When spec'ing Class 6 and larger trucks, you often need to choose from a menu of powertrain options with varying horsepower and torque ratings. Under-spec the powertrain and productivity suffers while the potential for premature failures increases. Over-spec and you will not maximize your return on investment from the increased purchase price and increased fuel usage.
You want the engine to operate within its peak efficiency power band. Except during initial launch, engine rpms should never drop below the maximum torque point and, in most cases, never rise above the maximum horsepower point. The engine must have the necessary torque to launch the vehicle in a worst-case scenario, and enough horsepower to achieve and maintain the desired road speed with a reasonable reserve.
Torque vs. Horsepower
The two measures that define the engine's power band are horsepower and torque.
"Torque is the most important engine power characteristic for an on/off-highway truck," says Tim Shick, director of engine sales and marketing, Navistar. "Engine torque, or twisting force, launches the truck, pulls the truck up out of an excavating site, accelerates away from a stoplight and up through the gears with city traffic. Horsepower primarily contributes to keeping loads moving at higher speeds such as on an expressway. So horsepower is more valuable to on-highway applications."
Dave McKenna, director of powertrain sales and marketing, Mack Trucks, adds, "Both horsepower and torque are important in an on/off-highway duty cycle operation. Consider a logging tractor that operates between a logging camp and a lumber mill. Gross Combination Weight Ratings can easily be in the 250,000-lb.-plus range. A massive amount of clutch engagement and almost immediate peak torque is required to get the combination moving. Then, a tremendous amount of horsepower is needed to keep it moving at an acceptable road speed."
It is really a balancing act. "Consider both the torque and horsepower, with the torque rise being the primary consideration followed by adequate horsepower to keep the job moving," says McKenna.
"Peak or maximum torque sets the limit on the vehicle's ability to move a load, so it is obviously critical," says Shick. "Assuming the truck has been properly spec'd for the load with appropriate engine peak torque capability, the shape of the torque curve makes the difference between a truck that is easy and satisfying to drive and a truck that merely gets the job done. The broader and flatter the torque curve and the lower the engine speed at torque peak, the better the throttle response."
Compare torque ratings. "Maximum torque is very important and so is where the torque actually occurs," says McKenna. "Between these two points, you would develop the start of a curve followed by the net torque, which is measured at the maximum rated engine speed. These three points largely dictate the shape of the power curve."
For best results, you need an optimized power system with gearing, horsepower and torque matched to your application. Many of the OEMs have become vertically integrated, offering balanced designs with components engineered to work together. But it is not a one-size-fits-all approach.
For example, Mack offers three different engine performance profiles to match a given application: the Maxidyne, MaxiCruise and Econodyne.
"Maxidyne is the most accepted profile in the vocational truck business, with an extremely high torque rise of 60%," says McKenna. "This means high levels of torque at lower engine speeds (around 1,100 rpm), with the right amount of horsepower to keep the truck moving around an unfinished jobsite and for power take-off hydraulic demands. Maximum horsepower is at 1,500 rpm.
"The MaxiCruise is a higher constant speed version of the MaxiDyne," he continues. "We developed this profile for on/off-road use where most of the work is at highway speeds and low-end brute torque and lower engine speed horsepower is not the priority. The engine offers what we call 'hump horsepower,' where there is much more available horsepower at engine cruise speed rpm than at maximum rpm." This profile significantly reduces fuel consumption at cruise rpm.