[X5plug-in]

Being a fan of cruise control, I spotted for the first time that the energy recovery blue bar does not appear on the same stretch of road under the same conditions where as it does if the car is driven manually. Does this mean this aspect of ED never works with cruise, with the resulting efficiency hit?

[JOHNBMWM5]

Originally Posted by X3buyerScotland

Being a fan of cruise control, I spotted for the first time that the energy recovery blue bar does not appear on the same stretch of road under the same conditions where as it does if the car is driven manually. Does this mean this aspect of ED never works with cruise, with the resulting efficiency hit?

Basically Cruise is not the most efficient mode to be in, it will apply the brakes down hill and use excessive throttle on many occasions up hill. It has been proven on many an occasion. Use it when in a steady cruise and when going down hill disengage it. You will see an MPG improvement .

[robwoods]

Interesting concept this thing about cruise control being less efficient. I disagree entirely.

I agree that applying the brake is inefficient for fuel, but if you have the cruise control set to the national speed limit (or whatever speed you think you can get away with), then surely you would have applied the brake going down that same hill in the same way if driving manually? So assuming you care about not getting a speeding ticket, and assuming you are 'cruising' at the maximum allowable speed, the cruise control is no less efficient here.

As for applying extra throttle up hill causing lower overall fuel efficiency, that is not true. It applies the exact same amount of throttle as you would have done manually if you had been trying to maintain the same speed. I agree that cruise control maintains the same speed at all times, and as a human being you would tend to slow down up hills, but that is an unfair comparison. Because as a human you would also tend to speed up a bit down hills, which the cruise control does not. Cruise uses more fuel up hills than a human and less fuel downhills.

A fair statement would be to say 'for the same average speed for a journey cruise control uses no more fuel than a human would'. If there are steep downhill sections then of course cruise might be less efficient due to braking assuming the human would have been happy to exceed the set limit.

In the end, cruise control only uses the same amount of fuel that a human would assuming the human wanted to keep the car at that same speed.

Go on, shoot me down

[Steve30dMSport]

Originally Posted by johnbmw6

Basically Cruise is not the most efficient mode to be in, it will apply the brakes down hill and use excessive throttle on many occasions up hill. It has been proven on many an occasion. Use it when in a steady cruise and when going down hill disengage it. You will see an MPG improvement .

This is correct but it doesn't explain it all. The reason you see a difference is because when you drive manually you actually modulate the pedal without realising. when you lift off you get the blue bar indicating the alternator is being charged and the reason you don't get the KERS effect is because you are not manually braking as with cruise control on the car automatically applies the rear brakes only to modulate the speed. You need full brake application to all four wheels (ie a manual braking) to active the KERS.

John is correct when he says that cruise is excellent on a basically flat terrain. And on hilly terrain cruise can be very uneconomical as well as dangerous; particularly on uphill blind bends when the car accelerates when you would be slowing down a little if driving without cruise.

Hope this helps.

[Bimar]

curious, does the X3 cruise control have sensor which approximate distance between cars to automatically decelerate like the new 7 ?

[Rodion]

Originally Posted by inamik75

curious, does the X3 cruise control have sensor which approximate distance between cars to automatically decelerate like the new 7 ?

No and it is not as even an option. Very disappointing But was not a deal breaker.

[Want the thrill]

Originally Posted by inamik75

curious, does the X3 cruise control have sensor which approximate distance between cars to automatically decelerate like the new 7 ?

No

[sfax]

Re. argument on cruise control. Think of a basic example where you have a big hill. If you take this hill at a constant speed of 70mph going up and then down again, you will need to apply the brakes on the way down the other side to stop the speed going over 70mph. If you gradually decrease your speed to say 65mph going up the hill and then let your speed gradually increase again to say 75mph going back down the hill using gravity rather than the throttle, you could still maintain the same average speed and use less fuel. CC also sometimes applies the brake unnecessarily where you could get the same deceleration by simply taking your foot off the accelerator and not applying the brake at all.

More on wiki if you're interested

Driving over "rolling" terrain, with gentle up and down portions, can usually be done more economically (using less fuel) by a skilled driver viewing the approaching terrain, by maintaining a relatively constant throttle position and allowing the vehicle to accelerate on the downgrades and decelerate on upgrades, while reducing power when cresting a rise and adding a bit before an upgrade is reached. Cruise control will tend to overthrottle on the upgrades and retard on the downgrades, wasting the energy storage capabilities available from the inertia of the vehicle.

http://en.wikipedia.org/wiki/Cruise_control

[Steve30dMSport]

Spot on Sfax. Rather more succinct than my roundabout answer!

[Radioactive]

Originally Posted by sfax

Re. argument on cruise control. Think of a basic example where you have a big hill. If you take this hill at a constant speed of 70mph going up and then down again, you will need to apply the brakes on the way down the other side to stop the speed going over 70mph. If you gradually decrease your speed to say 65mph going up the hill and then let your speed gradually increase again to say 75mph going back down the hill using gravity rather than the throttle, you could still maintain the same average speed and use less fuel. CC also sometimes applies the brake unnecessarily where you could get the same deceleration by simply taking your foot off the accelerator and not applying the brake at all.

Not only this, you can ease into your acceleration going uphill, whereas the cruise control tends to "floor it" going uphill to maintain speed.

[robwoods]

sfax, your 'simple big hill' argument is wrong.

You say that the car would decelerate to 65mph going up the hill and naturally accelerate to 75mph going back down the hill. Thats not true, unless the downhill slope were much longer than the uphill slope. It might naturally get back to where you started, i.e. 70mph. So the average speed would be 67.5mph-ish. So whilst I agree you would use less fuel than cruise control set at 70mph, you would also average a lower speed.

I suggest that if you had set the cruise control to 67.5mph all the way through, and compared the fuel usage to your human who starts at 70mph, ends at 70mph but bottoms out at 65mph at the top of the hill, the fuel usage would be more or less the same.

So my point still holds that assuming the same average speed, you will use the same amount of fuel whether using your foot to open the throttle or using a computer to open the same throttle. The laws of physics are at work here, and assuming energy lost to friction and wind resistance are equal, you need the same amount of energy to keep the same mass going at the same average speed for the same amount of time. And the energy is only coming from the fuel, nowhere else.

So unless someone can tell me where the cruise control is sending this extra energy that it is using due to its lower efficiency, then I'm not believing it. No-one is saying that the cruise control has some sort of energy overhead I assume, like older automatic gearboxes used to have compared to manual?

Some of the comments about anticipating the upcoming road conditions, the cruise control 'flooring it' etc are all generalisations that don't mean very much.

I do of course agree that if the downhill slope is harsh enough to require actual braking rather than just engine braking, then cruise control will use a bit more fuel due to wasting some energy as heat in the brakes, since we established earlier that KERS doesn't work for cruise control braking.

Wikipedia is wrong too!

[Rodion]

When you drive uphill, your kinetic energy is converted to potential energy. These are the two key words to resolve the disbelief you have. Wikipedia is not wrong.

Your example of "going downhill 75 mph is impossible" is wrong. It applies only if you do not use any extra force, ie. engines work that is convirted to forward force by transmission and wheels. You are thinking as if this was a rollercoaster which has no own energy creator. And therefore your thinking is wrong.

When going uphill so that your max speed at the top of the hill is 65MPH instead of 70MPH, your potential energy is the same but kinetic energy is less than going 70MPH at the top of the hill. Now where does the extra energy come from if you have 70MPH at the top? Bingo, it comes from fuel!

Now, 70MPH@top means that going downhill, your potential energy will convert back to kinetic energy, ie. your speed must accelerate given all other factors "fixed". But cruisecontrol does not allow that to happen but activates brakes and there goes the energy created by excessive fuel usage uphill... it reforms to heat on brake discs.

The most efficient way (put it simple) is to allow speed reduce as much as possible while going uphill and then downhill the speed would pick up again thanks to kinetic->potential->kinetic energy changes.

[sfax]

Originally Posted by Radioactive

Not only this, you can ease into your acceleration going uphill, whereas the cruise control tends to "floor it" going uphill to maintain speed.

True, it fights hard to keep the car at the set speed when going uphill whereas you could generally achieve the same speed with less gas by applying the gas more moderately

The problem with using fuel going up hills too fast is exacerbated with heavier cars too and the X3 'aint light at 1800kg.

It seems obvious to me but not only that I have spent a lot of time on the same stretch of motorway with and without cruise control and I can always drive more efficiently without it - going purely by the car's mpg reading. Try it

[X5plug-in]

What have I started?! I never liked physics! Has anybody confirmed whether cruise does not / cannot activate "KERS" then in the X3?

[Radioactive]

Just another reason I choose geology! Me thinkth that there are too many variables in the car for a straight textbook answer. Let's go for some empirical evidence.

[sfax]

Originally Posted by robwoods

sfax, your 'simple big hill' argument is wrong.

You say that the car would decelerate to 65mph going up the hill and naturally accelerate to 75mph going back down the hill. Thats not true, unless the downhill slope were much longer than the uphill slope. It might naturally get back to where you started, i.e. 70mph. So the average speed would be 67.5mph-ish. So whilst I agree you would use less fuel than cruise control set at 70mph, you would also average a lower speed.

I suggest that if you had set the cruise control to 67.5mph all the way through, and compared the fuel usage to your human who starts at 70mph, ends at 70mph but bottoms out at 65mph at the top of the hill, the fuel usage would be more or less the same.

So my point still holds that assuming the same average speed, you will use the same amount of fuel whether using your foot to open the throttle or using a computer to open the same throttle. The laws of physics are at work here, and assuming energy lost to friction and wind resistance are equal, you need the same amount of energy to keep the same mass going at the same average speed for the same amount of time. And the energy is only coming from the fuel, nowhere else.

So unless someone can tell me where the cruise control is sending this extra energy that it is using due to its lower efficiency, then I'm not believing it. No-one is saying that the cruise control has some sort of energy overhead I assume, like older automatic gearboxes used to have compared to manual?

Some of the comments about anticipating the upcoming road conditions, the cruise control 'flooring it' etc are all generalisations that don't mean very much.

I do of course agree that if the downhill slope is harsh enough to require actual braking rather than just engine braking, then cruise control will use a bit more fuel due to wasting some energy as heat in the brakes, since we established earlier that KERS doesn't work for cruise control braking.

Wikipedia is wrong too!

My point is that fighting to maintain a constant speed going uphill uses disproportionately more fuel than any gains you get by the fact you've tackled the uphill section slightly faster (but then unnecessarily held your top-of-hill speed at 70mph all the way down the downhill section). Just as accelerating like a lunatic everywhere rather than accelerating moderately uses disproportionately more fuel than any efficiency gains that come from your average speed going up due to the increased acceleration

But this is all theory, if you try it you'll find you can improve your mpg and yet maintain the same average speed. But no cheating to prove your point please!

[Radioactive]

Originally Posted by sfax

But this is all theory, if you try it you'll find you can improve your mpg and yet maintain the same average speed. But no cheating to prove your point please!

Let's not forget about 'significant figures" rules.

[sfax]

Originally Posted by Rodion

When you drive uphill, your kinetic energy is converted to potential energy. These are the two key words to resolve the disbelief you have. Wikipedia is not wrong.

Your example of "going downhill 75 mph is impossible" is wrong. It applies only if you do not use any extra force, ie. engines work that is convirted to forward force by transmission and wheels. You are thinking as if this was a rollercoaster which has no own energy creator. And therefore your thinking is wrong.

When going uphill so that your max speed at the top of the hill is 65MPH instead of 70MPH, your potential energy is the same but kinetic energy is less than going 70MPH at the top of the hill. Now where does the extra energy come from if you have 70MPH at the top? Bingo, it comes from fuel!

Now, 70MPH@top means that going downhill, your potential energy will convert back to kinetic energy, ie. your speed must accelerate given all other factors "fixed". But cruisecontrol does not allow that to happen but activates brakes and there goes the energy created by excessive fuel usage uphill... it reforms to heat on brake discs.

The most efficient way (put it simple) is to allow speed reduce as much as possible while going uphill and then downhill the speed would pick up again thanks to kinetic->potential->kinetic energy changes.

Great explanation and although this is true, you still need to maintain the same average speed so you can't allow your speed to reduce too much or you'll need more fuel afterwards to compensate for the lower speed. Balancing it is a skill and takes concentration but it keeps you busy on long journeys...

[clivem2]

Originally Posted by sfax

Great explanation and although this is true, you still need to maintain the same average speed so you can't allow your speed to reduce too much or you'll need more fuel afterwards to compensate for the lower speed. Balancing it is a skill and takes concentration but it keeps you busy on long journeys...

Might be worth balancing the fuel saving vs aggro for other drivers, slowing down & speeding up results in more overtaking. Just apply common sense when there's other traffic around, which I'm sure everyone here does! :-)