Clutched Air Compressor

Traditional air compressors for brakes turn constantly and pump air when necessary. The air pressure is monitored and controlled typically by a governor valve that maintains the pressure between two set limits. The upper pressure is normally termed the “cut-out” pressure and the lower pressure limit is called the “cut-in” pressure, typically set at 130 psi and 110 psi respectively. The ratio of time at cut-out to cut-in is called the duty cycle, and this can vary between 10–30% depending on the type of operation and the amount of air the system consumes.

​New clutched air compressors eliminate the robbing of engine power when the air tanks are already at required pressure levels. These designs typically have a multi-plate clutch arrangement mounted on the input drive to the compressor. When the air system pressure reaches its cut-out pressure, in addition to a signal sent to the compressor cylinder-head, there is also a signal sent to the clutch, which essentially disengages the compressor from the gear drive and therefore minimizes power consumption when the compressor is not pumping. When the system air pressure drops, the clutch is re-engaged and the compressor starts to deliver air again.

​It is important to note that air compressors typically consume 0.25–0.8% of the truck’s fuel to perform their function, so any fuel economy gains from new technology will be modest.

Engine Driven Accessories

What Others Are Saying


  • From 2009 to 2015, the Department of Energy sponsored a program with industry manufacturers to demonstrate at least a 50% improvement in freight efficiency. The project leads were Daimler, Cummins/Peterbilt, Volvo Trucks USA, and Navistar. Each of the four teams created demonstration vehicles incorporating a variety of new technologies to highlight the feasibility of meeting this aggressive goal.
  • As part of researching this subject, the NACFE study team interviewed each SuperTruck team to understand its analysis and choices relative to variable engine-driven accessories. It is important to note that most of the fuel economy and freight efficiency gains were achieved through improvements in aerodynamics, weight reduction, rolling resistance reduction, and powertrain improvements.
  • Note that all four teams chose to implement a form of waste heat recovery as part of their technology choices. The fuel economy gains for this technology are far higher than those available through improvements in accessory systems.


  • On reliability: “The past few years have given us a lot of new challenges to manage due to the introduction of less-than-reliable new technology-like aftertreatment systems or electric APUs. When it comes to accessories technology improvements, the new equipment must be proven reliable before I’ll be willing to implement it in my fleet across the board.”
  • On payback: “Any optional new technology that we implement must meet return on investment guidelines before it will be considered as part of a large purchase. Given that the fuel economy savings from new accessories are generally small, this makes measuring the potential improvement from anything new very difficult for us. This means it’s a harder sell to our management to make the investment.”

Decision-Making Tools

​The study team developed a Confidence Matrix, an OEM Availability Matrix and an Engine-driven Accessories Power Use Chart to assist fleets. The Confidence Matrix is designed to inform fleets of the study team’s confidence in the technology being studied vs. the payback the fleet should expect to receive from the technology. Technologies in the top right of the matrix have a short payback, usually thanks to their low upfront cost, and moreover are found to have enough performance data that fleets can be highly confident in those short payback times, usually because the technology is more mature or otherwise has a more substantial track record of results. The OEM Availability Matrix shows the current availability (as of March 2017) of these engine-driven accessories at the various truck builders. The Engine-driven Accessories Power Use Chart shows the various places the energy from fuel is used in a Class 8 truck.


  • Fuel economy gains from currently available variable engine-driven accessory technologies are modest.
  • The payback for these devices is extended based on today’s fuel prices.
  • Duty cycle of each accessory is critical to the ROI calculation.
  • There is a significant concern for subsystem reliability of these new variable engine-driven accessory technologies.
  • Payback of many of the new accessory technologies is currently insufficient to result in high levels of adoption.
  • Future enabling technologies like higher vehicle system voltage, increased energy storage, and waste heat recovery will likely improve ROI of new variable engine-driven accessories.