B180 over revving on downhill cruise

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notable
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B180 over revving on downhill cruise

Post by notable » Thu 25 Nov, 2010 9:22 pm

Hi Guys
My B180 CDI is fantastic, but on a recent trip to east Gippsland, I noticed that when on cruise, after a steep hill, on the downhill side of the hill, the tacho revs up to 3500rpm, although the speed remains constant at the set 100kmh.
The car has done 40K, next service at 45K, hadn't noticed this previously because all running is generally Hume Freeway where there are no steep hills.
Any comments??

Also, comment about the comfort of these sports cloth trim seats: Merimbula to Frankston, about 650 kilometres, took 9 hours and got out feeling totally relaxed, no back pain, even wife who has arthritis, had no discomfort.

thanks

CraigB
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Re: B180 over revving on downhill cruise

Post by CraigB » Fri 26 Nov, 2010 12:00 am

Welcome. This is way too new for my expertise but in general 'vintage' cars like mine have trouble holding back the speed of a car on a steep downhill and end up just running over the speed if the engine braking doesn't do it. Sounds like yours automatically changes down a gear to increase engine braking and therefore hold the car at the required speed (ie. cruise controls don't apply the brakes to slow a car.) Sounds pretty clever to me and I wouldn't be worried about it. I bet John Green knows the real answer!
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Moopere
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Re: B180 over revving on downhill cruise

Post by Moopere » Fri 26 Nov, 2010 2:16 am

CraigB wrote: (ie. cruise controls don't apply the brakes to slow a car.) Sounds pretty clever to me and I wouldn't be worried about it.
Sounds pretty clever to me too. In my 2008 CLC I'm pretty sure the cruise control applies the brakes on the decline side of a hill. I can't say for sure I've ever noticed this in a car before and I'm not sure I like it as the brakes appear to be applied with only a very minimal increase in vehicle speed (ie; 3km/hr or so judging by my GPS).

I have not yet had someone tail me and tell me, so perhaps I'm worrying for no reason, but at the moment I -do- worry that drivers behind me will see my brake lights blink on every time I decline a hill (**boggle**!!) as the car adjusts from say 103km/hr to 100km/hr. Why do I think its doing this? Well, not long after taking delivery I felt a movement near my foot. Investigation of this showed that the brake pedal is being moved towards the firewall (ie; brakes on) when cruise control is on and the vehicle is doing more than a little bit above its set speed. You can feel the pedal moving in and out by a few mm.

Up til now I've assumed speeding type legal reasons would have pushed such a design idea ... I don't like it, I'd much prefer to have the engine brake do the job, with perhaps a failsafe I guess at some set point that has a suitable hysteresis and won't cause a form of tight loop acceleration (up hill) followed immediately by braking (over the hill and down).

Moop

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petermc236
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Re: B180 over revving on downhill cruise

Post by petermc236 » Fri 26 Nov, 2010 10:34 am

Hi notable, welcome.

I regularly use cruise control on freeway runs. Changing down a couple of gears and the revving engine is the way the CC slows the car down - or to keep it at constant speed on downhill runs.
Our B200T has the 7 speed CVT gearbox and it will rarely revs over 2000rpm. With CC on it will go from 7th to 5th - even 4th to maintain constant speed - as the graduations between gears are not that significant.

As the car is very quite during normal operation with CC is on you will often hear the engine revving - which can cause some concern in the cabin.... I often get a "what are you doing to my car" comment.

If your B180 is an auto you can use the tip shift to manually control the gear box. If you think the engine is too noisy while CC is slowing the car, you can manually change the gear back to 6th or 7th for the duration of the hill etc and then hold gear lever to the right again until it selects auto mode.

Hope this helps
regards
-petermc


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Ivanerrol
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Re: B180 over revving on downhill cruise

Post by Ivanerrol » Fri 26 Nov, 2010 10:44 am

Computerised stability control uses small applications of the brakes to do it's job.
There would probably be a program to prevent the brake lights coming on when this feature is activated.
Probably would be the same for computerised C.C. braking.
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notable
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Re: B180 over revving on downhill cruise

Post by notable » Fri 26 Nov, 2010 4:09 pm

Thanks Guys
The general thought is its just the CVT adjusting to maintain road speed and engine braking, and that sounds feasible.
Next time I will try the manual override to see exactly what is happening on the dash display.

Thanks again for your comments
Rodney.

and a final comment about fuel usage. long term average is 5.8 that's nearly 1000km from a 54 litre tank and more economical than a Hybrid!!

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Moopere
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Re: B180 over revving on downhill cruise

Post by Moopere » Sun 28 Nov, 2010 3:30 pm

notable wrote: and a final comment about fuel usage. long term average is 5.8 that's nearly 1000km from a 54 litre tank and more economical than a Hybrid!!
Wow!

Thats really impressive. My CLC, despite being able to get down to high 6's on long trips, tends to do all its miles around town. I like to think I keep it down below mid 8's on the way to work, 7.6-8.5 seems common when I check the dash, yet the long term average the car keeps tells the real story :) 9.2 and 47km / hour

notable
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Re: B180 over revving on downhill cruise

Post by notable » Tue 07 Dec, 2010 8:25 am

Oh the advantage of country living!!
As you can see from the attached pic, 5.8 litres per 100 at an average speed of 71 over the last 12000 kilometres.
Most trips in my B Class are minimum 50 km on flat country roads, so it rarely gets over 2000 rpm. A trip to Melbourne is 140 km each way on the Hume Freeway, on cruise all the way.

On the overrevving concept, I tested the theory on a hilly stretch to Merton, and yes, guys you were correct. On a steep downhill run I moved the gear selector to M, and found that it had downshifted to 4th and 5th to increase engine braking.

cheers
Rodney
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