronic systems and its operation can be considered common to all of them. MPS's are a vital communications link between the engine crankshaft speed and the onboard computer (ECM). The MPS is installed next to a drive shaft gear made of a material that reacts to a magnetic field. As each gear tooth passes the MPS, the gear interrupts the MPS's magnetic field. This, in turn, produces an ac current signal, which corresponds to the rpm of the engine. This signal is sent to the ECM to establish the amount of fuel that should be injected into the combustion chambers of the engine. Electronic speed governing systems are set up to provide six basic governing modes:

1. Idle speed control

2. Maximum speed control

3. Power takeoff speed control

4. Vehicle speed cruise control

5. Engine speed cruise control

6. Road speed limiting

Each of the control modes above is described in more detail below.

1. The idle speed control provides fixed speed control over the entire torque capability of the engine. Also, the idle speed set point is calculated as a function of the engine temperature to provide an optional cold idle speed, which is usually several hundred rpm higher than normal operating temperature.

2. The engine maximum rpm setting can be programmed for different settings. This can improve fuel economy by eliminating engine overspeed in all gear ranges.

3. The power takeoff speed control setting can operate at any speed between idle and maximum. The operator uses rotary control or a toggle switch in the cab to vary electronically the engine power to the PTO from idle to the preset rpm.

4. Vehicle and engine cruise control includes set, resume, and coast features similar to that of a passenger car, as well as an accelerate (ACCEL) mode to provide a fixed speed increase each time the control switch is activated.

5. The road speed limiting function allows the organization assigned to determine what maximum vehicle road speed they desire independent of the maximum governed speed setting of the engine. Road speed governing provides the best method for ensuring ideal fuel economy.

The major advantage of the electronic governor over the mechanical governor lies in its ability to modify speed reference easily by various means to control such things as acceleration and deceleration, as well as load.