Electronic Governors

The 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.