A number of CDL holders today came into the industry learning certain fundamentals about operating a large truck. Beginning in the late 1970’s, we learned that if you drove above 1800 rpm, because of the governor, you were forced to either shift up or back off the throttle. At the lower end, there was a change from earlier practice. The power band was no longer between 1800 and 2100 rpm. Additionally, you didn’t have to wind up the engine and drive at the maximum revs.
The peak horsepower is still reached at the high end of the rpm range, and peak torque at the low end, but at even lower revs usually around 1200. By driving around the “midpoint” or 1500 and 1600 rpm, you could stay in gear longer as terrain changed. This is different from the previous, underpowered generation of engines that would reach peak horsepower at 2100 rpm and peak torque around 1800. These narrow power ranges caused drivers to constantly shift gears, and trucks would have to slow way down climbing hills that we might take today by just switching out of overdrive.
Finally, after nearly 40 years we are facing another driving style, and it has a name: downspeeding.
Downspeeding is operating an engine at a slower speed in order to save fuel. The less work an engine has to do in a given period of time, the less fuel it will use.
The given period of time part is important. It’s not about slowing down. You still have schedules and deliveries to make. It’s really about having the engine run slower while doing its work. That involves the engine, transmission and the entire drivetrain.
A Slower Engine
As we evolved into the era of electronically controlled engine, engineers developed indirect means to pressurize the fuel. Injection pressures grew to 40,000 psi and beyond. Electronic timing controls allowed engineers to create multiple injections per stroke, timed for maximum fuel efficiency. The engine of today are able to reach peak torque at speeds as low as 1000 rpm, and to maintain enough torque to keep a loaded truck rolling at clutch engagement speed of 800 rpm or less. This is where downspeeding comes in.
There a number of advantages to running engines slower. Number one it takes less energy to turn the engine’s moving parts. Pistons, rings, the crankshaft and camshafts, gears and timing chains, valves and all the driver accessories all have mass. We know that it takes energy to move mass. Slow the movement and save energy. Another advantage is heat. Heat transfers more slowly between two masses when they are moving slower in relation to each other. By reducing heat transfer to the clock and coolant, less energy is wasted.
Today’s automated manual transmissions (AMTs) are lightyears ahead of previous generations. The newest AMTs sense engine load, grade, truck weight and several other available sensor inputs that help predict gearing needs and make decisions to shift or stay in a given gear. This is necessary for a smoother operation of the transmission with benefits for downsped engines and their driveline components.
So why undergo this latest evolution in engine speeds? Because for every 100 rpm lower the engine speed, fuel economy improves by about 1%. If you drive 10,000 miles a month and you’re in the market for a new truck, the savings between one geared to cruise at 1450 rpm and another at 1150 rpm could be $1,600 annually (this is based on 7 mpg real world driving).
Downspeeding certainly has its advantages and as newer and more efficient technology comes out, trucks will continue to be more and more efficient.