I have been thinking about this for some time and wanted to toss this out for discussion.

Quote:

Newtons 2nd law is about conservation of energy

In physics, the law of conservation of energy states that the total amount of energy in any isolated system remains constant but cannot be recreated, although it may change forms, e.g. friction turns kinetic energy into thermal energy. In thermodynamics, the first law of thermodynamics is a statement of the conservation of energy for thermodynamic systems, and is the more encompassing version of the conservation of energy. In short, the law of conservation of energy states that energy can not be created or destroyed, it can only be changed from one form to another.

This would suggest that Pulse and Glide method can't result in better eco because the energy to get back up to speed will always be more than the energy saved coasting. That is the net energy will be more that just driving steady. This is based on sound physics. There is no such thing as free energy as this would sugest.

I know this flies in the face of some that do this and claim better eco. I would go as far as saying the data from P&G must be flawed.

I need to get registered and post this on the cleanmpg form, but figured I'd toss this out here for some lively discussion.

 

You're assuming 100% efficiency, which automobiles ain't. P&G is less inefficient than steady state. My ScanGauge says so!

 

I have often wondered about this because the reasoning behind didn't make sense. The only way it would make sense is if you didn't increase your speed back to the original starting speed.

Now, what I typically do is FAS down to desired lower speed and maintain that lower speed when I pop the clutch. I typically only increase my speed if going down hill which will use less gas than on flat ground.

Also, I have demonstrated that CC in the FIT does an excellent job and it is less work for the individual.

 

Pulse and glide is mainly for hybrids, where they have regenerative braking. I don't pulse and glide. I just glide but sometimes it's better to stay in gear. Engine braking uses zero gas, while idling uses a little fuel.

 

I'm on their side.
You have misinterpreted regenerative energy recovery. Its not supposed to be, nor is it, 100% efficient. Where now 100% of all braking efforts are lost thru heat to the atmosphere, regenerative braking merely recovers some of the energy used to slow the vehicle and stores it so it can be used later to improve acceleration or mere coasting as long as it requires energy.
Next year F1 requires energy recovery systems up to a limit and dictates it may be used at driver's discretion, like overtaking at the end of the straight. Now those will be very efficient.
In either case, fuel is saved. But if all you do is speed up to use regenerative braking it will cost you more to speed up than you get back in slowing down. You use more fuel than necessary.

 

No reread what I wrote.

Never said a word about brakes.

This was P&G as some do on here. They drive up to a speed (like 65mph) and than coasts down to a lower speed (like 45mph) with eng off than repeat.

 

Anyway I looked up P&G and it is not only for hybrids as I originally thought. Here's what I thought:

When you think about conservation of energy, it depends what you're talking about. Kinetic? Chemical potential energy (mpg)?

If you're talking about the movement of the car (KE), yes more energy was involved accelerating the car than had you slowly accelerate it.

But if you talk about fuel efficiency, you cannot think this way. You must consider the "torque per gallon" value. Perhaps "torque per gallon" is higher on hard acceleration than slow acceleration. One gallon of gas can either become heat, or torque. The ratio is varies depending on the engine conditions, which makes this problem unintuitive.

This is my only guess why P&G is gas saving.

 

In normal, nonhybrid cars, the mechanical inefficiencies swamp any of the theoretical aspects of this (second law of thermo etc). Bryanback's quoted article was well done in showing how P&G works.

I find it impractical on my base AT so I just use DFCO, accelerate slowly, and try to maintain constant speed when I can.

 

Thanks for the education on P&Ging. Having run a 1989 Geo Metro XFi for 195,000 miles for an average 48.8 mpg (best 61 worst 38 in snow) I understand the P&G principle, having used it in economy runs around Pittsburgh in the sixties with C&T SCC. Coasting with the engine off of course saves gas simply because a good part of the travel uses no gasoline. And restarting doesn't consume that much extra. If you keep average speed down, too to reduce aero resistance we easily doubled our normal gas mileage, some even tripled. But as the article points out you are endangering your life by driving thusly.
Yes that works. But has nothing to do with hybrid performance. Nor does it avoid any thermodynamic principles. You simply build up kinetic energy and use it to get somewhere for nothing. The net is of course zero overall but enerfgy efficiency is greatly enhanced. And that is what changes. Nowhere in thermodynamics is it required to have equal energy efficiency, just the amount of work done for a given situation.

It is my understanding that energy recovery on a Prius only works when brakes are applied; the drive is run 'backwards' to produce current to the battery similar to an alternator and assist the brakes in slowing the Prius. Coasting down would not seem to initiate the regenerative function as it would slow the vehicle without reason as acceleration would be required to get back to 65 mph and a repeat of that cycle would be gas-consuming, not gas saving. There is more gas used to accelerate than is saved. By P&Ging naturally you will save gas. Hills can make a difference tho.
I have a neighbor with a Prius so I'll have to go try that P&G thing if she lets me. She's a bit miffed that my wife's Honda Civic HX (not a hybrid but the last of Honda's econo HX models) get's as good gas mileage (40-44mpg overall) as her Prius so she may not cooperate. Still I know another one across town.
Its an interesting proposition. Thanks.

 

Paul,

I think you should really test out the theory and see if it works(I have my doubts) like in this picture:


I found some more info from(http://www.greenhybrid.com/discuss/f...se-glide-3700/):

Acceleration on a level road is the same energy-wise (in energy consumed per mile of travel) as climbing a hill at constant speed if the acceleration in g's is equal to the slope of the hill, for example an acceleration of 0.02g is equivalent to climbing a 2% grade. Similarly, decelerating on a level road is the same as coasting down a hill at constant speed. Coasting at constant speed of course will only happen if the downslope is just right; too steep and you will accelerate, too shallow and you will slow. In the glide phase of P&G the time taken to fall from say 44 to 22mph defines the deceleration rate: it would be 1.0g if the time was one second, and .02g if the time taken was 50sec. My HCH takes about 60 seconds on a level road to drop from 44 to 22, so I should be able to coast downhill at constant speed on a downslope of 1/60 or .0167. The problem I have is there are very few level roads around here, especially ones that I can use to dink around at 22mph. So I don't have a strong feeling for the accuracy of that 60 seconds. I do have some pieces of road that I have determined from GPS data have a slope of about .017 and indeed I can coast at nearly constant speed on them.

So where is all this leading? Well, I have used my Trip A meter to measure fuel consumption while climbing long hills of various slope and I've found that my fuel consumption is something like

FC = B(1+C*S) gallons per mile

where B and C are constants and S is the slope (.02 for a 2% grade). B is the fuel consumption you get at constant speed on the level, and C is a number somewhere between 30 and 50. Now let's imagine a P&G session where on a level road we pulse from 22 to 44mph in P seconds and then glide from 44 to 22mph in G seconds. The slope (S) in the equation can be replaced by acceleration (1/P), so the fuel consumed in pulse is B(1+C/P) multiplied by the distance traveled in pulse. The average speed is 33mph, so the distance traveled is 33P/3600. Fuel consumed in glide is zero and the distance traveled is 33G/3600. Taking total fuel used divided by total distance traveled we get

FC = B(P+C)/(P+G) gpm

You can play with this result but as long as C is numerically less than G you'll get lower FC with very small values of P - meaning hard acceleration is better. You have no choice with G - you get whatever your tire pressure dictates. Finding C is a little tricky because some of the hill climbs I made were assisted by the battery pack. If I started with low SOC I got values of C near the high end of the 30-50 range mentioned above.

I would dearly love to hear what others get for G - the number of seconds to glide from 44 to 22mph, and how it depends on tire pressure. You should go both directions to average out wind and slope (if there is any).

Dave (your friendly geezer physicist)

 

Very nice Dave. I do have a possible deviation in your position. Acceleration of a powered vehicle is a ramp or convex curve; deceleration is a concave curve on level ground and thus the average speed is not halfway between 22 and 44. The average must be obtained from the area under the plotted curve.
You could make acceleration a concave curve but that imposes other caveats. And I think your trip meter is bogus if its like mine; I wouldn't trust the data as far as I can sing and thats not far by mine and others definitions.
I have tried the burn and coast technique with my Geo Metro xFi and it truly does increase mpg significantly We accelerated gently (as if there was a choice in a Metro) from 40 to 60 mph and coasted to about the same 40 mph on a level road in SC. Did it both ways for insurance. Naturally it took a lot longer to coast down than it did to accelerate thus the average speed was different. However the measured fuel (obtained by topping off the tank) used was nearly half the normal, nearly 80 mpg vs 50 normally. I haven't had time to check your imposed calculations but that is where I think you went astray. Perhaps the widths of your two 'points' should be different causing considerable difference in areas under the curve.
Quick logic says the energy to accelerate and hold will always yield lesser energy per mile than acceleration plus coasting without using energy. And experience proves the logic.

 

education? huuu My response to your note was merely correcting a comment that implied I said something I did not.

So are you saying that under identical conditions with the P&G avg speed the same as doing that speed the MPG is 2X? If not it look like your saying the 2x is from what I totally understand, going slower with 2X MPG.

BTW
Energy is energy and the notion that you can do the same work. ie travel at the same avg speed with P&G in a Fit or othe small gas car totally conflicts with some pretty basic physics. I'd say any gains observed are just from going slower and if you are using a SG to take credit, you have bad data as the SG does not see the fuel used to start and restart over and over. That is at least 7 sec of idle fuel by one estimate.

Show me some real A_B-A testing with data and make me ashamed of my education.

What does some credible org like Consumers Reports or EPA have to say about this so called energy (GAS) saving method? Yea I thought so.

Shawn,
The link was of course to a discussion from hybrid owners.

 

The burn and coast, or pulse and glide, is not a good technique to use for normal driving but it does improve mpg, hybrid or not. I know you believe the gas used in acceleration plus not used in gliding is the same as just riding along at the average speed of the two other conditions in the cycle. It was not and I attributed it to the fuel used to accelerate the Geo from 40 to 60 and coasting in neutral to 40repeatedly was less than the fuel used to merely drive along at the same average speed constantly.
I suspect the energy to accelerate was more efficient than was the constant speed efficiency. And that may be due to the Geo having a carb rather than fuel injection. Think of it as acceleration at .2 g and not the same as coast downg.. If memory serves this ccycle required nearly a whole mile to accomplish.
If you were to accelerate and decelerate at the same rate I agree there should be little difference in mpg. Tomorrow I'll work on my math model to see if science can back up so=called experimentation or did we just prove what we already 'knew'.
cheers.

 

Paul, you are right but I just thought it would be interesting. I still think you should test it out to either prove it or disprove it.

 

mahout, Go back to my last note and look at the questions for you. Your statement about what gains you saw from P&G all by itself are not clear from what you wrote. In fact, I have yet to read of any real results from P&G testing in a gas eng Fit or other small car like ours ..... back to back testing.

Id also like to see you describe as an expert, so maybe some other Fit user could try, how you say it should be done in the Fit. I mean the details. The basic part is clear to most.

1. What speed to start and stop the acceleration?
2. What acceleration rate, in an easy to calc format like 20mph in 5 min?
3. How do you coast down. I'll assume eng off and coast down to the min start speed?
4. How to restart with MT clutch or starter? Understand with AT starter is needed.
5. And most important what are the expected gain in mpg doing this over steady cruse at the avg mph?

I'd like to test with my Fit, but my commute roads do not lend them self to doing that safe. I have tried a few times.

 

huuuuu Hey it's way more basic than that.


Getting from point A to point B, in the same amount of time is the work done.

Gas used to do that work is the real energy involved.

Can't possibly be more clear than that

My statement stands. P&G will not do that work in a Fit better, in fact it should be worse unless the test results are bogus.

 

I have no idea what the correct answer is, but here's my guess:

When accelerating, it is impossible to get 100% friction so that all 100% of the energy is turns into motion. While accelerating, heat is still generated within the tires. So I guess in a real life situation, energy is lost through both acceleration and deceleration in P&G compared to just deceleration while coasting. I think that's the initial point being made.

 

Dave
I believe we must consider the work involved in pushing the air out of the way.There are two places where that happens to consume fuel. The first is on acceleration and the second for the constant speed case. The sum of the acceleration for 40 to 60 mph (58.67 to 88'/sec), or force =(2500/32.16)6.432 ft/sec2 accel plus moving about 330 x30 cuft of air about 3 ft in about 0.2 sec yields the forces and thereafter the work involved for the P&G case, about 200,000 lb-ft.Using coast down numbers from R&T yielded a decel reate of about .02g so the coast-down would take about 45 sec. and 3000 ft. Total cycle 3360 ft accel + coast.

Pushing 30 x3360 cu ft (the entire length of the P&G cycle) of air at .075 lb/ft3 about 3 ft in about .2 sec yields high acceleration as well as a large amount of work, say some 400,000 lb-ft. of work.
Theoretically, if you didn't idle in neutral while coasting (9which we believe is abslotely essential) there is a substantial difference between P&G and constant speed.
Even tho these are table cloth calculations (hey the speaker was very dull) I think that shows the reason P&G works. While the air pushed only consumes fuel during accelerastion the constant speed vehicle must continue to push all the way through the case. And yes I used the same average speed for both cases.
Hey, doesn't mean they are right.
Dave: see what you come up with. And anybody else.
PS I used the cross section area of the Fit as 30 sqft. The air 'tunnel is 330 ft for accel and 3030 ft coast down and these equations: v=at and d=.5at2+VoTo as calculations, W=FxD and F=ma. Long time since HS.
Pretty obvious this analysis takes a lot more than tablecloth. So many assumptions.

 

FYI Dave is not on this form, Shawn copied that from another Eco site....

 

You can't judge it that way. Here's an example: Do 50 squats. When you're finished, you're in the same position you started at, but given your logic, you shouldn't be tired since energy = F*d, and d=0 since you're where you began, yet you're tired. How do you explain the fatigue, or where that energy went?

You're misinterpreting the 1st law of thermodynamics.

My only problem with P&G is it's dangerous to drive a gasoline car with the engine off for many reasons. Steering, brakes, all stiffen up.