The myth of pulse and glide

Bob,

It would take mountains of data gathering and multiple iterations of multi-variable regression analysis to come close to a reasonable approximation of a mathematical model showing the effects of P&G on fuel economy.

While I agree with you that on this forum we should not have knee-jerk reactions such "throw the bum out" when a difficult question is posed, I also think we should provide real comments to difficult questions. The real comments come from empricial knowledge, and that knowledge is gained through real-life experience.

Perhaps everything can be described by mathematical formulas, but the mathematical "descriptions" are not necesarily the proper (or even best) ways to describe not only what is happening but whey something is happening.

 

Come on, doesn't anyone else see a catepillar with boobs here? What? There isn't a Freudian in the room?

 

My limited P & G experience

I have found that P & G does marginally better than Cruise Control, but just driving smart (monitoring the traffic and topography, coast when traffic is slowing down, coast down a big hill) does better than rigorous P & G. Of course, it could be me.

In the city, my approach to P & G is again based on traffic, lights, and hills. Pulse going up hills, glide down, and glide approaching a stale green light (or red). In this, I get in the low 60's in the city. The one thing I have not figured out is am I better off accelerating hard for less time, or lighter for longer.

I suspect if I was driving at low speed (<40) without traffic, I could hit the big numbers, but in the suburbs of Washington DC, that never happens.

 

Re: My limited P & G experience

I'd make small adjustments/articulations to your factoids here:

First, the "lighter for longer" works best, because accelerating hard takes away from battery surplus ... simply to help you get up to speed quick. The "lighter for longer" lets you increase traction battery surplus ... especially important when doing P&G during dark driving/head lights runnnig, which draws off power / drops battery surplus. You can't P&G for long, if you don't let the traction pack take advantage of wasted ICE energy. "lighter for longer" keeps the traction pack charge higher, which in turn keeps the ICE from staying on.

Lastly, "coast downhill" needs to actually be done as a "GLIDE downhill". If you're on a long down hill run, eventually the ICE kicks on simply to burn off excess traction pack charge. It's ok to recapture SOME regen on long down hills, but not too much.



There are a bunch of us Gen II'ers already doing 70mpg-80mpg via P&G. When the Gen II first came out, it took about 6 months before PC'ers really began showing posts similar to mine above. I'm expecting the same from all you seruiys Gen III'ers as well. You folks'll figure it out.

To the myth / nay-sayers of P&G, you need to explain the bumble bee. Good luck with that!


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Re: My limited P & G experience

I'd say it likely is you, but if you are pleased with it and don't want to work the "rigorous P & G," then don't worry about anyone else's word on it. As a side note, I'd agree with hill that P&G is going to beat out "just driving smart" -- though I'm closer to the latter as opposed to full-out P&G.

On the downhill/uphill world: hill -- I could never really argue with you based on your MPGs, but I've felt it is better to do a bit of accelerating on downhills and gliding in uphills. I'll change if someone is on my tail, of course, but I prefer to let gravity take its course. Gliding on the downhill is great if I am gaining speed so as to make the next climb easier, but if the glide is not giving sufficient gain in speed, I'll look to accelerate at very low ICE cost. (These hills are ones we find in "rolling countryside" -- not mountains!)

 

Would you please make the graph up to 2000 glide mpg?

Ken@Japan

 

I think there is a better approach when someone posts something we disagree with.

Another set of real-life experiences are called 'experiments' and to the maximum extent possible, we need to control all variables and document the test environment. The real proof is finding the results are reproducible. Although SAE 2009-01-1322 gets higher values than my field experiments, the differences in SOC documented in SAE 2009-01-1322 could easily explain the difference.

I'm a fan of models backed up by experimental data and the original poster needs to do some field work to support his model. Then everyone gains but to propose a model w/o data, well that leaves a big hole.

Bob Wilson

 

I am loathe to argue with you Bob, because I respect and enjoy your posts here immensely. But how much empiric data is needed to agree that running the ICE in a more efficient band is going to lead to higher MPG in all but very unusual cases ?

Now, if I wanted to be contrarian I could with effort probably figure out how to pulse the engine badly, or run down the traction battery so low that the ICE turned on and added conversion losses. And if the efficient pulse band was extremely narrow that would merit discussion. But none of this is true, so I also find this thread uninformative. People *should* understand that P&G at highway speeds is not much gain, since the ICE is operating pretty efficiently already.

FWIW, my own little contribution regarding P&G: it is not going to overcome poor driving habits.

 

Re: My limited P & G experience

I did an experiment yesterday....driving from Ocean City, MD to Northern Virginia....First 60 miles, strict P & G. I would go from Speedlimit +10 to Speedlimit -5, typically 45-60 range. I accellerated as fast as I could while keeping the 'ECO' light on. In that mode, I was getting 64 mpg, according to the trip computer. Temp was 82, and I was driving into a 5-10 mph head wind. Note that my pulses approached my glide length.

After about 1 hr of this, my wife said, "What the h*** are you doing, I want to get home" -- it was about 9:00 PM, and we were near Denton, MD on MD 404. For the next hour, I had cruise set at 55. I was pulling in 56 MPG, or about 14% better with P & G.

After that, I just wanted to get home, and drove home the last 1 1/2 hr between 65 and 80 MPH, and got 52 MPH, according to the computer.



A next experiment would be to look at how fast the the speed bleeds off. That, plus modeling the efficiency of the ICE in different pulse combinations should allow for an accurate model to determine the optimal pulse and glide. I think the pulse combinations would be: Floor it (less time, more gas per second), accelleration at the peak with eco on, accelleration 1/2 the way up the meter.

This is what happens when a scientist gets a prius.

 

Re: My limited P & G experience

A pure P & G experiment should be done below 46 mph barrier speed.
http://priuschat.com/forums/gen-iii-2010-prius-main-forum/62635-46-mph-new-threshold-stealth.html

Ken@Japan

 

check my post #28 on this thread

http://priuschat.com/forums/gen-iii-2010-prius-fuel-economy/65353-determining-glide-empg-2.html

as bob alluded to, P&G is a guideline. its a technique...not a quantifiable thing, its a process with a result that GREATLY varies according to conditions.

 

Re: My limited P & G experience

That is an excellent area for such tests due to the flatness of that area. Just a couple of quick follow-up questions:

• Ah, Route 13/113 - If I remember correctly that stretch has a 55 mph posted speed limit except for the small towns.
• So it was 50 mph along that stretch so you were running between 60 mph and 45 mph plus the lower speed limits of the towns you passed through?
• Do you have the exact distance or entrance and exits of the run and elapsed time of the total, P&G run? This allows calculation of the equivalent, constant speed. Google Maps shows a route via Roxana DE to Denton MD of 48.4 miles and a predicted time of 1 hour 21 minutes suggesting an average speed of less than 55 mph accounting for the small towns in between.
• Could you provide the local towns that bound the P&G versus cruise control sessions?
• How did you handle the "glide" part, "N" or some setting of the power bar on the ECO display?
• Do you possibly have a Garmin nuvi that might still have the trip data? If so, the data track would make it much easier to replicate your protocol.
• You might want to double-check the weather station reported values at Cambridge and other points along the way. Cambridge reported winds of 5 mph during the 80F part of July 19 from the west. Easton reported even less.

We've got a similar, divided highway west of Decatur AL, Route 72 / 20, over to Muscle Shoals, just south of the Tennessee River and another, shorter stretch by the Flint River between Huntsville and Scottsboro. These segments aren't quite as long but the traffic is predictable, warm weather is returning, and I can run 10-15 mile, bi-directional tests to minimize wind effects.

I find it interesting that on cruise control at 55 mph you were under 60 MPG. Were you running AC or open windows? Was there any unusual traffic or speed changes?

BTW, just looking at my initial set of 2010 data, steady speed, cruise control:

Your reported "64 MPG" seems to correspond very closely with my steady speed of 55 mph. To get in the "52 MPG" range, I have to drive at 70 mph and/or deal with large hills, like the Bay Bridge and hills along Route 50 by Annapolis or a lot of stop-and-go traffic.

It will be interesting to see how to get "52 MPG at 55 mph on cruise control." Running parallel to the Piedmont, we have a few roads with many short, steep hills. Your 52 MPG does seem close to the 52 MPG I got on Monday driving from Huntsville to Ardmore, which includes not only short steep hills but an altitude increase. We were using AC in the 88-90F weather.

Bob Wilson

 

P & G is a technique....it has to be quantifiable. There are reasons why it works, for example, shutting down the ICE. If we can understand (reverse engineer) the variables, we can model the expected performance.

(The length of the pulse + the length of the glide)/fuel consumed=MPG


To the first order, the fuel consumed in the glide approaches 0 (compared to that consumed in the pulse). That assumption is reasonable because in a glide, even if the ICE is running the consumption is > 100 MPG, but in the pulse, it will be less than 40 MPG. So, the MPG is approximately

(Length of the pulse + length of the glide)/fuel used in the pulse.

Now let PL=Pulse Length (in range)
F=Fuel used in the pulse


F will depend on the PL, as faster acceleration will reduce PL, and increase the rate of F.

At one extreme, we can minimize the rate of fuel consumption in the pulse by decreasing the rate of acceleration (the end case is 100% constant speed pulse, or 'cruise control'). Empirical evidence proves that this is not optimal.

At the other extreme, We pulse for the shortest amount of time, slamming the foot on the gas, then have the longest glide possible. My experience is that drops the MPG to around 10. Using motor trends 1/4th mile of 17.9 seconds (77.7MPH), we would consume 1/40th of a gallon (assuming 10 mpg). How long of a glide from 77 MPH? I am guessing in flat terrain, we are looking at at least 2 miles, probably more, possibly less (need a measurement!).

That would mean the extreme pulse went 2 1/4th mile in 1/40 of a gallon, or 90 MPG. If the glide was 1 mile (I can not imagine it would be less), it would be 50 MPH, and if it were 4 miles, it would be 170 MPG.

Now, all of my assumptions (lead foot MPG, glide length) can be measured.

Anyway just some thoughts.

 

ok...not gonna dispute this since you said the same thing i said. anything that can be measured is quantifiable.

but there is no set parameters for P&G. depending on conditions, the parameters change but the process does not. the ratio of P&G is not a set thing or even a parameter.

now if i say "i P&G on a 1 to 3 basis, i will get 150 mpg" that is a goal to shoot for but may or not be possible depending on conditions. in reality, there is no upper limit for P&G efficiency and that is why records continue to be broken.

 

That is true...For example, going down wind in a hurricane will probably give great MPG. I am still experimenting with my car to try to figure out how this works. I might just try a 1/4 mile, then coast to a stop, just to see how I do.

 

There are two, and only two, possible advantages of P&G over cruise control:

1. More efficient engine use
2. Periods of time during glide when the engine is not spinning

If the car is a G3 and travelling faster than 46 mph, or is a G2 and travelling faster than 42 mph, (2) is negated

Speeds over ~ 50 mph put the ICE in a very efficient band using cruise control.

 

Can you confirm how this relates to the Hybrid System Indicator (HSI)? I'm trying to understand how each of these zones maps to the different[FONT=&quot] [/FONT]parts of the HSI.

• Does the 1600-2700 rpm zone map from the middle of the HSI up to the left of the PWR zone?
• When does 1200-1600 rpm happen (knowing the ICE will run in that part of the HSI when you go at higher speed)? Is it mapped from the middle of the HSI to its left? Does the car by itself avoid this inefficient[FONT=&quot] [/FONT]zone as much as it can?
• The PWR zone of the HSI would start at 2700 and end at? (knowing you can push beyond the top of the PWR zone). Why Toyota chose that particular rpm to end the visual PWR zone?

I think this would help taking this great technical knowledge and ground it to a useful form for the non technically savvy.

 

Assuming you're using "cruise control" to mean "steady speed", because once you remove the human from the equation -- either for P&G or something else -- you lose a good chunk of efficiency.

I agree with the idea that at 50 or more, the engine can be run in highly efficient ways. I had a thread here a while back about how, at 50+ you can see a full range of MPG's indicated on the instantaneous display, while at lower speeds you never see anything in the 55-95 range, only 50 and below, or 100.

I'm not very skilled at repetitive P&G, but I've found that a single pulse and glide has allowed me to get very good mileage in the kinds of traffic backups I encounter every day during my commute. Totally dependent on human judgement to make it work.

 

While I can't really answer your questions to Bob, I can add a few comments --

These figures are for the older Prius, the 2010 model will be somewhat different.

On the 2010, the boundary between the HSI display ECO and PWR zones varies with speed, and possibly with other factors. I've seen the boundary at 1660 rpm at street speeds, ~2400 rpm at 60 mph, and slightly above 3200 rpm at 'higher' speed.