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Does cruise control save energy, or not?

Discussion in 'Model S: Battery & Charging' started by Panoz, Apr 26, 2015.

  1. Panoz

    Panoz Member

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    I've read posts on the Tesla Motors website that state that cruise control actually wastes energy maintaining a specific speed. That seems counterintuitive to me, but I thought I'd ask. When cruise control came out on ICE vehicles (yes, I'm that old!), it was touted as a fuel saver because the driver no longer sped up/slowed down needlessly.

    However, I was towing a car behind my SUV a few years ago while on cruise control, and I watched the car floor it, downshift and race up a small hill just to maintain the speed I'd set. Had the vehicle been able to let the set speed vary by 10mph, it would've allowed the speed to slow, crest the hill, coast back up to the stated speed on the downside. With that behavior, I can see where it would use more energy maintaining an exact speed (but only on hilly areas).

    So what's the consensus? Would you recommend cruise control if you're low on battery power?
     
  2. CHG-ON

    CHG-ON Still in love after all these miles

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    According to Tesla, the energy usage is about the same. However, I find that it brakes much harder than I would using regen and end braking. So I think costs will go up in the brake dept. As long as we are under the service agreement, no problem. Afterwards, I bet it is not cheap.
     
  3. Saghost

    Saghost Active Member

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    Cruise control is likely about equal to the average driver in efficiency. If you're low on energy and worried about getting to the next charging spot, slow down and hypermile by hand (well, foot.) By minimizing the power and regen you should be able to produce results much better than the cruise control will.
    Walter
     
  4. wycolo

    wycolo Active Member

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    #4 wycolo, Apr 26, 2015
    Last edited: Apr 26, 2015
    CC does not apply the brakes so it should not increase brake wear.

    In a Tesla it can use more energy/mile in hilly terrain than a mild mannered driver would. But on flat terrain I can't see how a hyper-miler could beat it for efficiency, assuming you choose a proper set point.

    CC causes excessive downshifting/engine racing when ICE power is limited such as towing or high altitude applications, so only use it on long level stretches. I would like to see an 'econo-cruise' mode choice whereby it would limit top speed but be more elastic on the downside of SET SPEED point similar to normal use of foot pedal.
    --
     
  5. jgs

    jgs Member

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    TACC absolutely does apply brakes when needed. I've observed this on many occasions, including today. That said, I'm not at all concerned about brake wear -- the brakes are still getting much less wear than on an ICE vehicle. It's not like TACC rides the brakes or something, it just applies them if required.
     
  6. David99

    David99 Active Member

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    There is no clear answer to that question. It depends on how your driving style is compared to the constant speed of cruise control.
     
  7. trils0n

    trils0n 2013 P85

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    For me, regular cruise control (no TACC) uses much more power than controlling the speed myself, if there are any hills. Flat ground it is pretty much the same. CC uses tons of power keeping a set speed going up the hill, and regens too much coming down. I get much better results slowing down when going up hills, and speeding up going down (gravity assist). If I use cruise control on my regular trip from Palo Alto to Sacramento, I will use 350 wh/mile. Without it, <300. Pretty significant. Trips with little elevation change, the CC number and non CC number are about the same.
     
  8. FlasherZ

    FlasherZ Sig Model S + Sig Model X + Model 3 Resv

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    If you're trying to hypermile, cruise control will be a bit wasteful. At 50+ mph it will use up to 80 kW or so to increase speed if you hit resume. By using your foot and holding to 20-30 kW, you'll get better range.
     
  9. Saghost

    Saghost Active Member

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    This is clearly an opportunity for Tesla. When they get bored - after they get all of the Autopilot software working - they can give us a new firmware that has an Eco cruise option. :)

    I'm thinking you would enable it in the settings screen (or it could be tied to range mode?) and choose an acceptable range there (10 mph below, 5 above for defaults?) and then the car would use small but progressive amounts of power or regen to try to hold speed, not using most of it until/unless it gets near the edges of your defined speed range (or TACC detects something slower ahead, of course.) As I envision it, you would be free to stick with the existing cruise if keeping your speed is more of a priority than energy use.

    I don't know much about their implementation, but Ford told us they were doing something similar on the Energi twins...
    Walter
     
  10. jerry33

    jerry33 S85 - VIN:P05130 - 3/2/13

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    In almost all cases cruise control increases energy use. The reason is that cruise control tries to maintain a steady speed so it accelerates and decelerates too rapidly, doesn't slow down a bit going uphill, doesn't speed up a bit going downhill. Steady power always beats steady speed when it comes to energy use.
     
  11. Cottonwood

    Cottonwood Roadster#433, Model S#S37

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    #11 Cottonwood, Apr 27, 2015
    Last edited: Apr 27, 2015
    I beg to differ.

    In many situations, CC will use more energy than a good human driver, especially TACC. As noted these include starts, stops, active following of busy traffic, and going up and down over short, steep hills. The TACC is just too aggressive in starts, waits too long to start slowing, etc.

    In many situations, CC will do better than the average, and even a good, human driver. An extreme example of this is a long, flat, open road with no traffic. The CC can maintain even power with no waste, and has a much better attention span than the human.

    Hills are an interesting controversy. At highway speeds, many hills never put an MS into regen on the downhill. If you are not going into regen, then the least energy to cover a distance is at a constant speed. If that means using more power on the uphill, and less on the downhill, that is OK because constant speed is the best. If you need to use regen on the downhills, then for short hills, then coasting down the hill and then back up the other side can be a more efficient driving technique. On long, tall hills (e.g. Colorado mountains), it is far better to use more power to climb the hill and then regen down the other side. An extreme example of this is going west on I-70 from the Eisenhower Tunnel to the Silverthorne Supercharger. I can use regen and descend at a constant 60 or so MPH, to put 6-7 rated miles back into the battery. If I coasted, I could probably reach 90-100 MPH at the exit and put no rated miles back into the battery; more miles back into the battery mean more efficiency.
     
  12. jbcarioca

    jbcarioca Active Member

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    Good idea. A while ago in Spain I drove a BMW 5 series with fairly advanced adaptive cruise control. It was settable for distance following vehicles, speed sensitive, as well as several settings for responsiveness. I have forgotten what they were called but they ranged from quite gradual (i.e. ECO IIRC to quite aggressive, similar to the 6.2 Tesla version, called Sport IIRC). I thought that was an excellent system. At this point such adjustments should be available as a matter of course. While we're at it, in Tesla's case the regenerative braking I suspect is part of the difficulty in smoothing adaptive cruise control, so the combination of aggressive acceleration settings could benefit greatly by a less dramatic deceleration mode. For some reason both BMW and MB systems, the only two I have used, are far smoother than is the Tesla.

    Although I have used those two systems a bit, I am not intimately familiar with either of them.

    Does anybody else know of other current production examples that compare with the 6.2? How about the hybrid versions of the MB S class, for example?
     
  13. jgs

    jgs Member

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    Just because Tesla's regen braking is capable of slowing the vehicle aggressively, doesn't mean it has to. It only applies as much braking as the control system asks it to; as such, I think it's all about the control system and not about the regen system as such. Consider that if the vehicle had no regen whatsoever, all that would happen is that when the control system called for deceleration it would be provided by friction braking instead.

    FWIW I find TACC as it stands to be adequate to my simple needs on the highway, but I certainly wouldn't object to more tuning of the system and in particular a mode optimized for maximum range.
     
  14. scottm

    scottm Active Member

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    #14 scottm, Apr 27, 2015
    Last edited: Apr 27, 2015
    Setting cruise at a lower speed helps stretch energy over longer distances, as you would expect.

    So, yes, it can help save energy if you are unable to keep speeds down it will do that.



    When Tesla smartens up TACC so that it follows the ideally eco driven virtual car ahead of you, instead of an actual physical car ahead of you... then it will optimize energy efficiency. Of course, the algorithm will accept and adapt to physical cars intruding on that space.

    The new algorithm will be paying more attention to incline & decline (accelerometer), terrain maps (topographically encoded maps), and will use the radar (and new laser ranging) to detect "hilliness" of upcoming roadway. It will also have a preference to lock on and travel behind near-speed semi's at reduced car lengths for drafting.

    When that happens, you'll get 25% increase in range for any given battery.
     
  15. LetsGoFast

    LetsGoFast Active Member

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    This is purely speculation, but I think it is possible that on dual motor cars that cruise control can be more efficient. Perhaps it knows how to sleep the rear motor more often or perhaps it is a placebo effect, but it seems to me that I get better efficiency from TACC cruise so long as traffic is light. I do agree that TACC is too aggressive with the late braking and that you can get higher efficiency yourself in stop and go traffic.
     
  16. brianman

    brianman Burrito Founder

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  17. majorlance

    majorlance Member

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    The best I've seen for TACC is to pull behind an 18 wheeler that's cruising and then set distance at 1 and turn on TACC. The drafting effect is huge.

    But, as others have said, easy terrain, flat, no stop lights, etc. TACC will do a very good job. Add in variables and you can't beat the human element.
     
  18. Galve2000

    Galve2000 Member

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    As a current Chevy Volt driver it makes me sad to hear that on a Model S CC (and now TACC) accelerate very quickly when resuming the pre-set CC speed. One of the great things about the Volt is that when you hit resume it will gently accelerate to the pre-set speed. personally I always get much better milage when I use CC and don't make too many changes (even when there are lots of hills) than I do when I drive without CC.

    I was hoping (and am still expecting) the same to be true in my Volt.
     
  19. KurtR

    KurtR Member

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    I agree. I've been experimenting with cruise control on basically flat terrain for a 20+ mile stretch of highway to and from work, and have noticed that with just a moderate amount of attention I can keep the orange line much steadier than cruise control will, which jitters quite a bit. My guess is that the cruise control is so finely calibrated that it tries to keep exactly the same speed, to something like a tenth of a mile per hour, over every little bump, rise, etc. which wastes energy. Having said that, I'm not sure that the difference is worth the inconvenience of the attention and the effort of feathering the pedal. Cruise control at 55mph has been getting me about 270-280 wh/m while manually I can get about 260-270 depending on the conditions I've experienced so far.
     
  20. David99

    David99 Active Member

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    I really wish there was a constant power cruise control setting. Instead of keeping the speed constant it keeps the power constant.

    This used to be possible if you were able to keep the accelerator at a certain position. They changed that with one of the software updates. Now the power increases slightly with speed even when you keep the pedal perfectly constant.
     

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