Cruise Control Brake Overuse - 2015 Kia K900 Long-Term Road Test

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Cruise Control Brake Overuse - 2015 Kia K900 Long-Term Road Test

Our 2015 Kia K900 regulates speed closely when descending grades on cruise control, but ours seems to rely too heavily on the brakes to do so.

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ckuers

I find that odd, but something I would expect from a company that hasn't been in the luxury market for very long. Have you taken this up with Kia (not the dealer, but Kia North America)?

This strikes me as a calibration/strategy issue that could be fairly easily remedied by their engineers.

I'd also be interested to see if there are any lasting consequences to the brakes from the run. Perhaps check the discs for runout? It would make for a cool Edmunds Garage piece, something that's been sorely lacking as of late.

benson2175

"Grade Logic" has been on auto transmissions since the 90's. Funny KIA didn't include it in their "luxury" car.

kirkhilles1

Yet another "to do" on the list for semi-automated systems. I think auto manufacturers need to get together to get these systems outsourced to a company that can integrate all of this logic. This becomes a serious safety issue and is going to result in a lawsuit someday (or a massive recall) if the brakes fail due to high temperatures.

legacygt

I'll give the Kia a pass because this is an extremely challenging thing to program into the cruise control. If a hill is short you may not want the downshift. How is the car going to know you have 6 miles of downhill ahead of you vs. a short downhill followed by flat road ahead? The car would need to have detailed elevation maps AND know where you're going. I think Mercedes has map-based cruise control but to me, there is a point at which the driver needs to take responsibility for something. When the entire car is automated, then the car should be responsible for knowing the route and terrain and it can shift/brake accordingly.

chol92594

kirkhilles1 said:

Yet another "to do" on the list for semi-automated systems. I think auto manufacturers need to get together to get these systems outsourced to a company that can integrate all of this logic. This becomes a serious safety issue and is going to result in a lawsuit someday (or a massive recall) if the brakes fail due to high temperatures.

While I can definitely understand the frustration here (hell, even my 09 Civic with a 5AT downshifts competently enough on grades when cruise is set), I think it's important to remember that cruise control has always been touted as something meant to decrease the need for driver inputs, not eliminate them entirely. Just as you shouldn't really use cruise control during rain, snow, etc, you probably should't solely rely on cruise control to slow a car on long, steep grades. It would probably be best to simply put the transmission in manual mode and shift accordingly to maintain a certain speed without excessive braking.

Also, maybe I've just never driven down a steep enough grade, but I don't think I've ever witnessed a cruise control system shutting off as a reminder to the driver to take control.

yellowbal

I thought you weren't supposed to use cruise control on hills and slippery surfaces. Has that changed?

In the K900 owners' manual: Advanced Smart Cruise Control system (ASCC)
Do not use the ASCC when it may not be safe to keep the car at a constant speed, for instance, driving in heavy or varying traffic, or on slippery (rainy, icy or snow-covered) or winding roads or over 6% uphill or down-hill roads.

zoomzoomn

It is really strange that Kia relies solely on it's braking system. Grade logic integrated cruise control is pretty commonplace in newer cars, but the ones that I have experienced rely solely on downsifts to maintain speeds which sometimes has its limitations, brakes overheating not being one of them. My '13 TSX is rather aggresive, albeit smooth in its downshift to maintains speeds both going up and down hills never wavering more than a few mph either way. Our '14 Durango does the same, but is not as aggresive and does not maintain speed as well, but still uses only gearing to attempt to do so.

I am certain that you could actually over-ride the transmission manually with the cruise engaged so as not to rely solely on the brakes under those steeper grade situations. Also, 6th gear would likely hold the speed with almost no other intervention needed.

rwatson

Another reason I like manual transmissions. Seriously, I can set the cruise on my VW and can not see a difference of 1 mph on all surfaces. I realize there will be a difference and none are "perfect," but I can't notice it on my speedometer. I've never had an automatic that hasn't fluctuated several mph while seeking gears from one grade to the next.

actualsize

Yes, a lot of us instinctively regulate speed downhill manually, mostly because the first cruise control systems didn't talk to the transmission. They just worked the throttle, sometimes in very rudimentary ways. If you didn't anticipate it yourself, downhill speed would climb until a certain point where the cruise self-cancelled to let the driver know it was time to get busy.

But the issue was not as acute back then because the automatics only had three or four gears. They couldn't accommodate insane overdrive ratios like this 8-speed. And when a cruise control system a) has adaptive capability meant to control speed and maintain safe following distances as traffic gaps grow and shrink and b) seems to have downhill speed logic because it regulates speed so effectively, the implication is this is a smarter cruise control that can handle downgrades. From where I sit this one is not smart enough.

actualsize

legacygt said:

I'll give the Kia a pass because this is an extremely challenging thing to program into the cruise control. If a hill is short you may not want the downshift. How is the car going to know you have 6 miles of downhill ahead of you vs. a short downhill followed by flat road ahead?

Others manage it. And what's wrong with a downshift followed by an upshift ten seconds later? We're not talking about the sort of annoying "busy shift" outcome that is so noticeable when a loaded engine is struggling to climb a hill. And shift shock is a lesser issue when the gear splits are as narrow as they in a transmission that has eight ratios. I think this needs to be a priority for development engineers when ultra-overdrive cruiser gears get added to transmissions in pursuit of increased mpg. The transmission has got to participate in order to bring in some engine braking and give the brakes a rest.

gslippy

Neither my 09 Kia Sedona nor my 13 Optima Hybrid act this way.

funcarguy

Using the brakes takes less fuel!

colorado kid

Using brakes or engine braking takes the same amount of fuel because fuel injected engines do not inject fuel during engine braking. Cruise on my F-150 does this perfectly - it will downshift one gear at a time until it reaches equilibrium. If I tap teh brakes cruise disengages but it holds the same gear until I press the throttle (or shifts down another gear with the next tap of the brakes). Wife's Enclave shifts down from 6th to 5th, which is not enough. It won't shift down again no matter how fast it gets going, and as soon as I touch the brakes it shifts back to 6th as it disengages cruise.

allthingshonda

colorado kid said:

Using brakes or engine braking takes the same amount of fuel because fuel injected engines do not inject fuel during engine braking.

If fuel is not constantly injected into the engine it will stall. Grade Logic is not a common term used throughout the industry but is the name of the algorithm programmed into the transmission by Honda. According to Honda service manuals the computer uses throttle position, speed sensors, ATF temp, coolant temp, RPM, brake pedal position, shift position cruise control engagement and to determine if engine braking is needed and how aggressive it should be. It can downshift down to 3rd gear if needed and works even when cruise is not on. It works well and I notice that it will usually shift down to 4th when coasting and if you use the brakes it will drop down to 3rd. What is interesting is the engineers also programmed it sense heavy braking to anticipate cornering. When heavy braking is sensed the computer downshifts to help slow down but also to be in the correct lower gear for accelerating out of a corner. I guess the Honda engineers still like to have a little fun.

veedubber86

allthingshonda said:

If fuel is not constantly injected into the engine it will stall.

Actually the load on the engine from the car's momentum keeps it spinning even when fuel isn't being injected. If you shift the transmission into neutral the fuel injectors will engage again to maintain idle. In my VW the instant fuel economy readout would show "---" when I took my foot off the throttle while leaving the car in gear (because no fuel was going to the engine), but something like "126mpg" if I shifted the car into neutral while in motion.

random_shots

yellowbal said:

I thought you weren't supposed to use cruise control on hills and slippery surfaces. Has that changed?

In the K900 owners' manual: Advanced Smart Cruise Control system (ASCC)
Do not use the ASCC when it may not be safe to keep the car at a constant speed, for instance, driving in heavy or varying traffic, or on slippery (rainy, icy or snow-covered) or winding roads or over 6% uphill or down-hill roads.

Still shows the same thing on the Kia website...
http://www.k900kia.com/advanced_smart_cruise_control_system-368.html

Dan,
You used the ASCC in a manner the owner's manual explicitly states not to use. It is only fair you update the post to include the driver's error omission from your earlier post.

actualsize

Actually, no. Six percent is the maximum gradient per US interstate highway construction standards, so the grade could not have exceeded the manual's six-percent limit. The grade likely varied between 5.5 and 6-percent, and most US interstate grades do. Kia's 6-percent cutoff was probably chosen with this fact of American road construction in mind, I'd wager. An adaptive cruise control system that can't be used on the interstate highway system isn't of much use.

Besides, the system wasn't in adaptive cruise control mode. I dropped it into normal cruise mode (easy to do) because I don't like the way this car's ASCC system behaves.

And this issue of grade steepness is no excuse for the system choosing to ride the brakes instead of downshifting. Maybe I should try this on Baker Grade, a 22-mile 4% downgrade on the way back from Las Vegas--in summer.

actualsize

Like many modern cars, this one goes into fuel cut mode on downgrades. The road keeps the engine turning at the speed dictated by the transmission gear selected. The only times fuel would be used are 1) if the selected gear was too low and provided too much engine braking or 2) if the grade weren't steep enough to maintain speed on its own. In those cases fuel would be burned to power the vehicle up to the desired speed. Obviously neither was the case here. The K900 had too much speed in 8th gear but would not downshift.

Jay Kavanagh (our resident engine development engineer) and I used this fact (along with other tips) to beat others in the Audi Mileage Marathon mpg competition in a diesel Q7 some years back. Others coasted down long grades in neutral, but we always coasted in drive. The former requires fuel to keep the engine turning, the latter does not.

texases

Dan - I agree, shouldn't be using the brakes like that. How about combining the cruise with the downshift, leaving cruise on while downshifting the transmission?