Dual Displacement Power Steering Pump

Power steering pumps typically run all the time to handle worst-case needs of low-speed vehicle turning and maneuvering. Virtually all heavy-duty trucks have power steering systems to assist with the turning of the vehicle when necessary. The power steering system consists of a hydraulic engine-driven power steering pump, a steering gear, control valves to route the hydraulic fluid depending on the direction of the turn, a small fluid reservoir, and hydraulic hoses to route fluid to the appropriate components. In addition, a fluid cooler is sometimes required to keep the hydraulic fluid in the proper temperature range. The power steering pump turns and generates pressure/flow any time the engine is running.

New systems take into account the fact that line-haul tractors spend the vast majority of their operating hours going nearly straight down a highway when little steering effort is necessary. The dual displacement power steering pump works by using the full displacement available when the engine is at a low speed. Once the engine/pump speed is sufficient, the dual displacement power steering pump automatically switches to the pump with smaller displacement that takes less horsepower to turn.

It is important to note that power steering pumps typically consume 0.3–0.6% 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.