MPG with C/Control on

[wemorgan]

Do you think Top Gear just decided to drive from London to Scotland & back on a full tank of fuel without researching or analysing how it was likely to turn out?

It may be true, it may be false, but Top Gear is primarily an entertainment program not a scientific test-house like MIRA and TRL et al.

 

[hawk20]

Sorry, you are wrong on this one.
Anyone who still has usable brain is smarter than your cruise control.
I'll have to agree to disagree with you also.

Russ

Really? Do they use some telepathic process to adjust the engine so that at a steady 70 it uses less fuel with them on the accelerator rather than cruise.

All physics should tell what brain they have left, that doing a STEADY 70 will use the same amount of fuel.

 

[del320]

I'm pretty certain that, theoretically, a better mpg will be gained with the CC off. But it would be tiresome forever driving on the lightest possible throttle at all times.

At steady speeds with the CC on, fuel will surely be metered out in the least possible quantities, whilst maintaining a constant engine speed.

I use it as often as possible, even using it as a sort of hand throttle at lowish speeds on the really quiet roads we have here. The only exception, as already mentioned, is avoiding the vigorous resumption after a significant reduction from the set speed.

This, along with correctly inflated tyres, explains why I regularly exceed 32 mpg.

 

[Dieselman]

At steady speeds with the CC on, fuel will surely be metered out in the least possible quantities, whilst maintaining a constant engine speed.

Road speed.
This is why it uses a bit more, it keeps the speed constant wheras a driver would allow the speed to dip a bit then be resumed more gradually.

 

[Roger Jones]

"avoiding the vigorous resumption after a significant reduction from the set speed."

Except that all the serious advice on minimizing fuel consumption recommends getting up to the desired speed briskly -- briskly, rather than flooring it. It seems to me that this is exactly what CC does.

 

[glojo]

"avoiding the vigorous resumption after a significant reduction from the set speed."

Except that all the serious advice on minimizing fuel consumption recommends getting up to the desired speed briskly -- briskly, rather than flooring it. It seems to me that this is exactly what CC does.

The word brisk to me means quickly or lively. To get up to the speed we are happy with then Cruise Control will accelerate 'briskly' which means putting far more fuel into the system and then when it reaches the desired speed, if the vehicle starts going down hill, the stem will regard this acceleration, thus wasting all the fuel that was needed to briskly get up to the designated speed. Those of us that use forward observation will see that the road is about to gradually descend and make allowances for this.

I really don't see the point in discussing this issue as it is a no brainer. The instant anyone gets better consumption when not using g cruise control, must surely answer this very simple question?

Cruise Control works the same on all our modern vehicles, so if all the cars are the same and they are on the same stretch of road (not a theoretical highway that has no winds, gradients, traffic or hazard) then every car will return the same mpg. No one can say there car gets a better return. However as soon as the Cruise Control is switched off, then a drivers style of driving comes into the equation. We are not discussing speed of driving, as a slow driver that accelerates briskly at every opportunity will burn\waste far more fuel than a competent driver that is skilled in the art of reading the road and making safe progress by using good observation.

To highlight the difference between 'brisk' acceleration and competent driving, I will simply suggest driving along a strip of road with a large number of traffic lights. The Barry Boy with his 'hot mobile' accelerates briskly away from the first set of lights, brakes like an idiot for the next set, accelerates briskly away as soon as this set changes green and so on. The competent driver will accelerate away from the first set, adjust their speed because they can see in the distance Barry Boy, stomping on their brakes, and as the lights change, Captain Sensible drives through these lights whilst Barry Boy is briskly accelerating, trying their hardest to catch up with our driver that is merely using their driving skills to drive sensibly.

I am a great fan of cruise control and as I keep saying we regularly use it, but on our Sprinter you can hear how hard the engine works maintaining the required speed, that noise is not my imagination; it is quite clearly working that much harder, just because it uses more revs when pushing our van.

I would very respectfully suggest those folks that get less mpg with the Cruise Control disengaged ask any of us that get a better return to accompany them on a drive and help to improve their observational skills. I say this with the greatest of respect and appreciate just how folks get very protective when it comes to observations about our driving abilities.

I'll now put on my steel helmet

Regards
John

 

[Dryce]

Do you think Top Gear just decided to drive from London to Scotland & back on a full tank of fuel without researching or analysing how it was likely to turn out?

They're journalists.

It was stated and not explained or verified.

Put kindly: The media are not good with science and numbers.

Sadly the same is also true of the majority of the audience these days.

 

[stevesey]

Really? Do they use some telepathic process to adjust the engine so that at a steady 70 it uses less fuel with them on the accelerator rather than cruise.

All physics should tell what brain they have left, that doing a STEADY 70 will use the same amount of fuel.

But nobody, cruise on or off, does bang on a steady 70. Your speed will always be fluctuating slightly 69.9, 70.2, 70.1, 69.8 etc (still a steady 70 as far as the speedo goes though). And CC and a human have different feedback loops that they use to make minor adjustments to the throttle.

The CC will algorithim will measure the speed difference to the target speed, factor in some integration/differentation and come up with minor throttle movement, apply a little hystersis to this, and decide if needs be to adjust the throttle. And it will do this 100 or so times a second.

The human however, will sense their speed, look at the speedo, and the road ahead for dips/hills and bends, then adjust the thottle appropriately (we will see the hill ahead and attempt to feed in that little extra throttle as we start the climb, so our speed will probably be slight on the high side, say 70.1, 70.2 as we start to climb, the CC can only react to the speed slowing so will undershoot slightly, 69.8 say, as it starts the climb).

Even if the human is trying for a constant speed (and not picking up a little momentum downhill) these are two different feedabck arrangements so will result in different throttle movements and therefore different MPG figures.

 

[Dieter]

Hi,

I've just typed "cruise control and fuel efficiency" into google and the concensus (first few hits) is that the appropriate use of cruise control does save fuel:

http://www.edmunds.com/advice/fueleconomy/articles/106842/article.html#test3Test #3

Use Cruise Control

Result: Surprisingly effective way to save gas

Cold Hard Facts: Up to 14-percent savings, average savings of 7 percent

Recommendation: If you've got it, use it.

(proviso that this is only effective in non-mountainous areas).

http://budgettravel.about.com/od/cheapgroundtransportation/ss/step_gasoline_6.htm

economy tips - no. 6 'use cruise control'

http://ezinearticles.com/?How-To-In...ey-By-Using-Cruise-Control-Properly&id=343820

But despite its advantages, cruise control isn't for everyone and isn't for all roads. It's best for open city driving, preferably flat roads, without hills where there's little need for a driver to stop and go. Using cruise control ensures that a driver maintains a consistent speed, helping to improve gas mileage and overall safety on the road, as the need to put on the brakes is lessened. When tested, a Land Rover LR3 got close to 14 percent better mileage when using cruise control on the highway than it did when the driver adjusted the car speed manually. This test shows how the use of cruise control can impact a car's efficiency.

Thus my conclusion are:

Use cruise control to cruise....and/or use it appropriately (gentle undulations) to boost fuel economy so as usual the answer to the original question (hence the 9 pages of comments) is '6 of one and half a dozen of the other' .

Good night.

 

[stevesey]

Method: We did this test twice with four different cars each time driving the 55-mile loop. The first time we set cruise control to 70 mph. The second time, with the cruise control off, we varied our speed between 65 mph and 75 mph. We tried to mimic the driving style of a person who is in moderate freeway traffic.

Not the discussion we are having here at present though - we are focusing on the driver maintain a steady speed vs CC doing it.

Also: Cruise control ensures that a driver maintains a common, steady speed while on the road, helping to eliminate much of the stopping and starting that unnecessarily burns up expensive fuel.

So crusie control, smooths out other traffic and make lights change in your favour.

Not exactly well constructed articles.

 

[hawk20]

Road speed.
This is why it uses a bit more, it keeps the speed constant wheras a driver would allow the speed to dip a bit then be resumed more gradually.

Then the driver would not be maintaining a steady 70 would he? So very annoying for other traffic to follow someone endlessly speeding up and slowing down. And definitely not what cruise is for so it is totally irrelevant.

 

[Corrado]

oorjim, that a bit of a hornet's nest you've stirred up . Re your OP, the difference in mpg on a journey like that could simply have been down to a combination of
i small differences between the speed setting of your cruise control
ii small differences in head/tail wind
iii other (tyre pressure, fuel quality, headlights, aircon)
iv use of cruise control

i and ii have seeming disproportionate effects on mpg, but if on both journeys you had a (hardly noticeable) 5kt southerly breeze, the effective difference seen by the car on the two journeys will be 11mph. It wouldn't have needed much more to have entirely accounted for the difference in mpg of your two journeys.

All other things being equal it's become clear I'll get near equivalent mpg with cc on a clear motorway, but on a road with more undulations and other lumps and bumps the mk1 eyeball + brain + right foot provides the more safe, comfortable and economical journey.

 

[rf065]

Funny how some people believe a device solely invented to measure crankshaft speed, and fuel an engine to keep turning over at that speed regardless of any other criteria, can be more economical than the human brain? By all means use CC for the convenience it was designed for, but saving fuel was not part of the remit.

Russ

 

[Diesel Benz]

Funny how some people believe a device solely invented to measure crankshaft speed, and fuel an engine to keep turning over at that speed regardless of any other criteria, can be more economical than the human brain? By all means use CC for the convenience it was designed for, but saving fuel was not part of the remit.

Russ

I don't want to continue giving opinions one or the other way but the above is not right in the sense that CC is not making use of the crankshaft speed. A modern CC is built into the engine ECU and gets all other info like from ESP about cornering etc. etc. A lot that the human brain does not actually have access to and even if it had, would not be able to process it every millisecond.

The human brain is very good in anticipating traffic but is someone seriously considering using CC in a traffic jam in the centre of London? CC is intended for stretches where constant speed is good (depends on the driver if he wants to use it for short or long shots). The human brain should be used to switch off and on the cruise as appropriate. In my opinion the discussion here should be limited to the constant speed sections (I mean it only makes sense there, no intention to oppose others giving any opinions they want to).

 

[wemorgan]

Funny how some people believe a device solely invented to measure crankshaft speed, and fuel an engine to keep turning over at that speed regardless of any other criteria, can be more economical than the human brain? By all means use CC for the convenience it was designed for, but saving fuel was not part of the remit.

The consensus seems to be leaning towards that a skilled driver in real world conditions can give better MPG than CC. I'm happy to accept that view.

My twist on this debate is how many of us are actually skilled drivers? I guess being a member of a MB forum makes us more than averagely interested in driving and therefore possibly 'better' drivers than Joe Bloggs. But looking across the driving population I don't see too many skilled drivers on the road. How many drivers do you see having animated conversation with their passengers causing them to slow down or have that great driving tune come on the radio that causes them to speed up? The human brain is undeniably more advanced than CC but equally more distractable.

 

[BTB 500]

The average driver is not cabable of maintaining a steady speed. Anyone who uses cruise control on a reasonably flat & clear motorway sees evidence of that all the time.

 

[rf065]

I don't want to continue giving opinions one or the other way but the above is not right in the sense that CC is not making use of the crankshaft speed. A modern CC is built into the engine ECU and gets all other info like from ESP about cornering etc. etc. .

CC was invented in 1945, it measured the speed of the crankshaft and kept the engine turning over at that speed, regardless off any other criteria.

To my knowledge the majority of systems still work on the same principle.
The only reason they may have a link to ecu's or esp, is to turn them off if a problem is detected in another system as it may be unsafe to continue on CC.
In much the same way as applying the brakes will switch it off.

If it was linked to any of the cars other systems for any other reason, it would detect cornering or uphill/downhill sections and act in the most practical manner, just like a human would. The fact is, it is only there to keep the engine rotating at the same speed, nothing more, nothing less, it is simply a convenience feature.

In the US, manufacturers are now aware of the increased likelyhood of lawsuits from drivers who will blame CC for accidents as the roads become more congested making it's use dangerous. They are very careful not to call it a safety feature or an economy mode, as they would leave themselves open to lawsuits they could not defend.

Their answer was to develop the new breed of CC which detects you are gaining on the car in front and applies the brakes. I wonder if that will use more fuel than doing it yourself?

Russ

 

[Dieselman]

The fact is, it is only there to keep the engine rotating at the same speed, nothing more, nothing less, it is simply a convenience feature.

It's linked to the road wheels, not the crankshaft. The engine speed does indeed vary according to load and gradient.

 

[stevesey]

CC was invented in 1945, it measured the speed of the crankshaft and kept the engine turning over at that speed, regardless off any other criteria.

To my knowledge the majority of systems still work on the same principle.

That sounds like just using a good old fashion Watt's govenor as a crude CC, they must have moved on pretty quickly.

Won't work very well a torque convertor (let alone the fact that an auto box might decided to change down going uphill).

 

[rf065]

It's linked to the road wheels, not the crankshaft. The engine speed does indeed vary according to load and gradient.

It was linked to the crankshaft when first invented. I said modern systems still worked on the same principal, where they measure the speed from has changed but that is mostly irrelevant, they are still maintaining the same speed regardless of any other input.

Russ