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Old 07-22-2007, 12:39 PM   #1
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More questions about P&G w/ an auto tranny

Before I heard about P & G, I have been accustomed to the belief that maintaining a steady speed on a highway was the best way to save fuel. Do the two theories contradict each other? Why can P & G benefit fuel economy (for non-hybrid cars)?
Please enlighten me! Many thanks in anticipation!
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Old 07-22-2007, 12:48 PM   #2
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The primary benefit comes down to rpm/mile. While you may trade off some increased consumption to accelerate to speed, the glide portion consumes less fuel overall because it reduces the number of rpm/mile and engine drag loss vs. holding a steady cruise rpm for the same distance. This is why it doesn't really work if you are cruising at or near an idle speed rpm, as the motor will continue to rotate at the same speed whether you are gliding or cruising and wasting the same energy just to overcome that drag.
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Old 07-22-2007, 02:59 PM   #3
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Thank you Snax. Your explanations make very good sense. I asked about why P & G was good for F.E. after reading some articles similar to this one - http://www.extragasmileage.com/gas-p...ng-gas-prices/, which more or less said amongst other things that one should drive steadily and that sudden changes in speed was not good for F.E. I think those articles make good sense on the whole. In a way, I am torn between two schools of thought. However, your explanations about P & G has definitely interested me. I will try some P & G so see if it further improves my F.E..
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Old 07-22-2007, 06:18 PM   #4
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The theory of P&G

I was forever wondering if P&G was better then a steady speed before I knew it was a term, and always thought it equalled out. It just seems like if the engine is pumping for some distance, then you would glide some distance, but to get anywhere it would all cancel out. What I realized is that to keep the average speed the same, it probably would cancel out. I think P&G techniques work because I can always keep the RPMs the same when pulsing but they sink way down on the glide (725 in N, 0 in EOC), and it may take longer to get somewhere, but the glides are far more efficient than the pulses.

Looking at the ScanGauge, I might get 20mpg at the instant while pulsing, but then the glide can be 300mpg, which affects the overall average very well.

I think this spreadsheat is correct (tell me if I'm wrong) and it shows that RPM / distance is much lower in P&G than steady speed, but time is longer. I picked what I thought was reasonable figures, but it's probably not quite that perfect.

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Old 07-22-2007, 07:33 PM   #5
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Well, I mean, 2083 is a ton more than 974. So even if you pulsed to 56 and glided to 44, it seems like you would still have fewer revs.

The # of revs thing is easy to get your head around, and explains part of why p&g works, but it still assumes that every rev is at the same efficiency, which isn't true. An added factor is that the engine is operating at a greater efficiency during the "p" than it is in steady state cruising. (insert BSFC chart here)
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Old 07-23-2007, 06:41 AM   #6
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I disagree with the notion that your average speed with P&G has to be any different. I regularly pulse up to at least 5 mph over the speed limit and glide to 5 mph under. That ends up with an average very close to steady cruising at the speed limit.

Yesterday I had one of my best trips ever over a distance of about 25 miles at 40.5 MPG two-way average on the freeway with about 1/10 city driving. What I am finding is that it's the 1/10th before and after highway travel that is most critical to overall economy. So by 25 MPH I already have it in 5th gear where it will pull a slow steady acceleration up to whatever top speed I am shooting for.
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Old 07-23-2007, 03:54 PM   #7
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Nice and useful chart at Post # 4. I notice that the rpm at "G" is 725, which is only achievable by gliding in neutral (N). So I really don't know if gliding in whatever gear my automatic transmission chooses qualifies as "G" for the purpose of P & G.
Frankly, I still find it uncomfortable to glide in N because I am not confident enough to switch back from neutral to D when the car is going quite fast. Therefore, all I can do is glide with the right foot lifted completely from the throttle pedal and let the automatic transmission choose the gear it wants. It is hard to really reduce the rpm significantly by doing merely this. Understandably, I benefit much less than those who glide in neutral.
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Old 07-23-2007, 04:12 PM   #8
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Hi swng... AHA. You're driving an auto. There's no guarantee that P&G in an auto will net as much of a benefit as a manual transmission. The reason it works so well in the manual is that the driver has far more control over engine loading vs RPM during the pulse, which is critical for making it work.

In most automatics, generally speaking, the harder you press the accelerator, the more the torque converter slips and the higher your RPM goes, making the pulse much less efficient.

I'm not saying it can't be done in an auto, but I suspect it's not going to make much of a difference doing engine-on N P&G in an automatic vs. holding a constant speed at the same average speed.
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Old 07-23-2007, 04:19 PM   #9
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Swng, I was once worried more about shifting to N especially at speed, but I'm not concerned quite as much now. As long as I rev up to match the rpms in gear (usually around 1800-2000) then it transitions smoothly. If I shifted to N and didn't rev to match before shifting back, the engine jumps and does a big pull to get going the speed it needs.
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Old 07-23-2007, 04:31 PM   #10
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MetroMPG,
Message understood. Anyhow, I will try some P & G with my auto Yaris (in my own way) and if my fuel economy does not improve, I will switch back to maintaining a steady speed. Many thanks!

GeekGuyAndy,
I wish I had chances to copy your advanced techniques. Still thanks for the useful message.

My wife drives the car at least half of the time. I cannot switch to a manual transmission at least for the time being. However, I still think that there is a lot to learn about fuel economy on this nice forum, even with an automatic transmission.

Gentlemen, it's a pleasure reading your good advice.
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