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Old 08-06-2008, 06:56 PM   #1
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Pulse and Glide question

in which circumstances is it more efficient to pulse and glide vs. driving at my car's optimum highway speed (like 85kph for example)?
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Old 08-07-2008, 03:31 AM   #2
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Personally, I P&G in the city with my small car. I think I get about ~5MPG out of that. I'd have to force myself to drive back in my old ways to confirm, but I used to get less than 30MPG I recall (lost my gas log spreadsheet ). Now I'm like 35MPG.

Highway:
On highway with my bigger CR-V, I just use cruise control on flat grounds, even uphills, and coast in neutral in downhills or in gear if the slope is too steep. Since I don't have DFCO with my CR-V, optimal is drving neutral on like 5% grade downhill at high speed, to reach like 210MPG while it lasts.

P&G doesn't really work on highway with the CR-V, it decelerates too fast. I could try to do it staying in drive (it's an auto).
P&G seems to work with the smaller car but still totally impractical to do on long drives.

Downhills:
Last week-end I drove down from Tahoe, CA to Placerville, and averaged almost 40MPG with the CR-V (About 60 miles), with a lot of neutral coasting, and some smart driving, not passing people when it was pointless, but aggressive when the road divided. For info, on flat HW my CR-V averages 25MPG~26MPG with my driving style.
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Old 08-07-2008, 03:34 AM   #3
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Pulse and glide, where the pulse is at about 80% throttle, and across the peak torque part of the rpm band, has your motor returning about 35% efficiency, then you glide at idle or engine off.

Cruising at "max mpg" speed has the motor in a very inefficient operating region, and you're just doing the "least worst" you can with it, efficiency may be as low as 15-20%

So done correctly P+G should be more efficient in just about any vehicle, the only exception to this will be something that is "correctly powered" or as most people would say "dangerously underpowered" meaning that you're running it at 80% throttle near the torque peak just to maintain highway speed.
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Old 08-07-2008, 05:10 AM   #4
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Originally Posted by RoadWarrior View Post
Pulse and glide, where the pulse is at about 80% throttle, and across the peak torque part of the rpm band, has your motor returning about 35% efficiency, then you glide at idle or engine off.

Cruising at "max mpg" speed has the motor in a very inefficient operating region, and you're just doing the "least worst" you can with it, efficiency may be as low as 15-20%

So done correctly P+G should be more efficient in just about any vehicle, the only exception to this will be something that is "correctly powered" or as most people would say "dangerously underpowered" meaning that you're running it at 80% throttle near the torque peak just to maintain highway speed.
I've noticed that my truck will P&G all by itself on a level road w/ the cruise set @ 60 MPH. Every time the cruise lets off the throttle for 3 seconds it will DFCO for a moment. Lower than 60 and you're probably not going fast enough to DFCO (at least in OD), faster than 60 generally does not give many opportunities to let off the throttle for more than 3 seconds, unless going downhill. I think this is probably key to that one 30 MPG tank I did in it years ago, and have never been able to duplicate. I was driving down to Richmond to visit my best friend's graduation from medical school. With about a 2 hour drive I lefft early and took my time. I probably had the cruise set on 55 or 60. Don't remember now. Rusty used to do quite well on the highway. I used to fill him up in Fredericksburg, VA and then not stop for gas again until I was in Savannah, GA. I'd then continue on to my grandparent's house (They lived 5 miles outside of Daytona). I'd drive around FL for a week, and then fillup my tank again in Georgia.

The last time I drove The Beast to FL I planned my fuel stops ahead of time to take advantage of the cheapest fuel along the route. I fueled in Caramel Chruch, VA, Florence, SC, Brunswick, GA. With normal driving it would be questionable whether or not I could make it from Caramel Church to Brunswick, but with a little help from some LRR tires, (I need tires soon) an extended airdam, and cruise set @ 60 I think I can do it. I picked up an airdam from a Chevy Tahoe @ the scrapyard the other day for $10. I'm going to flip it upside down and bolt it to my existing airdam. Should look pretty close to stock, and most people probably won't notice.

-Jay
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Old 08-07-2008, 05:41 AM   #5
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Originally Posted by RoadWarrior View Post
Cruising at "max mpg" speed has the motor in a very inefficient operating region, and you're just doing the "least worst" you can with it, efficiency may be as low as 15-20%
Hmmm...

CR-V's should have a flat torque curve from 2K to 6K with the VTec engine design. If that's the case I should be better off cruising at 2000K RPMs, right? Peak torque is at 4500 RPMs I believe. Impossible to find a CR-V dyno...

How about doing P&G staying in drive, not going to Neutral? You pay the extra drag from the engine in G. That might be acceptable for highway driving... I don't have DFCO on the CR-V.

P&G trades off the cost of accelerating vs staying at constant speed, for usage of the engine at a higher efficiency for short intense work bursts, and low consumption for the glide.
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Old 08-07-2008, 06:19 AM   #6
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Quote:
Originally Posted by Jay2TheRescue View Post
I've noticed that my truck will P&G all by itself on a level road w/ the cruise set @ 60 MPH. Every time the cruise lets off the throttle for 3 seconds it will DFCO for a moment.
Quote:
Originally Posted by sonyhome View Post
How about doing P&G staying in drive, not going to Neutral? You pay the extra drag from the engine in G. That might be acceptable for highway driving... I don't have DFCO on the CR-V.
You've both described what I call "Pulse & DFCO". It's not gliding if you're not in neutral. If your torque converter doesn't stay locked then you are actually at a hybrid of glide and DFCO.

Pulse & DFCO does not work with a manual transmission*, but with a torque converter that unlocks during highway speed engine braking I imagine it could partially work.

I'm amazed that a CR-V doesn't DFCO.

*: It's even worse than just cruising; I gained 3mpg by using cruise at 70mph instead of Pulse & DFCO. I believe that this is because you're still subject to the same amount of engine friction, same amount of drivetrain loss, same everything; and it all adds up to the same energy used to get to your destination, but may be made less efficiently. I'm not sure why it would be made less efficiently, though, since when the engine is making power it's doing so at increased BSFC efficiency.
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Old 08-07-2008, 06:24 AM   #7
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Hmmm, Hybrid glide... I like the sound of that... so its extending the "glide" by not letting the full momentum of the vehicle get eaten up by the compression of the engine... Anyway, like I said earlier, I would like to be able to reproduce my 30 MPG highway trip again.


-Jay
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Old 08-07-2008, 06:31 AM   #8
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Yeah if you've got a flatter torque curve, you can just get on the edge of it, I think around 85-90% of peak torque is where it starts to be good.
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Old 08-07-2008, 06:46 AM   #9
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I went from ~18mpg to 29mpg in town on my 1990 Accord with P&G. I accelerate at near WOT till I hit around 2200rpm, then shift. Given that my peak torque is in the 4.5krpm area, I can't see accelerating to that point as efficient, especially considering that's where I shifted before I started hypermiling, and it resulted in my awesome 18mpg.

I guess I wont know for sure till I get my carputer going, and can log fuel consumption and acceleration together and generate a rough BSFC map...
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Old 08-07-2008, 08:18 AM   #10
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Are you sure the CRV doesn't have DFCO? My 1996 civic does.

I also say that unless it's in Neutral, it's not a glide. I doubt you'll see much gain from pulse-and-DFCO. Neutral is where the gains are. If you can get the transmission to unlock, you're most of the way there without shifting to N. You might try a tiny blip of the throttle right before letting off, to encourage it to unlock. The rpm should drop to near idle level.
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