Downspeeding heavy duty trucks


Engine efficiency is defined as the ratio of useful work done to the heat provided, which translates into fuel burned. This boils down to how much work can be completed per gallon of fuel consumed.

Typically, diesel engines have provided efficiencies between 40 and 50 percent at speeds up to 1,800 rpm. But the latest technologies, including advanced, common-rail fuel injection systems and injection algorithms, improvements in internal combustion efficiency aided by such modifications as better combustion chambers and electronic integration into the machines, translate into increased efficiency.

Many of these advancements have been driven by U.S. EPA-mandated emissions regulations.

The movement toward increased power densities – more power output per cubic inch displacement – is not new. However, the latest designs are pushing the boundaries, resulting in even more efficient engines.

This makes it necessary to carefully evaluate engine and drivetrain components before making a selection.



A recent trend in the ever-increasing demand to save weight, lower cost and increase operating efficiency of heavy trucks is downspeeding.

Downspeeding involves making the engine run at low speeds and with high torques. This allows the engine to spend more time operating in the load and speed where they yield higher efficiency and reduced fuel consumption (brake specific fuel consumption, or BSFC, which is a measure of the fuel efficiency).

When this occurs, the result is reduced engine friction due to low piston speeds, reduced relative heat transfer and increased thermodynamic efficiency.

The challenge with downspeeding is that it creates the risk of durability and reliability problems.

The faster axle ratios needed to accommodate the lower rpms generate additional torque in the drivetrain. This places much higher stresses on the main driveshaft and interaxle shaft, and this can reduce the life of driveshaft components.

Consequently, the necessity to specify the appropriate drivetrain components takes on greater significance.



The special supplement to Fleet Maintenance looks at the concept of downspeeding, the potential issues associated with it and how those risks can be avoided.

This supplement is sponsored by Dana Holding Corporation Dana (, a global leader in the supply of highly engineered driveline, sealing and thermal-management technologies for vehicles with both conventional and alternative-energy powertrains.