The air compressor is an engine-driven, piston-type compressor which supplies compressed air to operate air powered
devices. The compressor runs continuously but has loaded and unloaded operating modes. The operating mode is
controlled by a pressure activated governor and the compressor unloading assembly. When the air system reaches
cut-out pressure, the governor applies an air signal to the air compressor unloader assembly causing the unloader cap
valve to activate and stops compressed air from flowing into the air system. As the air in the air system is used, the
pressure drops. At cut-in pressure, the governor exhausts the air signal to the compressor unloader assembly,
allowing the compressor to again pump compressed air into the air system.
The E-Type (E for economy) unloader differs in that the intake is closed by the unloader cap during the
unloaded mode. Back pressure on the exhaust valve at the same time traps air in the cylinder causing the
piston to move against an air spring, thereby reducing parasitic horsepower draw in the unloaded mode.
Additional benefits include reduced oil passage, cooler exhaust air temperature and unlimited turbocharger
boosting capability. For more information on E-type systems, refer to Service Bulletin No. 3666104, Air
Compressor Familiarization Holset E-Type.
The E-Type System can require modification to the vehicle air system plumbing if an air
dryer vented to the atmosphere during the unloaded mode is used on the vehicle. All QE
Model Air Compressors utilize the E-Type unloader.