Speed limiter

Discussion in 'Gen 4 Prius Technical Discussion' started by KV55, May 19, 2017.

  1. KV55

    KV55 Member

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    My UK Gen 4 Prius has a speed limiter. It is activated using a steering wheel button and the speed is then set by a push down on the cruise control stub lever. I have noticed that when it is active the blue guidance bar on the HSI is not shown. It works well, stopping the car from exceeding the set speed under normal conditions however it feels like it does this by countering the accelerator with the brakes or at least regen. If so, holding the car to the set limit is not going to be good for fuel consumption. Does anyone know how it works?
     
  2. bhtooefr

    bhtooefr Senior Member

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    The accelerator guidance going away tells me that it's engaging part of the cruise control, just setting an upper limit, rather than a fixed target.

    Does it feel like what happens if the cruise control is engaged and the setpoint is significantly below the current speed, and you lift off the accelerator? (One way to test this is to set the cruise control at a low speed, then get on the accelerator and speed up, then lift off.) Alternately, does it feel like what happens when descending a mountain on cruise control? If so, that will do regen and engine braking, but not friction braking. (That said, because you have the speed limiter functionality, I wonder if that's different on UK Priuses...)

    That said, I'm surprised if it actually has to go to that extent to limit the speed - I'd expect that in most cases, except downhill, just reducing power would be enough. If the HSI doesn't go negative, it's not doing any form of braking.

    We don't have the speed limiter in the US, though, and I've never seen any car (from any manufacturer) with one.
     
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  3. kithmo

    kithmo Couch Potato

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    I can't see the point of it TBH.
     
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  4. bhtooefr

    bhtooefr Senior Member

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    I can see it as a lower-cost way to implement part of what dynamic radar cruise control gets you.

    So, with regular cruise, you set a setpoint, that's it, you go that speed until you disengage it. If there's a car ahead, you have to disengage it yourself.

    With radar cruise, you set a maximum speed, and it goes that speed unless there's a car ahead.

    With a speed limiter, you set a maximum speed, and then you can drive in variable-speed traffic while being held to a maximum speed (for efficiency, legal compliance, or other reasons).
     
  5. kithmo

    kithmo Couch Potato

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    But full speed radar cruise does almost the same thing as the speed limiter with the added advantage that you don't have to keep accelerating yourself to keep up with traffic. I can see it may be of use if you don't have radar cruise but why have both (as mine does) ?
    I think in some situations limiting speed could be dangerous, e.g. if you set the limiter, then go to overtake and run out of speed.
     
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  6. alanclarkeau

    alanclarkeau Senior Member

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    I've driven a couple of cars with a speed limiter on a standard Cruise Control, one was a Renault, can't remember the other - a VOLVO? You set the cruise speed (wasn't digital display, just press set at say 100km/hr), and if the car tried to speed up going down a hill, it would apply the brakes, keep it to the set (eg 100km/hr) speed, and then when it flattened out the Cruise would just continue cruising. So accelerator up hill, brake downhill on steep undulating territory - it worked well.

    The DRCC on my PRIUS doesn't have the speed limiter either.

    It's dreadful in keeping to speed - one descent at 70km/hr I drive regularly, the PRIUS will speed up to almost 80 unless I intervene, then going up the other side it will be back at 70 again, and then overshoot as it crests the hill by about 8km/hr. With POLICE here very trigger happy with RADAR, it's $162 for up to 13km/hr over the speed limit - no I haven't been done - yet, but the PRIUS Cruise is much worse than my previous cars.
     
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  7. DonDNH

    DonDNH Senior Member

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    Have you asked your Toyota dealer to check the operation of the cruise control? The CC shouldn't be that inaccurate; maybe something is wrong that can be corrected.
     
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  8. kithmo

    kithmo Couch Potato

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    My radar cruise doesn't go over the set speed (although I haven't tried it down a steep hill), which is why I question the need to have a speed limiter as well as radar cruise.
     
  9. alanclarkeau

    alanclarkeau Senior Member

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    I just assumed it wasn't real good. I'll get them to check it at next service. My previous 2 cars were diesel Manuals and with high compression engines would keep to the speed really well unless it was an extreme slope.
     
  10. raspy

    raspy Senior Member

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    Much of the decision will be based on where you drive, how you drive, and how others around you drive. I find DRCC isn't that useful in a place like London where others drive very aggressively and are constantly changing lanes and squeezing into any gap available. I tend to use DRCC on the motorway in light to medium traffic.

    I've seen the speed limiter on Mercedes before. The decision whether to use the speed limiter or DRCC will also be influenced by how much control you want to cede to the car's computer. DRCC has flaws, and some may prefer to have control over gas and brake inputs, or even just may prefer the tactile sensation of pushing the pedals.

    I do know some folks in the UK who use speed limiter on their vehicles when they are driving in unfamiliar locations, and/or those motorways where you have construction for miles and 50mph limits.

    Finally, in an emergency. if you have the speed limiter set in the Gen 4, and you press the gas pedal all the way down, the speed limiter would be over-ridden and you could accelerate above the speed you'd limited it to. I tried it this morning.
     
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  11. Prodigyplace

    Prodigyplace Senior Member

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    Mine will go over by 1 or 2 mph on some downhill sections. If that is a concern I can set the cruise control speed a little lower.
     
  12. kithmo

    kithmo Couch Potato

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    I'm glad to hear it does that, maybe I'll give it a go now before I finally condemn it to the back burner. (y)
     
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  13. RCO

    RCO Senior Member

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    The Bristol area is like Cornwall inasmuch as it is rather hilly with steep gradients along the roads. My DRCC will overrun the set speed by more than 5 mph down some of these, so I too set the speed 5 below my target mph to avoid the average speed cameras with numberplate recognition. Another thing I discovered is that of you punch Sport Mode in, the DRCC is much more aggressive in holding back from overrun. @KV55 could try that out as IMO it is better than CC.
     
    #13 RCO, May 21, 2017
    Last edited: May 21, 2017
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  14. Prodigyplace

    Prodigyplace Senior Member

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    So you guys have an added Sport mode? We have Eco, Normal & PWR modes.
     
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  15. kithmo

    kithmo Couch Potato

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    I think he means PWR whilst eating a pastie.
     
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  16. RCO

    RCO Senior Member

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    There are just no secrets left on this site!!! :ROFLMAO:
     
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  17. KV55

    KV55 Member

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    I used the energy flow screen to see what was happening. Some times the ICE is running and the regen is happening. I didn't notice any increase in battery charge but perhaps I did not hold that state long enough. At other times the car was running in EV mode. Unlike DRCC the car can be trying to override the driver input, even at constant speed on a flat road it feels as if regen is happening when pushing against the limited speed. It instinctively feels wrong. On a downhill stretch the car does over run the limit and the set speed changes on the display to a more prominent figure. I think the speed limiter is not an ideal (lazy) way to hold the local speed limit where DRCC is not practical and that in use it should be set to stop the speed drifting too high, eg set it to 22mph and hold 20 using the pedal. Perhaps if you need to deliberately add some battery charge it could be used for that purpose.
     
  18. pakitt

    pakitt Senior Member

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    DRCC is quite useless here in Germany/Bavaria too. Continuously changing speed limits and very aggressive driving, do not allow the DRCC to do a good job as it has to change widely the speed either because people cutting the space in front of you, driving 80km/h when the limit is either nothing (i.e. 130km/h advised) or 120km/h in some sections, or because of a tunnel (typically set at 80km/h or 100km/h).
    I was 2 weeks on vacation just recently in the USA and the DRCC was working much better on my rental Prius (I suppose a 2 since it had practically no options on it).
    The speed limiter I think it is useful to regulate yourself speed vs the DRCC and to avoid getting speeding tickets in sections of road where the speed limit is constant or changes to slower speeds (e.g. regional roads).
    Another thing I don't like of the DRCC, is that if the car is in Eco mode, it will accelerate very slowly back to speed when you need to change lanes because there is yet another driver going 90km/h in the middle lane, with the Prius braking hard almost all of a sudden to match its speed. This irritates other oncoming drivers that don't see you gathering speed fast enough.
    If you set the driving mode to Normal or even PWR, it is way faster, but then sometimes it accelerates so fast, you wonder if fuel consumption is considered at all.
    Moreover, with DRCC and a speed limit comes up and you don't see it or more simply rely on the DRCC to match the preceding vehicle, if the driver is very slow in matching the new speed limit, or drives aggressively, you end up being an aggressive driver yourself as the Prius will brake last minute just as the preceding vehicle will.
    I find DRCC only really useful in traffic jams....
     
  19. Ogo

    Ogo Prius Owner since 2008

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    I use my speed limiter mostly in city traffic, where I want to have manual control over how car drives, yet still have a upper speed limit enforcing in place.

    DRCC is on other hand quite handy for highway traffic up to the level of light to moderate traffic.
     
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