Given recent record diesel fuel prices, fuel economy has a great impact on truck transmission selection. Yet, you need to look at all performance criteria before jumping to a conclusion. ?Fuel price certainly has become a key factor, but transmission selection based on fuel economy alone may [mean] a trade-off with productivity,? cautions Brian Daniels, product manager powertrain, Daimler Trucks North America.
?The most critical component of achieving maximum fuel economy during cruising is engine RPM,? says Lou Gilbert, Allison Transmission. ?Whether a truck cruises at 10, 40 or 65 mph, having the engine operating in its recommended area is the best way to reduce fuel consumption and increase MPG. Fleet managers understand how their trucks are used and should write their specs according to the duty cycle, not truck performance at 65 mph on the highway, where it may spend a very small amount of time.?
As such, you need to think through all of the duty cycle requirements. ?Acceleration, cruise speeds, deceleration and idle time are all factors,? says Gilbert. ?The amount of time each of these duty-cycle components is used will be specific to a total duty cycle.?
Once you establish the duty cycle requirements, you can then select the appropriate axle ratio. ?Following the axle selection, available shift schedules should be used to further refine engine RPM for the primary operating parameter,? says Gilbert. He adds that Allison Transmission offers a variety of shift schedules that can be tailored to the duty cycle and overall truck specs. ?These shift schedules will help refine vehicle operation and optimize fuel consumption.?
The right ratio range
A transmission must enable the engine to work in its ?sweet spot? while traversing the jobsite and cruising down the road, plus provide startability for severe grades and ground conditions. ?The sweet spot is certainly a factor. However, it is also a function of the ability to shift (grab gears) while maintaining momentum,? says Daniels.
Jobsite conditions play a key role. Consider grades and soil conditions when looking at items such as gear selection (missing gears); torque interruption (vehicles getting stuck in poor soil); and launch behavior.
?The overall powertrain combination should be selected to drive at the desired speed without being near a gear shift point,? says Daniels. ?With automatic and automated transmissions, it is important to have the ability to lock into a specific gear (manual mode).?
Also factor in engine type (heavy duty vs. medium duty), desired maneuvering speed and desired cruising speed. ?The transmissions that offer the best combination of startability and ratio range are those that are optimized with the vehicle, engine, axle, etc.,? Daniels asserts.
In addition, consider the importance of a power take-off (PTO) in your application. ?While most transmissions offer a PTO... certain buyers will purchase transmissions based on PTO capacity (torque and power output), PTO engine speed coverage (as related to engine speed) and the ability to shift the engine while the PTO is being operated,? says Daniels.
Manuals offer multiple choices
Before you get too involved in matching transmission gear ratios to a particular application, you need to choose the type of transmission that best meets your application. There are three basic types: manual, automated mechanical and automatic.
Manual transmissions come in variations to suit virtually any application from 5 to 18 speeds. For example, a dump truck application offers dozens of available choices.
?One of the most popular is an ?LL? transmission, equipped with two deep low gears for the work site, yet very capable on the highway when hauling from the site,? says Gerard DeVito, director of engineering for heavy-duty transmissions, Eaton. ?The 8LL is still very popular because of its ruggedness, durability, simplicity of repair and outstanding performance.