To enhance safety and provide for ease of operation, especially with non-traditional drivers, there is growing interest in and increased use of automatic transmissions in medium-duty trucks. But is there a fuel economy benefit to be gained as well? According to recent testing by Allison Transmission Inc., the answer to that question is a resounding yes.
Commissioned by Allison, fuel consumption testing at the independent Transportation Research Center (TRC) facility in East Liberty, Ohio, sought to determine how and why a fully automatic truck transmission could provide superior fuel efficiency. For the evaluations, TRC was provided with two identically specified medium-duty trucks: one equipped with an Allison 2200 HS automatic and the other with an automated manual gearbox.
Several tests under a variety of conditions were conducted at TRC. First were acceleration tests, related Steve Spurlin, executive director, 3000/4000 Series Transmissions and Application Engineering at Allison, which showed that the automatic transmission-equipped truck used less fuel than its counterpart by achieving a higher average speed faster.
“During acceleration,” Spurlin explained further, “an automatic provides smooth, seamless full-power shifts to put engine power to the ground in the most fuel-efficient way. Manual and automated manual transmissions interrupt engine power every time a shift is made. Any interruption in engine power during a shift creates powertrain inefficiencies, loss of vehicle energy and lower average speeds. Ultimately, less work is accomplished with the fuel consumed.”
Cruise tests conducted by TRC at speeds below 40 MPH, where medium-duty trucks tend to spend a majority of their time, also revealed better fuel efficiency. “Fully automatic transmission internal gear ratios produce lower engine RPM in many ranges below 40 MPH,” Spurlin noted, “resulting in less fuel used and better MPG. In upper speed ranges, where the two transmissions have nearly the same internal ratio, comparable engine RPM resulted in comparable fuel consumption.
“The cruise test illustrated that the most critical component to achieving maximum fuel economy during cruising is engine RPM,” Spurlin continued. “Whether a truck cruises at 10, 40 or 65 MPH, having the engine operating at its recommended RPM is the best way to optimize fuel consumption. If a truck is operated locally and spends 60% of its time at 40 MPH, it doesn’t make sense to spec that truck to cruise at 65 MPH at the engine manufacturer’s recommended RPM rating.”
In the future, medium-duty fleet managers may also have another tool at their disposal to help determine and achieve fuel efficiency objectives when writing vehicle specifications. According to the Allison test report, the U.S. Environmental Protection Agency, as part of its SmartWay Program, is now developing for the first time a test protocol proposal to measure the fuel efficiency of medium- and heavy-duty commercial vehicles.
“The proposed fuel consumption metric for this test would be fuel consumed per amount of work performed, which is not the same thing as simply measuring MPG,” Spurlin pointed out. “A truck’s duty-cycle can be broken into four components, including acceleration, cruise speed, deceleration and idle. Acceleration and cruising are the main factors impacting fuel consumption, so fleet managers should know how their trucks are used and write specs according to that duty cycle.”
