While fuel economy in Class 8 vehicles is impacted by a variety of specification and operating factors, driver technique can account for a large amount of variation in fuel efficiency. Although the driver primarily controls engine torque, it is drivetrain specifications that control engine speed in any given condition and affect fuel economy and performance.
Transmission overall gear ratio range should be selected to provide the required startability in the application, while still providing top end gear ratio for the cruise speed operation. The number of gears and step size should then be selected to keep the engine operating in its best fuel efficiency area at speeds below cruise speeds.
With an automated transmission and the burden of shifting removed, gearing the vehicle faster allows the engine to operate closer to its “sweet spot,” thereby improving fuel economy. Benefits of automated transmissions include lower fuel costs from improved fuel efficiency, reduced wear and tear on the transmission and extended brake life.
The UltraShift Plus family of fully automated transmissions from Eaton is a two pedal solution featuring automated clutch technology. The units’ intelligent shift selection software employs grade sensing, weight computation and driver throttle commands to make shift decisions for efficient performance. The family includes 10- and 18-speed heavy-duty models in six platforms with thousands of configurations available. The transmissions can also enhance safety with features like Hill Start Aid, which provides a consistent controlled launch, helping to prevent rolling when stopped on a grade. UltraShift Plus also provides enhanced maneuverability at lower and reverse speeds, giving the driver added control in parking lots, loading docks, and other tight spaces.
Mack mDrive from Mack Trucks is an automated manual transmission (AMT) available on the OEM’s Pinnacle model highway tractors powered by the MP7 and MP8 engines. The mDrive has 12 forward and four reverse speeds, and is available in direct drive or overdrive versions. It has a torque capacity of 1,920 ft./lbs. and a shifting strategy based on cruise speeds of 1,350 to 1,450 RPM. The transmission’s Economy Mode maximizes time spent in top
gear, while its Performance Mode maximizes gradeability. An additional MackCellerator or kick-down feature drops the transmission one gear (while synchronizing engine RPM) when the accelerator pedal is sharply depressed to maximize acceleration in passing situations. mDrive also provides an optional Grade Gripper or hill start assist when launching the truck on an incline, to prevent the vehicle from rolling backward.
Volvo I-Shift is a 12-speed, two-pedal, automated mechanical transmission that is designed to integrate with all Volvo diesel truck engines. Using microprocessor technology, I-Shift continuously monitors changes in grade, vehicle speed, acceleration, torque demand, weight and air resistance and automatically selects the best gear for the engine. Available in five packages, I-Shift features include a grade sensor, Kick-Down Mode for instant power and maximum acceleration, Performance Mode for maximum gradeability, and a Brake Program Mode for maximum retardation. An optional feature called Eco-Roll also improves fuel economy in the top gear. In addition, there is an optional Performance Plus (P+) mode that includes functions that adapt gearshifts to off-road conditions and offer extra torque when starting.
Allison Rugged Duty Series fully automatic transmissions employ Load-Based Shift Scheduling (LBSS) to optimize fuel economy. LBSS automatically selects between Economy and Performance shift schedules based on the vehicle’s actual payload and the grade on which it is operating. With the Reduced Engine Load at Stop (RELS) feature, the transmissions automatically reduce the load on the engine when the vehicle is at a full stop to save fuel. Shift Energy Management (SEM) provides better engine/transmission integration for faster, smoother, more consistent shifts, leading to greater fuel economy.
