Usage:
3408 Truck Engines
When engine governor stability at low idle rpm is a problem, an easy test can be made to check the air compressor as a possible cause. Disconnect the discharge line to remove the load from the air compressor. If this corrects the engine governor stability problem, air system leakage is the probable cause.
Air compressors used on truck engines generally operate in a pressure range from approximately 620 kPa (90 psi) cut-in (start) pressure to approximately 830 kPa (120 psi) cut-out (stop) pressure. The cut-in and cut-out pressures are controlled by the air compressor governor, according to the pressure in the truck air reservoirs. To control operation of the air compressor, the governor releases the compression pressure from the compressor cylinders to stop compressor action at the cut-out pressure. This removes the load from the compressor cylinders, until the system pressure is lowered to the governor cut-in pressure. The removal of compressor load at cut-out pressure and the addition of compressor load at cut-in pressure is the normal compressor cycle and does not cause a problem.
The development of leakage in the air system can prevent normal compressor cycles, and possibly cause engine governor problems. When there is a leak in the system, the rate of leakage increases rapidly with system pressure. If the rate of leakage becomes equal (similar) to the capacity of the compressor at a pressure less than the cut-out pressure, the compressor will operate constantly. When this condition causes the compressor to operate constantly at a system pressure of approximately 690 kPa (100 psi), the characteristics of compressor governor operation can permit the governor to remove the load from one cylinder of the compressor but not the other cylinder. The result is constant compressor operation with a load on only one cylinder. This changes the stress loads on the drive gears, and can cause a gear vibration which has an effect on the engine governor. For this reason, problems with engine governor stability at low idle rpm can be caused by leakage in the truck air system.