Cruise Control Less Efficient?

[gge5]

I've noticed that my MPG goes down with the use of cruise control. With the car already warmed up and on a flat highway, going at the speed limit of 55 I get around 48MPG with the cruise control activated. If I switch to manual control the display shoots up to around 63MPG. And no, this is not the instantaneous readout, but a value that holds steady over more than 3 minutes and still going 55. No change in elevation, speed, or direction. I'm not trying to pulse-and-glide (which I don't think is possible at this speed anyway). If my speed falls and I have to accelerate, it still only goes down to a minimum of 51MPG -- which is better than cruise control is at simply maintaining speed!!

I'm not quite sure why, but it seems like the yellow arrows to the battery tend to switch back and forth between charging/discharging more when I have manual control, and it says more on charging when I'm in cruise control. So does cruise control not take advantage of the battery or what?



In short:
Cruise Control - Steady at around 48MPG
Manual Control - Minimum of 51MPG while accelerating slightly
Manual Control - Average of 63MPG maintained

[Tideland Prius]

<div class='quotetop'>QUOTE(Alan D @ Jan 24 2007, 11:47 PM) [snapback]380513[/snapback]</div>

I've noticed that my MPG goes down with the use of cruise control. With the car already warmed up and on a flat highway, going at the speed limit of 55 I get around 48MPG with the cruise control activated. If I switch to manual control the display shoots up to around 63MPG. And no, this is not the instantaneous readout, but a value that holds steady over more than 3 minutes and still going 55. No change in elevation, speed, or direction. I'm not trying to pulse-and-glide (which I don't think is possible at this speed anyway). If my speed falls and I have to accelerate, it still only goes down to a minimum of 51MPG -- which is better than cruise control is at simply maintaining speed!!

I'm not quite sure why, but it seems like the yellow arrows to the battery tend to switch back and forth between charging/discharging more when I have manual control, and it says more on charging when I'm in cruise control. So does cruise control not take advantage of the battery or what?
In short:
Cruise Control - Steady at around 48MPG
Manual Control - Minimum of 51MPG while accelerating slightly
Manual Control - Average of 63MPG maintained
[/b]

Depends on the terrain (or rather, the incline of the road you're on).

Hell, I've had CC do EV mode for 4kms at 80km/h before (that was when I had 8 bars :P ).

Ok, beside that anomaly, I've had CC do hyper-EV whenever it deems necessary.

Also, like you, I've been able to maintain the same mileage with CC (i.e. slow lift the right foot until you just start losing speed, then press ever so slightly to maintain speed and that's the best mpg you can do and maintain your speed). Usually, I just let CC do its job as it's fairly smart, as least in my car in my local topo.

[JimboK]

Hobbit (another PC member) has researched optimal driving methods extensively, and among his many conclusions is that cruise control is way too aggressive for maximum fuel economy. Here is one of his papers, with links to others:

http://www.techno-fandom.org/~hobbit/cars/sweet/

It's pretty technical, and he has a lot of added instrumentation in his car, but the take-home lesson for the average Prius driver seems clear enough: During mid-range or high speed cruising or acceleration maintain instantaneous FE between 35 and 75 MPG as much as possible. Below that range, you're obviously accelerating more aggressively and pushing the ICE to operate less efficiently. Above that range is also inefficient as the engine "loafs." In the latter case, you might as well apply a technique instead called "warp stealth" -- a method of gliding with no fuel consumption and minimal electricity use. (One of the paper's links describes this technique.) The problem with CC is that in its effort to maintain constant speed it routinely (and unnecessarily, in my view) pushes the ICE well beyond this range in both directions.

I used CC routinely when I first bought the car; now it's rare. I've added a ScanGauge to my car to, among other things, monitor a variety of additional engine performance parameters. The most valuable one to me is RPM. (It's a shame the car doesn't have a tach!) Applying Hobbit's findings, I use the RPM to maintain the engine efficiency sweet spot.

[efusco]

The OP specifically said this was on flat terrain. I don't know how/why you'd see such a huge discrepancy in FE w/ or w/o CC on flat roads at hwy speed... In fact it's the only time that I expect FE to be better w/CC.

I suspect it's not somthing that he would find to hold true over longer periods of time. IOW...same road, same speed held as closely as possible over a period of 30 minutes. More than likely the difference in MPG wouldn't be measurable via the regular display, but if it were it would be lower and/or more variable for each 5 minute bar than w/ CC.

In my Prius when I use CC on relatively flat interstate i often see 30 minutes of identical 5 minute bars.

[dmckinstry]

<div class='quotetop'>QUOTE(efusco @ Jan 25 2007, 05:12 AM) [snapback]380537[/snapback]</div>

The OP specifically said this was on flat terrain. I don't know how/why you'd see such a huge discrepancy in FE w/ or w/o CC on flat roads at hwy speed... In fact it's the only time that I expect FE to be better w/CC.

I suspect it's not somthing that he would find to hold true over longer periods of time. IOW...same road, same speed held as closely as possible over a period of 30 minutes. More than likely the difference in MPG wouldn't be measurable via the regular display, but if it were it would be lower and/or more variable for each 5 minute bar than w/ CC.

In my Prius when I use CC on relatively flat interstate i often see 30 minutes of identical 5 minute bars.
[/b]

Hmm. Missouri. I see why you could to 30 min. without any variation. To drive across town, or anywhere around here, I can't go more than a few miles without a change in elevation of at least 100 ft. I have that much in getting from home to a little more than a mile away where I work.

Dave M.

[Warwind]

Well now I have to try this. I use my CC set to the speed limit every time I get on the interstate. During my daily 30-mile round trip to work, half the trip is interstate. The other half is a 45 mph road where I never use CC (and get much better FE).

I'm getting 45 mpg using CC (down from 51 when it was warmer out). I'll post whether or not it makes any difference.

[efusco]

<div class='quotetop'>QUOTE(dmckinstry @ Jan 25 2007, 08:22 AM) [snapback]380554[/snapback]</div>

Hmm. Missouri. I see why you could to 30 min. without any variation. To drive across town, or anywhere around here, I can't go more than a few miles without a change in elevation of at least 100 ft. I have that much in getting from home to a little more than a mile away where I work.

Dave M. [/b]

Ever heard of the Ozark Mountains? It's nowhere near flat around here...but I said relatively as the interstates are, for certain segments, 'relatively' flat meaning there are fairly consistant rolls as you go so the ups tend to balance the downs enough that every 5 minute bar tends to be about the same.



Going across Kansas & Eastern Colorado in our HiHy last week gave some nearly 1 hour time periods where the line across the consumption screen was absolutely level...

[dmckinstry]

<div class='quotetop'>QUOTE(efusco @ Jan 25 2007, 07:08 AM) [snapback]380567[/snapback]</div>

Going across Kansas & Eastern Colorado in our HiHy last week gave some nearly 1 hour time periods where the line across the consumption screen was absolutely level...
[/b]

Elevation changes are still fairly large on I-90 around here. I know there's a significant drop in elevation going from Cheney to Moses Lake (90 mile west of here). I get much better mileage going in that direction than coming back. It's not uncommon to see mpg vary from 99.9 down to 20 along that stretch just going up and down relativetly "mild" hills. I wish the Nav system had elevation (or angle of incline) included though.

I haven't driven through MO since I got my Prius. And most of that was just between Kansas City and Columbia.

It still would be interesting to see what affect lowering speed in cruise control on the up hill would have on mileage. With so many other variables, it would probably difficult to get meaningful results. Everytime I travel any distance along I-90, weather conditions are different.

Dave M.

[fbacher]

<div class='quotetop'>QUOTE(Tideland Prius @ Jan 25 2007, 01:52 AM) [snapback]380515[/snapback]</div>

Depends on the terrain (or rather, the incline of the road you're on).

Hell, I've had CC do EV mode for 4kms at 80km/h before (that was when I had 8 bars :P ).

Ok, beside that anomaly, I've had CC do hyper-EV whenever it deems necessary.

Also, like you, I've been able to maintain the same mileage with CC (i.e. slow lift the right foot until you just start losing speed, then press ever so slightly to maintain speed and that's the best mpg you can do and maintain your speed). Usually, I just let CC do its job as it's fairly smart, as least in my car in my local topo.
[/b]

Oh to dream of a day when there is a "fuel economy agressiveness control" where you could change your vehicle's behavior to be more fuel efficent, or more responsive. It needs to be more than a two position switch...

[Charles Suitt]

<_< I must agree that the Cruise Control programming is overly-AGRESSIVE and that, IMO, leads to a loss of fuel efficiency, particularly in rolling terrain.

If you take a highly technical approach, a trip in rolling terrain with, say, EQUAL up and down hills will not 'cancel out' due to the efficiency loss in recharging the traction battery when coasting. For over two years [2+ years in my 2004, 2 months in my 2007], I have tried both WITH C/C and WITHOUT C/C and cannot notice enough difference to be significant.

My conclusion: I drive my Prius in a way most comfortable to me in given circumstances.