Myth or fact? Does ACC lead to higher fuel consumption? Dependent on where it is used, eg. highway or "back roads"?

Higher in some circumstances. For example, when coming down hill in eco mode, the car will go out of gear and coast (DSG model), But when ACC is engaged, this doesn't happen.

It will depend on the type of roads/journeys and also how you normally drive.

 

By that I mean that if you normally drive poorly (in terms of fuel efficiency) then you may then get better fuel economy by using cruise/ACC.

 

If you are good at getting high MPG driving manually (hypermiler perhaps?!) then using cruise/ACC may achieve a lower MPG figure!

 

Also I find cruise control tends to apply more throttle up a hill then I would and (as above) doesn't ease off the throttle enough (or not coast as with the new DSG).

 

Phil

Yes, a dumb computer will never be as smart as a good driver.

Are you sure about the downhill coasting ? Mine (1.4 / DSG) doesn't go out of gear, but if I hit the brake pedal smoothly while going downhill,  throws in smaller gear to reduce speed. Consumption shows zero. Haven't got the drive mode selection though. I think it's a matter of ACC not being able to predict traffic as driver does.

Are you sure about the downhill coasting ? Mine (1.4 / DSG) doesn't go out of gear, but if I hit the brake pedal smoothly while going downhill,  throws in smaller gear to reduce speed. Consumption shows zero. Haven't got the drive mode selection though. I think it's a matter of ACC not being able to predict traffic as driver does.

If you don't drive mode you can't select Eco therefore, an Eco Mode function isn't gonna work.

My air con/climate is always in auto.In winter it helps keep the windows mist free, in summer, well you know that bit.

It disengages drive in down hill no throttle mode as it should (dsg).

Seen reports saying on long runs savings by switching air con off are negligible, in my town use it could help. Think engine size can also have a lot to do with it.

If you don't drive mode you can't select Eco therefore, an Eco Mode function isn't gonna work.

When going downhill and in ECO mode, revs = 0 or revs = idle ? If the answer is latter one, I would say that ECO is consuming more fuel by idling the engine rather than shutting the fuel injection. To my understanding coasting is allowing the car to roll freely with engine disengaged...or perhaps I missed something.

 

ps. Not trying to start an argument, just a matter of figuring it out

Edited 15 May, 201511 yr by Han2u

I would say that ECO is consuming more fuel by idling the engine rather than shutting the fuel injection. 

 

 

Some smarter engineers than us have figured out that coasting out of gear and using enough fuel to keep the engine ticking over, is even more efficient than rolling in-gear with a closed throttle and no fuelling

Some smarter engineers than us have figured out that coasting out of gear and using enough fuel to keep the engine ticking over, is even more efficient than rolling in-gear with a closed throttle and no fuelling

In theory this is true, the a car in Eco will actually increase in speed on a suffiently steep downhill road, far less steep than an in-gear car however, in the real world some muppet in an in-gear car will always get in your way and spoil your potential fuel saving although, with a lot of forethought it is possible on Motorways to gain from this function. 

 

The only PITA is that as in a standard car the throttle map of Eco is very difficult to live with in real world driving situations, it can also seem a little odd to be doing 80mph with only about 700 revs showing, I tend to use it when I see slower moving traffic in the distance, simply coast up behind them using the fuel I've already burnt in Sport Mode, without doing this my fuel avg. tends to be somewhere in the mid 20's !

In the last 15 years I have had 6 different cars all with cruise. On all of them I achieve better mpg when not using cruise.

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When going downhill and in ECO mode, revs = 0 or revs = idle ? If the answer is latter one, I would say that ECO is consuming more fuel by idling the engine rather than shutting the fuel injection. To my understanding coasting is allowing the car to roll freely with engine disengaged...or perhaps I missed something.

It would be revs = idle. I don't have a DSG, but I'm pretty sure the start-stop system isn't going to stop the engine while you're moving. Whether it's better better to coast in neutral with the engine idling or leave the engine in gear with no fuel depends on whether you want to speed up or slow down.

If you want to slow down then staying in gear with no fuel will be more efficient. If you want to speed up (or you just don't want any engine braking) then coasting and using only enough fuel for idle revs will be more efficient than staying in gear and pressing the accelerator enough to maintain speed at a couple of thousand or so rpm.

ACC certainly isn't as economical as a human driver can be, but I have to admit it's smarter than the cruise in my previous cars. For example it seems to be smart enough to drop down a gear and engine brake when going down a steep hill instead of braking.

 

And as for coasting vs. cutting fuel injection/ignition in Eco mode, coasting is more fuel efficient in situations where you'd have to start applying more throttle to gain lost speed if the engine started engine braking. But luckily it's easy to control which behavior it uses via the gear selector. When in Eco D coasts and manual mode doesn't.

Edited 15 May, 201511 yr by kallekilponen

Higher in some circumstances. For example, when coming down hill in eco mode, the car will go out of gear and coast (DSG model), But when ACC is engaged, this doesn't happen.

You will use more fuel freewheeling as you have to put fuel in to keep the engine running. If you leave it in gear the engine will stop the fuel.

Hod. Freewheeling you will use more fuel per second, i.e. some.

You will use less fuel per mile as running the engine at idle and coasting will slow the car less than the frictional losses of turning the engine over faster than tick over.

Hod. Freewheeling you will use more fuel per second, i.e. some.

You will use less fuel per mile as running the engine at idle and coasting will slow the car less than the frictional losses of turning the engine over faster than tick over.

In theory this is true, the a car in Eco will actually increase in speed on a suffiently steep downhill road, far less steep than an in-gear car however, in the real world some muppet in an in-gear car will always get in your way and spoil your potential fuel saving although, with a lot of forethought it is possible on Motorways to gain from this function. 

 

 

However, if holding it in gear and letting it coast in gear using 'zero' fuel is able to hold you perfectly at the speed limit, then you arent able to gain an advantage from coasting out of gear. Unless you want to speed of course. Not possible with cameras about. Then of course theres the extra fuel you burn putting it back in gear. Plus considering how quickly a car slows down when not going downhill, you wont be coasting that much longer if out of gear.

 

Sorry, i just cant see a benefit from coasting out of gear now. Putting it back in gear 2 seconds later than you would apply the throttle if in gear, surely cannot overcome the extra fuel you burn elsewhere. Thats not to mention possible wear and tear on brakes and gears.

I come across very few hills that you could coast down out of gear and maintain speed, but couldnt coast down in gear. Even the hills where you can, you burn so little fuel if in gear, it just cannot be worth taking it out of gear then putting it back in again.

Coasting in eco-mode can get me quite far with the terrain we have up here. There lots of hills wich is just perfect for coasting out of gear. In gear coasting will slow the car down too much.

But I havent tried how much difference it maken makes on fuel consumption since I usually have ACC enabled.

Coasting in eco-mode can get me quite far with the terrain we have up here. There lots of hills wich is just perfect for coasting out of gear. In gear coasting will slow the car down too much.

But I havent tried how much difference it maken makes on fuel consumption since I usually have ACC enabled.

In gear slowing the car down? Thats not much of a hill. Usually it holds you just nicely at the correct speed.

In gear slowing the car down? Thats not much of a hill. Usually it holds you just nicely at the correct speed.

In gear coasting slows the car quite much.

And has done so on all cars Ive had.

Some hills lets me Coast a kilometer if out of gear.

I come across very few hills that you could coast down out of gear and maintain speed, but couldnt coast down in gear. Even the hills where you can, you burn so little fuel if in gear, it just cannot be worth taking it out of gear then putting it back in again.

 James, the only hill where you live is called the "Isle of Kent" every winter it rains hard so, I don't think your qualified for this discussion anyway, there's no knocking it outta gear, in Eco Mode the DSG does it for you as soon as you lift off the throttle and simple re-engages the drivetrain when either the throttle or brake are applied.  Your only thought when gaining on traffic is "oh those poor souls don't have a Coasting Function what peasants"

 

Seriously though, when in Eco Mode the car just does it and you do notice it, a lot, the only real downside is when a busy pulls outta a side turning1 behind you, you've gotta flick the DSG to the side and pull back 2 or 3 times to get some engine braking so as not to show any brake lights.  Its just another thing about my O3 that makes me smile.

I really like ACC and probably could get better MPG myself, but bought ACC for easy lazy driving.

 

As for Eco mode, for me it is no Eco mode, I live in a hilly part of the country and the best MPG I get is from individual mode and no coasting out of gear.

Where you UK imperialists miss out is that the Octavia consumption display is optimised for litres and kilometres.

If you have it set up for litres/100km then you get a bit more information such as litres/hour consumption when stationary and idling or true consumption when coasting. The mpg display maxes out at 200mpg in the conversion process.

The Australian supplied versions of the base 1.4tsi manual do not have stop/start (or any sort of mode options) and I like it that it shows my consumption when caught in traffic.

Typically my idling consumption when warmed up is 0.4l/100 which then rises as various loads are applied.

DRLs will make it hover between 0.4 and 0.5, headlights 0.6 to 0.7, A/c will vary but typically between 0.8 and 1.2 when cooling and how much it has to cool, even the fan speed makes a difference.

Maintaining revs with no load at 2000 rpm increases consumption to 1.5l/hour.

So how does this translate into actual driving?

Well if you are coasting at about 50kph then consumption displays about 1 l/100 (280mpg), and at 100kph the display is 0.5 l/100 (about 560 mpg).

If you have to apply some throttle to maintain speed at about 100kph (2000 rpm) where you could coast freely then consumption will be approximately three times higher (180 mpg).

Ideally the hill is sufficiently steep that you actually need engine braking and zero consumption to maintain speed or engine braking for deceleration.

The 1.4 tsi has so little engine braking that I can use it a lot and sometimes wish it provided more (screaming the engine in 2nd gear down a 4 kilometre hill and still using brakes to hold at 90kph speed limit where my previous diesel 1.9d provided ample braking effort in 3rd gear)

Coasting is only beneficial under a small set of circumstances matching gradient/speed requirements or you are able to anticipate road conditions upto a kilometre in advance. Otherwise in normal (UK) traffic you are probably better off with maximising use of engine braking.

Incidentally the official VW fuel consumption guide recommends that climate control A/C is turned off where it is not required, even when not actively cooling there is an adverse effect on consumption.

Sorry, but I love this sort of stuff.

Edited 20 May, 201511 yr by Gerrycan

Myth or fact? Does ACC lead to higher fuel consumption? Dependent on where it is used, eg. highway or "back roads"?

 

As Dodgy & Phil said, if you are a good driver for fuel economy, you should be able to achieve better results than ACC.

e.g. in heavy traffic ACC will maintain a distance to the car in front.

If they brake & accelerate, brake & accelerate (as most people seem to do these days) the ACC will just follow this (using a lot of fuel).

A good driver would increase the gap & maintain a more constant speed thus giving better fuel economy.

But then ACC & CC are not designed for fuel economy they are designed for comfort.

 

Cruise control technology, in trucks for example, can be much more sophisticated where it uses gradient sensors & map data to know when to increase speed downhill to maintain momentum for the uphill.

I haven't seen this kind of implmentation in cars yet though.

 

however, in general I would say ACC & CC would on average give better results than many drivers because its anticipation is better & it is less aggressive.

 

Yes, a dumb computer will never be as smart as a good driver.

 

ACC & CC are not designed for fuel economy, they are designed for comfort.

Therefore, its logical that its not that hard to achieve the same figures.

Cruise control technology, in trucks for example, can be much more sophisticated where it uses gradient sensors & map data to know when to increase speed downhill to maintain momentum for the uphill.

I haven't seen this kind of implmentation in cars yet though.

 

however, in general I would say ACC & CC would on average give better results than many drivers because its anticipation is better & it is less aggressive.

 

 

ACC & CC are not designed for fuel economy, they are designed for comfort.

Therefore, its logical that its not that hard to achieve the same figures.

I'd agree with your summation of ACC & CC benefits/disadvantages.

CC works well enough on relatively flat country but in hilly terrain I can easily improve on it.

I found consumption on diesels were less affected than petrol though when using CC in hills. Diesels do it better when it gets harder.

I believe the latest Bentleys use GPS mapping for terrain anticipation for their gearboxes.

I cannot understand why it has not been applied to hybrids though. Nothing wrong in using some battery power to aid ascending a hill if there is a good gradient on the other side to recoup some of it back on the say down.