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!

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.

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 - https://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..

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.

https://geekguyandy.com/p&g.jpg

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)

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.

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.

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.

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.

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.

Swng, Even pulse and glide in gear will help your mileage a good amount I think. Watching the scangauge in neutral I can see 300 or 400 mpg, and in gear it would be less, but still much higher than 30, so it will help.

I will definitely give it a try:)!

swng - do you have a ScanGauge? I did a quick search of your posts, and nothing came up.

PS - Snax, sorry for the hi-jack. I'll split off the auto/p&G discussion into a new thread if you like.

Sorry I don't have that in my Yaris. However, I do have something similar (OEM trip computer) in my MB for some instant feedback. I may as well try some P & G with the MB. It is an auto car too.
Just do not want to do too many experiments with it:).

Can I reduce my idle RPM's by constricting the throttle body plate bypass hole?

One of the finite number of nice folk on saturnfans suggested something like that when I wanted to keep my saturn from racing up after restart. It should help by limiting how much the computer can rev it up (which is a lot sometimes).

I just unplugged the lead to the idle control motor for now (when it was at about 900 RPM). Naughty computer, no idle control for you! :)

No worries. I only started the thread to give Mazda5 owners a place to start.

I'd wager not. :(

In my car, using N and idling while gliding, I once measured an 8% FE increase over steady cruise at an average speed of about 80 km/h (70-90 pulse). When you factor in the less efficient pulse of an auto tranny (my car's got a manual) and on top of that the added deceleration from gliding in gear, I think you'd be hard pressed to come out in positive territory.

That's why I asked about FE instrumentation - I think it's going to cut it close enough, it's the only way to know for sure.

EDIT: of course, this all depends on the transmission. Perhaps it freewheels nicely once the torque converter locks up, or something like that, and I'll have to eat my words.

One of my complaints about P&G is that the idle "drifts" down after I pop out of gear, and settles around 1200 RPM while moving. At a stop, it drifts down to 750 RPM. I want it to go to 750 right away (as long as that won't hurt anything). With the RPM's drifting down, the Glide part of the P&G may not be as efficient as leaving it in gear and letting the ECU cut off fuel injection.

Can you tell me where the idle control motor is and what it looks like? I want to do what you did (as long as the engine doesn't idle rough when cold). I feel so sorry for the poor, confused computer though. It must be a rough life dealing with people like us!

The engine WILL idle rough when first started, sorry, thats the price you pay. I left the engine at about 1000rpm to compensate (I don't spend hardly any time idling anyway).

On my saturn (97 SW2), the plug is between the throttle body and the firewall. It is the bottom of the two plugs (top one is probably tps).

But, yah I thought that 1200 rpm thing while coasting was strange too, and don't count on it cutting the fuel off completely in overrun either. The only way to get it to stop wasting gas is to kill it completely (10 amp microswitch in-line with the FI fuse does a good job).

I also have an AT and am currently testing P&G in neutral, I previously glided in gear before and didn't have much of a increase to even notice. However I DON'T have a scanguage which I am sure would help in this experiment. I will post back my results if anyone is interested.

Yes, please post your results when you have them. Have you noticed any really useful drop in rpm when you glide in gear? I have not:o. Does your car jerk when you shift from N to D as you finish gliding and wish to start pulsing again. Actually, I personally think I should rather shift to the "3" position for gliding in the hope to make the car's computer shut off fuel supply to the engine.
Also, I wish to ask if I should accelerate to the desired speed fast or slowly at the "P" stage.
Any advice will be appreciated.

I have tried some P & G. I glided in gear and the lowest rpm I could go was about 1,100 rpm. The cars were slow at that rpm. I will try more.

I disconnected the throttle controller. It's nice because it idles at 650 RPM. We'll see how this tank turns out.

You bring up a good observation to comment on! There are AT's that WILL jerk when put back into gear (my Celebrity) and my opinion is that those that feel that with thier own cars will not see a noticible gain from p&g. I will say that there could very well have been other factors that killed the effort to get the gains of p&g, but still, it just didn't work with that car.

Now my 96' Monte is a different story, the tranny freewheel's really good while coasting in gear......while gliding I see 110mpg at 50mph(in gear) and 150mpg while gliding in neutral at 50mph. So I have picked up a conservative 2mpg using p&g with my auto tranny monte. Rpms are sometimes as low as 890.....plus its smooth as silk when going back into gear at any speed

I also have not figured out if its more efficient to pulse hard throttle to speed or to do it gently?

Dont think downshifting is a good idea. Just shift in neutral. Personally I think one of the BIGGEST BIGGEST advantages in P&G is the fact that you have eliminated engine braking for the duration of the glide. Leaving car in gear increases engine drag. The lower gear makes that loss even worse

WHY P&G beats steady speeds even for AT: When you drive in gear you are being affected by engine braking. For those that have ever driven manual, this is best experienced by trying to drive 50mph in 2nd gear. For an auto try doing 40mph+ in the '2' even 'L' position. The car CAN ACTUALLY attain those speeds but you have to 'force' the car to reach said speeds by revving the engine. As soon as you let the pedal go the car will experience a negative forward force and slow down. The faster you go the stronger this force is. Once you reach a certain rpm, tranny will upgrade to nexct higher gear tha is more suited for your speed. Problem is that tranny only has 4 or 5 speeds. So once you pass highway speed you are experiencing high engine braking after 55 mph, less at slower speeds when your car is in correct gear. So by GLIDING, you are not experiencing this braking which is eating your FE. Engine braking really kills you on hills. When I am at top of a hill doing 50 I might hit 70 by the time I coast to bottom of hill. If I coast in gear I might not even hit 60 by the time I reach bottom of hill. P&G also works better at higher speeds. P&G from 80/60 will be better than straight 70. P&G from 60/40 will also be better than straight 50, but the % increase will not be as high in the percent increase you experience at the higher speeds. This is because with high speed P&G there is more engine drag eliminated.
Personally I only P&G at high speeds and on hills where you MUST coast in downhills as oppossed to losing FE from engine drag

Your chart makes sense, because you're staying below a certain speed on average (that's why your trip time is longer), but I wonder how your P&G 50-44 compares to a steady speed of 47 or 45. Comparing to a speed of 50 it will always be lower MPG.

GeekGuyAndy, great chart! I'm jumping in late but want to add my .02 anyway.

swng, I'm glad you'll be testing P&G in your auto car. I've been doing P&G with my auto tranny, putting it in neutral for the glides. I can't use ScanGauge on my old beast but I'm pleased with the results from my version of P&G.

My 60 mile route is essentially a series of hills on a 65 mph limit road. Steeper than the usual interstate grades, so there is a definite climb and a possible good downhill coast.

True that an auto tranny will allow some slippage. What I do for that is, I don't accelerate hard at all. On the upgrades I peg my tach at about 2100, which is just enough in this car to climb the grade at 48-55 mph, depending on the particular grade. This is a "steady throttle" or "driving with load" technique but I only use it on the upgrades. Once I clear the peak I goose it up to 60-70 depending on conditions, but again I stay pretty light on the accelerator. Since it's a downhill it doesn't need much to get going. As soon as I hit the target speed I get off the gas and put it in N to coast. If my car had less aero drag, I could just stay off the gas and I'd by flying pretty soon just from gravity. But this boxy beast needs a boost to get going, even on a downhill.

This way I never (rarely, anyway!) step hard into the accelerator. I think wider throttle is where the auto tranny has most of its losses, so that's how I try to minimize those losses.

I agree....downshifting to the "3" position is a bad idea. Most auto transmissions are electronically controlled these days....WITH a manual override. That's usually pretty basic, especially if you have a floor mounted shifter, and RWD. (I believe this principle applies to all auto transmissions, however). The shift lever will usually allow you to over-ride which gear the computer thinks you should be in. However, this often disables/cripples the "freewheeling capability of the tranny. This leads to more forced engine braking, less gliding, and less FE.

Like I said above, I'm not sure about all transmissions, but I know this is how it works in my Jeep, as well as the wife's Blazer, and several other SUVs/PUs I've driven. I just don't get into cars that often =P.


*edit*
Also VERY interested to know y'alls thoughts.....Pulse at WOT, 80&#37;, 50%, 25%, or <25% throttle??

GeekGuyAndy's chart and explanation look really good to me. I'll attempt another explanation of why P&G works.

The engine is more efficient at producing motion from fuel at wider throttle openings. Again, it's mostly the reasons given in the other explanations - the "overhead" costs in spinning the engine and all the other mechanicals like alternator and water pump that spin along with it. Another reason is that in closed throttle it's trying to suck in fuel-air mixture but it is fighting against the closed throttle. I suspect that if you eliminate fuel + spark and just spin the engine, it spins more easily with an open throttle than closed.

OK, so it's more efficient with throttle more open. But you can't drive wide open all the time for various reasons. Mainly becuase you want to keep speeds down because what you lose to air resistance (aero drag) goes up hugely with increased speed. Not to mention the state troopers.

So you alternate a more open throttle with coasting. P&G.

Another good reason to avoid WOT (wide open throttle) is that many engine computers will enrich the mixture at WOT. So 3/4 or 7/8 throttle is probably a good max, as long as it's a standard tranny without the losses of an auto tranny.

I was wondering about this. The BSFC charts usually show the peak efficiency to be at about 80-90% of peak torque. BUT, I don't know how that translates to throttle position. I would not be surprised if 50% throttle gave 80% torque. I remember that the SG2 has a LOD parameter for engine load, but I don't know what exactly that means either.

Whatever you do, you want to stay away from open loop or some kind of WOT enrichment.

Thank you gentlemen for the input above.
I once thought that shifting to "3" when gliding may induce DFCO but now it seems that the consensus is gliding in "N" should save most fuel.
With your input, I will have lots of materials to try to understand and lots of experiments to carry out:). In other words, there will be enough to keep me busy in the coming weeks/tanks. There are legal and other limits as to how much I can do though.

p&g works awsome in my auto i wouldent break 30 before i startd and now my best p&g tank was in the 38 range

I haven't notice a significant drop in RPM's but enough to make it worth wild. If my RPM's are at 2K constantly when I put the car in N they usually go down to 1100 or 1200. It would be nice if they went do to 800 or so like they do when in drive and stopped. My car does NOT jerk when I put it back in drive to resume pulsing again, however since most of my commute is on city streets most of the gliding is just up to the next light which I resume pulsing after stopping.

Actually as of yet I have not seen an increase at ALL in MPG while P&G however I still have a 1/4 of a tank left. Although that could be to a few other issues and will continue to test as well as EOC ( Engine off Coasting ).

Thank you Zvolen.
Safe and happy motoring!

Throttle==Air==Torque, unless the car is drive-by-wire. Then the manufacturer has purposefully disconnected the throttle from the pedal in order to have lots of throttle tip in, so the engine will fo' sho' operate in those nicer BSFC regions when accelerating, and the EPA city mileage will increase. In effect, they figure that they have to automate acceleration to some degree for the consumer to accelerate efficiently.

Sorry, I wasn't clear. Is the air flow through the throttle definitely linear with respect to throttle position sensor reading?

Heh, yeah, well, probably not. You may be able to infer that from the fuel maps or some crazy fluid dynamics. The easiest thing would be to go on a dyno, and see if you can have the TPS signal and torque logged simultaneously.

Where I was going with the idea was that I might be at 80&#37; of available torque when the throttle is only (to my foot) only at 60%. And that pressing the gas to the point that feels like 80% might over shoot the max efficiency point. It would be nice to prove it with a dyno, for sure. Okay, I think I'm done thinking out loud now...

Hockey4mnhs,
Thats a conservative 26&#37; increase!! Really!? Do you have a four speed auto, or an auto with overdrive?!