On my new 2018 Cascadia Aero X, there are some additional control features in the driver menu which I needed to become familiarized. The feature that I want to discuss today is the over speed/under speed function. The over speed/under speed function works in conjunction with Detroit Intelligent Powertrain Management (IPM) this is standard on all Detroit DT12 Automated Manual Transmissions since March of 2015. The objective of IPM is to use the momentum of the truck most efficiently to reduce fuel consumption. This is achieved by preventing unnecessary shifts, predictively engine braking and fueling, shifting optimally, and precisely controlling eCoast events. If you click on the highlighted area above, you can view a blog in which I explain IPM in more detail.
On Today’s blog, I am going to discuss a new level of functionality which has been integrated into the drivers menu of settings to enhance the capability of IPM. This function is accessed by depressing the menu buttons at the far right hand side of the steering wheel control buttons. After pressing the buttons, you will see a screen which will display the over speed and under speed value.
The over speed function controls the amount of over speed that will be allowed before a combination of engine brake and/or downshifting will begin to hold the truck speed within the desired amount. The adjustability can be 2 to 15 mph over your cruise speed setting. These settings are made with the arrows on the left hand side of the steering wheel dashboard menu controls. Making a selection becomes important to be aware of the speed limit in relation to your selection. In addition, there is a setting just one past the 15 mph over speed which is called unlimited. Unlimited is not actually unlimited as it is typically set at 3 mph over the max road speed in the trucks parameters. For example, you could have over speed set at 10 mph over speed if you have a cruise speed setting at 65 mph and were operating in a 75 mph speed zone.
Now, let’s talk about under speed. Under speed enhances the manner in which the truck “crests” the top of any given grade. This feature has adjustability all the way down to minus 10. Going to extreme to explain how this works with a minus 10 setting goes like this… you’re climbing a grade with a cruise setting of 65 mph and a under speed setting of minus 10 mph. The IPM is reading the terrain ahead. Once the system determines the hill can be crested at a speed somewhere near the minus 10 mph figure, the DT12 transmission will enter into eCoast mode allowing the truck to overcome the last part of the grade and regain speed by simply using gravity on the down spoke side of the grade.
To see how the system would work, I set all settings to the extreme one day while on rolling hills with no traffic around me. The speed limit was 65 mph and I set the cruise at 55 mph. The over speed was set at 10 mph the under speed at minus 10 mph. In this particular location, I was also losing elevation on average. Each time I reached the top of a hill and could coast over it above 40 mph, the transmission shifted early into eCoast. Additionally, if it seems as though the truck would reach a speed above 65 mph, it would roll out of eCoast early and select a gear which would enable engine braking to hold the vehicle at the selected over speed setting of 10 mph.
The ups and downs of all this are the ability to save fuel by maximizing the abilities of IPM. I will add that it is seldom that I would ever set the controls to plus 15 and minus 10 as my interaction with other vehicles on the road could aggravate other motorists using the same highway. What became readily apparent was how little the engine was needed to maintain forward momentum and managed to its maximum value.
In practice, I generally cruise at 65 mph if allowed with an over speed value which does not exceed the speed limit and an under speed setting of 4 mph. When in heavy traffic, I reduce the under and over speed to prevent unnecessary interaction with other users of the highway. Additionally, I will override the default setting of minus 2 mph under speed if there is another vehicle directly behind me as I “crest” any given grade. The “cresting” feature is overridden by lightly engaging the throttle pedal while on cruise control. This is done to be polite and not aggravate my fellow drivers.
I encourage anyone who has this to experiment with different settings to maximize the benefit of this added feature.