Going back to its earliest incarnation, cruise control meant tugging on a dash- or floor-mounted mechanical throttle extension cable and giving it a twist to lock it in position. As rudimentary as it was — and definitely not a recommended practice — it set the engine to a given rpm and allowed the truck to cruise along at a given speed on level ground.
Modern-day cruise control works essentially the same way, but with the added benefit of electronic "eyes" and "brains." In some ways, though, modern cruise control still has some of the same limitations as the throttle lock.
It’s convenient for drivers and an easy way to maintain a certain speed on flat ground. Early versions of electronic cruise control did just that. Set it to 62 mph and that’s what the truck did — or tried to do. If you started climbing a grade or ran into a headwind, the computer opened up the fuel pump and in went more fuel, whatever was needed to maintain the set speed. It wasn’t subtle about it, either. Its mission was to maintain that set speed. Period.
This meant the engine often went from, say, a 70% engine load at the set speed to 100% engine load as it started up a grade. It wouldn’t really have mattered much if the engine maintained 70% power output and the speed dropped to 56 mph, but the programming in the early days wasn’t capable of discrimination. It got worse at the top of the grade.
Cruise control, hanging on for dear life, kept the engine output at 100% until the truck once again reached the set speed, meaning, it went over the top of the hill and part way down the other side, throttle wide open. And as the truck hit the set speed on the downside, the engine brake would often come on. You went from full throttle to idle in a matter of meters, and now the engine brake had to reign in the speed you just burned a bunch of fuel to achieve. Cruise control in that capacity didn’t do much for fuel conservation.
Some of today’s cruise controls still work like that, but they are tempered to some extent with engine parameter settings called “droop”.
Droop allows a slight variation in road speed without an instant response from the fuel pump. For example, if the lower droop is set 3 mph below the set speed (62 mph), the computer will not begin rolling on the throttle until the vehicle speed has drifted down to 59 mph. The same applies to an upper droop setting: the engine brake will not come on until the road speed reaches 65 mph. Droop settings vary by engine brand and are usually customer-programmable parameters rangng from 1-8 mph.
Some truck makers still offer this basic cruise as an option, and it’s still popular with some heavy haul fleets that are concerned more with maintaining road speed than saving fuel.
Droop adds a margin above and below the set speed to save fuel while climbing a hill and to preserve vehicle momentum when starting down the other side. Droop is still a part of the road-speed and cruise-control parameters, but it’s now a part of a much more sophisticated system that responds to dozens of external influences rather than a handful.
Predictive cruise control
There’s more going on with cruise control that you might gather from the way I’ve described it here. In most cases, recent-vintage cruise control systems can control engine torque to deliver extra hill-climbing power at lower engine speeds, which might forestall a down-shift compared to a cruise-control systems of just a few years ago. The next big thing in cruise control is called predictive cruise control. As the name implies, the system can “see” the road ahead and predict what the engine should be doing to get up or down a hill as efficiently as possible.
“The functionality of predictive cruise control compared to basic cruise control has reversed the best-use case for these different generations of cruise control,” says Volvo’s Peter Blonde, senior product marketing manager for fuel and transportation efficiency. “Standard cruise worked best on straight flat roads where keeping a steady speed was the primary function. Predictive cruise control, on the other hand, delivers the best value in rolling hills where the driver can’t necessarily see what’s over the next hill or around the next corner.”
Predictive cruise control combines a GPS receiver on the vehicle with onboard maps, and then adjusts vehicle speed to deliver optimal fuel economy according to its read of the road ahead. For example, on level ground when approaching a hill, predictive cruise may accelerate the truck slightly to gain a little momentum for the climb if it’s a longish hill, or it might let the truck slow slightly while climbing a short hill, because it knows it will make up some speed on the downside, maintaining an average speed over the hill without compromising the trip time. In other words, slow 6 mph going up, gain 6 mph coming down. It all works out in the wash.
“Unlike standard cruise control, which maintains a set speed regardless of the terrain, as a truck enters certain types of terrain, such as rolling hills, the system modulates cruising speed to correspond to the surrounding geography,” says Kurt Swihart, Kenworth marketing director. “For example, Kenworth’s predictive cruise control will hold the gear on the flat in anticipation of an upcoming grade. As the truck ascends and crests a hill, the system allows the Kenworth T680’s speed to drop slightly below the set cruise speed, which is an effective way to boost fuel economy.”
Another basic difference between standard and predictive cruise is the aggressiveness of the throttling and braking.
“Software is set up to gently accelerate and decelerate the vehicle, to maximize fuel economy and driver satisfaction,” says Jim Nachtman, Navistar’s heavy-duty marketing director. “By operating how the driver expects, the driver is more likely to use cruise control more often, further improving fuel economy.
“International Truck’s predictive cruise control system has fleet-adjustable programmable parameters that control how much the vehicle speed will go above and below the driver’s set speed. By allowing the speed range to deviate more, additional fuel efficiency can be achieved,” says Nachtman.
In Volvo and Mack trucks, the engine brake features an automatic position on top of settings one through three. This enables the engine brake to apply in stages as the truck begins its descent to allow the best use of vehicle momentum.
“Naturally we don’t want the truck to run free down the hill for safety reasons,” Blonde said. “By setting a droop into the engine brake, we manage the speed of the truck by modulating the engine brake, first Stage 1 and up to Stage 3 once the truck reaches the limit of the droop [up to 10 mph above the set speed, as set by the customer].”
All of these features are known by various names specific to different OEMs, but they function essentially the same way, with a few variations. Daimler, for example, calls its optional advanced cruise control Intelligent Powertrain Management. It offers, among other features, a predictive gear-shift strategy for upcoming terrain and predictive acceleration, which increases vehicle speed before a steep hill to prevent unnecessary downshifts and “dip” coasting, which allows the vehicle to increase speed at the end of the downhill in a dip before an uphill.
Mack Trucks offers two options for maximizing fuel efficiency with cruise control. Smooth Cruise acts like most cruise controls systems to keep a truck moving at a set speed, but it also has the ability to adjust and slow the vehicle when traveling up a grade, while also allowing the vehicle to travel slightly faster than normal on a downhill grade to help preserve trip time and maximize efficiency. The second is Mack Predictive Cruise, which combines input from sensors on the truck with GPS data to learn not only the topography of the truck’s routes, but also the right speed and gear to be in to avoid unnecessary downshifts for maximum efficiency. The system can store up to 4,500 hill profiles in its memory.
In various test drives I have done in recent years, I have noticed that cruise control doesn’t always work the way I expected it to. On one particular drive, I had come to the conclusion that the cruise control was malfunctioning, but when I spoke with an engineer afterward, I learned that buried within the cruise control’s electronic architecture were actually four distinct modes of operation, basically invisible to the driver. They reacted as they were supposed to in different scenarios – but not the way I expected.
Before fleets assign a truck to a driver, it’s worth doing some fairly extensive orientation with the new technology so that drivers won’t get confused or even reluctant to use cruise control. These new systems offer huge efficiency gains; it would be a shame to waste them.
Man vs. Machine
Last summer during an editor ride and drive event hosted by Mack Trucks, I posted an average fuel economy for the second leg of a three-day trip of 10.4 mpg. I was particularly proud of that because I believe the way I drove the truck added about half a mile per gallon to the score. An identical truck driven totally on cruise managed “only” 9 mpg.
While the Mack’s cruise control was limiting acceleration while climbing hills, I turned the cruise off and managed my own road speed by feathering the throttle and using the minimum amount of fuel needed to continue climbing. On some occasions I gave up 10 mph or more on a hill, sometimes dropping the engine revs to 900 before the truck automatically downshifted. I made it up on the downside, coasting up to 70 mph in some cases. Overall, I maintained 65 mph cruise speed where I could and my average speed for that leg of the trip was 56 mph.My elapsed time for the trip was 4:15, while the fastest truck made the 240-mile trip in 4:11. I consumed 23 gallons; the fastest truck burned 29 gallons.The point to all this is that I was able to out-drive the cruise control by managing the throttle pedal even more conservatively than the truck.
I’m not sure many drivers would have the patience to drive like that every hour of every day. I’m not sure I would, either. And that’s the beauty of today’s advanced cruise control systems — they drive like that all day, every day, because they don’t get tired or bored or cranky.