The preferred specs
“The recommended axle ratio is highly dependent on a number of factors, including customer preferences, operating area, truck specs and vehicle speed. We target engine RPM at cruise speed, taking into consideration the vehicle operating conditions,” notes Peterbilt’s product planning manager for powertrain, Trevor Lokie. “For line-haul customers that are looking for peak fuel efficiency, we typically target 1,150 to 1,200 RPM at cruise speed. The cruise speed is defined as the average vehicle speed in top gear as opposed to average vehicle speed overall or the maximum vehicle speed. For typical line-haul applications, we have seen great results with our APEX powertrain package which includes the PACCAR MX-13 engine and the Fuller Advantage automated transmission with a 2.64 or a 2.79 rear axle ratio. This will slow the engine down to 1,150 to 1,200 RPM to reduce parasitic and friction losses while providing peak torque of 1,000 RPM for plenty of power to get the job done.”
As for working with fleets to develop the right axle ratio/driveline specs, Lokie says, “Peterbilt and its dealer network work closely with customers to understand their operating conditions and truck requirements. In some cases, we will install a data logger on a vehicle in operation and collect a few weeks’ worth of data. Analyzing this data will allow us to determine what engine rating, transmission and rear axle ratio will provide the best fuel efficiency while still maintaining the required performance. Not being aggressive enough on downspeeding means less fuel efficiency; being too aggressive means the vehicle will not have the required performance and may spend too much time one gear down, which can also have a negative effect on fuel economy.”
All powertrain components have been designed to accommodate downspeeding, adds Lokie, who notes that Peterbilt has worked closely with all of its suppliers to create the optimal powertrain package that is designed for downspeeding.
“A unique clutch has been designed to reduce noise and vibration that can be caused from downspeeding,” he says. “The Fuller Advantage transmission has a unique gear set and shift strategy that is optimized for the PACCAR MX-13 engine, which constantly monitors grade, acceleration rate, torque output and vehicle load to ensure the most fuel efficient operation. The axle and driveline have also been designed to handle the increased torque load from downspeeding. Other vehicle components such as the cab mounts and engine mounts have also been redesigned to reduce noise and vibration and allow the engine to run at lower RPM.”
What to know about axle ratios
When recommending the best axle ratios for downsped engines, Kurt Swihart, Kenworth’s marketing director, says, “First, you need to know engine speed; Does the engine manufacturer allow the engine to be downsped? The PACCAR MX-13 engine allows downspeeding all the way to the 1,100 RPM range. The next thing to consider is the type of transmission: Kenworth offers both overdrive and direct drive transmissions, which play a big part in selecting the correct rear axle ratio. Lastly, the vehicle’s typical operating speed must be understood. The rear axle ratio should be chosen to keep the engine RPs at their lowest while the vehicle is operating at its typical speed. Pairing a PACCAR MX-13 engine with a direct drive transmission may require a rear axle ratio as low as 2.26. Kenworth offers a variety of super-fast ratios that can be paired with direct drive as well as overdrive transmissions in order to downspeed the engine and provide optimal fuel economy for every application.”
