MPG with C/Control on

[lynall]

I remember a few years ago and this was on full weight artics and the main objective of the cruise control was to get the truck up to max speed asap and keep it there and this was the most efficient for max mpg.

To encourage this even further the ecu could be programmed so that you could not get max speed unless you were in top gear (16th) to be the most fuel efficient.

IIRC the trucks were ERF ec11 with the Cummins engines.

I use my cruise all the time as i hate having to try and maintain a steady speed, best mpg over 300 miles on cruise was 31.50mpg which i thought was pretty amazing from such a big engine.



Lynall

 

[Dryce]

they are still maintaining the same speed regardless of any other input.

Which is pretty much what a driver does with their right foot but not quite as well.

 

[rf065]

Which is pretty much what a driver does with their right foot but not quite as well.

Only the driver gets more mpg.
Is that not where we started?

Russ

 

[Bobby Dazzler]

Fascinating thread.

PS Who's the person quoted with an MSC in Automotive Engineering?

 

[timskemp]

There's little or no difference in reality - if you drive to maximise the use of cruise by setting it at a steady average speed (50-60 on a busy motorway, 90-130 on an empty one for example) it's no different from driving without it as economy goes. What will affect it is having to stop harder because your attention wasn't on the traffic flow and the cruise runs you closer to the slowing traffic, especially when you have a rising and falling road.

 

[Diesel Benz]

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

I'm sorry but you do not know a lot if you don't know the relation of ECU and CC on modern cars (CC is actually implemented in the ECU computer as it anyway controls the engine). The CC intelligence and control capability is absolutely different from the W123/W124 times.

I would not refer to US law suits, everything is deadly dangerous if you ask US lawyers.

The comment on cruise using brakes was a good one, I forgot earlier to mention that this does kill fuel efficiency on a strong downhill, if you otherwise would be willing to accept over speeding. I'd like to have this controlled on modern cars (specifically if I had to drive roads with good downhill sections). One should really switch off CC in this case (again, if you accept the speed increase).

As far as I know, there are not too many MBs that apply brakes when CC asks for it, most use engine braking which is OK for fuel economy. The W211 with SBC does not use brakes even if it easily could. But the face lift W211 does, as well as the W221 with Distronic Plus, I guess all W221 cars do. I would expect cars of same age and newer would do the same.

 

[rf065]

I'm sorry but you do not know a lot if you don't know the relation of ECU and CC on modern cars (CC is actually implemented in the ECU computer as it anyway controls the engine). The CC intelligence and control capability is absolutely different from the W123/W124 times.

Taken in context, I was referring to your quote, "A modern CC is built into the engine ECU and gets all other info like from ESP about cornering etc."

The point I was trying (unsuccesfully) to make was that, no matter what it is connected to or part of, it's only function is to maintain a constant speed at all times. If it is connected to the ESP, it is only to switch itself off if it detects the car has lost control. It does not alter its behaviour for any other factor no matter what it is connected to. It is simply not that intelligent a system.

Russ

 

[hawk20]

Do tell me how a skilled driver holding a steady 70mph for long distances can possibly use less fuel than cruise control holding a steady 70mph for long distances.

However much fuel a car uses to maintain a speed of 70 will be needed by both.

 

[stevesey]

Do tell me how a skilled driver holding a steady 70mph for long distances can possibly use less fuel than cruise control holding a steady 70mph for long distances.

However much fuel a car uses to maintain a speed of 70 will be needed by both.

Didn't I answer that in post 128 - well the fact that you will get different mpg at least, which is better will vary between drivers and CC algorithums.

 

[Diesel Benz]

Didn't I answer that in post 128 - well the fact that you will get different mpg at least, which is better will vary between drivers and CC algorithums.

I guess most of us have repeated their opinionsseveral times. It is obvious that some with a specific view would not even take note of a comment that contradicts his view. An example I believe is that you consider it better for fuel economy (as per the post you refer) if the engine is not temporarily driven at high load, better to catch up a bit speed for an uphill section. This simply isn't right (excluding possibly glojo's stretched limo that may need to switch a lower gear in this case). All but very small engines do not reach the best engine efficiency load when driving uphill (I assume we are not talking about Alps but typical motor way sections where cruise would be used).

Someone already referred to training for good fuel efficiency. The rule is to gain the target speed reasonably fast (in exact terms, use the best engine efficiency point which is close to full load, not exactly full load) and then keep the speed steady, irrespectively if the engine load changes. This often sounds strange but it is backed up with pretty simple physics.

Unfortunately with an auto tranny the acceleration to best economy is quite difficult when you cannot force gear changes early enough (manual gear selection may help but not equally as for manual transmission).

I believe if I asked whether a car with a small engine consumes more fuel because it needs to work harder compared to a bigger engine on the same car body, we would get two groups of people that correlate pretty accurately with those for and against CC fuel efficiency.

 

[rf065]

Do tell me how a skilled driver holding a steady 70mph for long distances can possibly use less fuel than cruise control holding a steady 70mph for long distances.

However much fuel a car uses to maintain a speed of 70 will be needed by both.

As posted way back from the guy with the MSC in automotive engineering.

"As somewhat of an expert in automotive engineering (i hold an MSc in it) i can say that the use of cruise control does effect your MPG but in a negative way.

Yes staying at a constant speed is a good way to increase your MPG and cruise control would be fine if every road in the world was flat and level, however it is not and resistance from different road surfaces, weather and all sorts of other things causes problems.

The reason for this is that most cruise control systems work by operating your throttle control for you in order to stay at a regular speed, as your speed drops the cruise control opens up the throttle incrementally until the speed is brought back to its correct state. The problem is that because cruise control doesn't know about hills or different conditions it will start to 'feather' the throttle. This is to say keep coming on and off it in order to balance your speed. You might not feel it while driving but it is doing it.

This can seriously effect the fuel efficiency of your engine as it needs a constant and smooth throttle operation in order to work its best. Ultimately only humans at this point and provide this."

Russ

 

[280gee]

Not having an axe to grind either way.
It seems to me that about the only constant one could rely on (according to a civil engineer pal) is that motorway gradients are never more than 'x' percent in the UK. Everything else is a variable and so how can any definitive answer be found to the original question without controlled experiment.

Put another way, the discussion so far seems to be subjective opinion.

 

[hawk20]

As posted way back from the guy with the MSC in automotive engineering.

"As somewhat of an expert in automotive engineering (i hold an MSc in it) i can say that the use of cruise control does effect your MPG but in a negative way.

Yes staying at a constant speed is a good way to increase your MPG and cruise control would be fine if every road in the world was flat and level, however it is not and resistance from different road surfaces, weather and all sorts of other things causes problems.

The reason for this is that most cruise control systems work by operating your throttle control for you in order to stay at a regular speed, as your speed drops the cruise control opens up the throttle incrementally until the speed is brought back to its correct state. The problem is that because cruise control doesn't know about hills or different conditions it will start to 'feather' the throttle. This is to say keep coming on and off it in order to balance your speed. You might not feel it while driving but it is doing it.

This can seriously effect the fuel efficiency of your engine as it needs a constant and smooth throttle operation in order to work its best. Ultimately only humans at this point and provide this."

Russ

Yes, it was rubbish when posted earlier and is still rubbish now. If a car is driven at a steady 70 mph it will use the same fuel whether controlled by cruise or by your right foot.

 

[stevesey]

Yes, it was rubbish when posted earlier and is still rubbish now. If a car is driven at a steady 70 mph it will use the same fuel whether controlled by cruise or by your right foot.

Only if either system could drive at a perfectly steady 70.00000mph, neither can and they both differ in the way they respond to slight speed differences hence different mpg.

 

[coupe deville]

I was in a 30 mph limit so set the cc to about 32 but dont think its working properly [anyway thats my excuse officer] I thought these e320's were limited to 130 !

 

[hawk20]

Only if either system could drive at a perfectly steady 70.00000mph, neither can and they both differ in the way they respond to slight speed differences hence different mpg.

And we haven't a shred of evidence, other than endlessly conflicting experiences, that either is better than the other. Over hundreds of thousands of miles in many different Mercedes I have found no significant difference in consumption at a steady speed.

In fact in general use the cruise saves fuel -used properly- by keeping closer to the steady speed while most drivers find their speed varies up and down (as we can see daily on the motorways) and frequently gradually creeps up -which is bad for economy.

Where you can save is when held up in traffic: instead of using cruise to return to 70 -or whatever- you can return to cruising speed gently and then re-engage cruise. Can save a bit that way.

More interestingly cruise is quicker from A to B for almost all drivers on a long run. Why? Because having chosen your maximum safe cruising speed -say an indicated 75mph- cruise will keep you there consistently hour after hour. And if you get held up it will return you quickly and surely to your chosen speed without the need to look at the instruments. Humans concentration levels are rarely if ever able to equal electronics.

 

[rf065]

Interestingly, you haven't offered a shred of evidence to support your theory, yet you rubbish the statements of someone with a MSc in Automotive Engineering? That's my last word on the subject as it going nowhere.

Russ

 

[st4]

Coming up for one of the longest threads ever-apart from the "I'm embarressed to barter on...."

 

[BTB 500]

Coming up for one of the longest threads ever

Check out the numberplates thread

 

[Shude]

Check out the numberplates thread

The NW GTG thread is up there too!