Cruise Control / MPG

[guest806]

Having just purchased the Si after a long test drive and using the cruise control this brings me back to a question i have wanted to ask for ages.  Is it more fuel efficient to cruise at a set speed no matter what the road level is, or allow the car speed to fluctuate with the road level?  I.E. doing a constant 60 mph on the flat, up and down hills or 60 on the flat, 50ish going up the hill and 60 - 70 coming down the hill?

I thought the cruise control would be a gimmick but i gave it a full testing on the test drive and i think it will be great on long journey or variable speed limits around the M25.  Can't wait to pick the new car up in about 10 days time.

[eljuero]

I'm interested too.
But I guess going 50 up hill is more fuel efficient than 60.

[madasafish]

If you read about hypermiling and the  actions of the owners of Toyota hybrids, the key way to save fuel is:

1. accelerate promptly to a given speed and then coast backing off the throttle as far as possible
2. don't accelerate UP hills
3. anticipate hold ups in traffic by slowing down and not braking unless essential.


Cruise control fails on all three issues..

I had it a couple of cars: ideal in the US and South Africa on roads hundreds of miles long with little traffic  - and gentle hills.  (Not the Rockies or the Drakensbergs)

[olduser1]

Cruise control works best on flat USA & Continental roads , if you want to max MPG then use your right foot.

Over the years I've found cruise control can provide a more relaxed drive down to Menton.

[eljuero]

If you read about hypermiling and the  actions of the owners of Toyota hybrids, the key way to save fuel is:

1. accelerate promptly to a given speed and then coast backing off the throttle as far as possible
2. don't accelerate UP hills
3. anticipate hold ups in traffic by slowing down and not braking unless essential.


Cruise control fails on all three issues..

I had it a couple of cars: ideal in the US and South Africa on roads hundreds of miles long with little traffic  - and gentle hills.  (Not the Rockies or the Drakensbergs)

I think that all of listed issues don't make cruise control less efficient.
It just must be used properly.
Working with right foot to get the best mpg can be very hard on long distance :-P

[caseyjones]

It is beyond question that using a cruise control is less fuel-efficient than driving 'manually'. This is because the cruise control forces the car to maintain its speed up hills, regardless of how much extra throttle opening this requires, and fails to take advantage of downhill sections by allowing the car to speed up. As any engineer will tell you, the most economical way to run an internal combustion engine is with a constant throttle opening, though the wide fluctuations in speed that this would cause would be rather impractical in reality. Cruise controls are useful in that they allow you to rest your right leg, and they let you maintain a steady, known speed without constantly having to take your eyes off the road to look at the speedo, but you do pay a small price in terms of fuel economy.

[guest806]

conversely, they must close to reduce the throttling when going down though ?

[caseyjones]

Yes they do, but whilst this saves a bit of petrol in the short term, continual changes in throttle opening are not the most economical policy in the long term. As I said above, the most economical way to run an internal combustion engine (any engine, probably) is with a constant throttle setting, allowing the car to speed up on downhills and slow down on uphills, though this is not always very practical on real roads with other traffic.

Years ago I used to have a little Fiat 126, that actually had a hand throttle, and I occasionally used to experiment with it on motorways (and annoy other road users, probably). I also remember a device called a "Holdspeed" I think, that had some of the features of a modern cruise control (like auto switch off on braking) but held a constant throttle opening rather than a constant speed.

[dg]

not sure how full throttle correlates with economy, maybe with efficiency of converting fuel into power, which is not the same. full throttle at 100mph won't save you any money

as for the hills, for me:
drop speed approaching steep downhill so you won't brake, no throttle, no fuel burnt
uphill close to pick torque rpm

I wish I could have cruise control, sitting position kills my leg in 3 hours.

it should be easy to (dis)engage cruise control quickly, so I'm not sure why you guys are worried about hills.

EDIT:
if I understand this correctly: http://en.wikipedia.org/wiki/Brake_specific_fuel_consumption
most effective power extraction is within effective torque range at around 80% throttle, but we don't have that kind of map for jazz
if it makes car accelerate uphill, then it's probably wrong approaching speed

EDIT2:
so caseyjones is right as long as we can ignore air resistance and speed limits

[caseyjones]

Not sure I managed to follow all of that, but a couple of points come out...

No-one ever mentioned the use of 'full' throttle, or suggested that it correlates to good fuel economy. I said constant throttle opening.

Yes, one could disengage the cruise control on uphill sections to prevent it forcing the car to maintain its speed, and thereby avoid excessive use of fuel, but if you're going to do that all the time it rather defeats the object of a cruise control.

Naturally, once you start thinking about the real world, with considerations like wind resistance, speed limits, bends, having to fit in with other traffic, different driving styles etc., the subject gets very complicated and subjective, but the original question, as I read it, was whether it is generally more fuel efficient, assuming conditions allow, to maintain a set speed regardless of the topograpy, or to allow the car's speed to fluctuate with the hills. This is essentially a simple question, and is the question I answered.

[dg]

right, sorry, I misread your post
not sure I understand why constant throttle position is better for fuel economy though

[guest3250]

Pretty much covered above - Cruise control is not, in my opinion, as Fuel efficient
but does rest the leg/foot.
I do a mixture on a long journey - using Cruise control on the flats.
Coast downhill as much as poss - anticipate uphills and try to pitch the speed/revs at a
good point in the torque curve (tricky with max torque at 4800RPM!!) so just over 3000rpm is pretty good.
Don't force the issue up hills - let it slow a little if it wants to.
Driving like that I did once achieve 63MPG with my 1.4 EX - but I kept it mostly below 70MPH