The Fit has electric power steering (EPS); as long as the ignition switch is in the "on" position, the steering will feel normal.

"P&G" means different things to different people. For me it means what the hypermilers call "NICE-on coasting" most of the time, meaning that I slide the car into neutral and coast with the engine running. I have only a very few spots in my normal commute that lend themselves to safely turning the engine off while the car is moving, mostly because I am not willing to risk having the braking ability of the car compromised. Also true P&G (powering down while coasting) seems to me to be a bad tradeoff in the long run; a couple of MPG saved now, versus the probability of shortening the lifespan of several expensive components, doesn't make a lot of sense the cheapskate in me.

 

Ah, but you did travel "up" and "down"

 

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.

P&G should be done in 'glide' with the engine running at idle and the gear in neutral !

Shutting the engine off does not necessarily 'stiffen' brakes etc but it may lock the steering wheel. No matter, P&G with engine shut off is DANGEROUS.

 

So far I'm correct and P&G is a myth as far as any proven Eco gains proven with test results on a gas car like our Fit.

1. Not one scrap of real test results.
2. Not a single link EPA or trusted source with results

P&G for the Honda Fit results is..... BUSTED

 

My scribbling was based on a 2500 lb Fit starting at 40 mph (59 fps) and accelerate at 0.2g (6.4 fps2) to 60 mph (88fps) then coast down at .02g (0.64 fps2) back to 40 mph and repeat. I declared only the work performed by the acceleration consumed fuel and none on deceleration. To that I added the work to push 30 sqft of air column of the length of the accleration for 3 ft - I should have used 1.5 ft) in .02 sec based on a Cd of .3.
I compared that to moving that column of air 30 sqft by the length of the steady state velocity equal to that of the average speed from acceleration plus deceleration and moved again 3 ft Again I believe 1.5' is more accurate; the mean time for the air column to be pushed aside..
The premise is only acceleration consumes fuel while the entire length of the steady state velocity consumes fuel. The work done in both cases should be equivalent to the fuel requirement. In this comparison the work of acceleration only plus that length of air column compared to pushing the column of air equal to the length of both acceleration and deceleration pushed aside 3 ft (again I think 1.5' is more realistic).

See anything that needs to be changed? Or I missed?

I ignored restarting. In our trials we simply left the engine running at idle with clutch dropped and restarted by re-engaging the clutch.

I did intend to go out today and actually measure a coast down fron 60 to 40 for verification but didn't have the chance. Obviously that is a crucial data point. Tomorrow.

Having found my trusty slide rule lets do the calcs:

accel dv of 59 to 88fps at .2g with v=at yields t = 4.5 sec; Fit csa =30 ft2
d=.5at2 + voto yields 330 ft to accelerate from 40 to 60 mph(59 to 88 fps)
coast down at .02 g is (88-59)=.64(t) and t = 45 sec.
So the cycle is 4.5 sec accel and 45 sec decel.
The distance of decel is .5(.64)(45)2 + 59(45) or 3303 ft. Total cycle distance = 3303+330
ft or 3633 ft. Thats the total steady state power path.
The accel only pushes air out of the way during the 330 ft so we 'assume' an air cylinder 30 ft2 x330 ft x .075 lb/ft3 for a total of 742.5 lb air that must be moved to the side. A distance nominally 1.5 ft for the whole section of air. Now that air has to be moved aside in .02 sec at a Fit avg speed of 70 fps. Thats an accel of 1.5=.5(0 .02)2 or 7500 fps2
That needs a force of 742.5/32.16 x 7500 = 173178 lbf. Work is 1.5(173178) lb-ft
Add the accel force of 2500/32.16 x 6.4 = 497 lbf. Work is 497(330) lb-ft

The steady state involves a cylinder of air 3630 ft long by 30 ft2 pushed aside again 1.5 ft in .02 seconds. Thats 8168 lb air pushed 1.5 ft to the side in .02 seconds which we already know is 7500 fps2 accel. The force then is 8168/32.16 x 7500 = 1.904,734 lb f

The work, or fuel required, for accel case: (497 x 330) + 173178(1.5) = 423,777 lb-ft
The work for the steady state case is 1.904,734 x 1.5 = 2,857101 lb-ft

That seems a bit opimistic probably because the coasting time must be considerably less than 45 seconds or .02 g. That penalizes the steady state unduly.
For those who like me suspect that .02 second time to push the air aside note that at 70 fps (50 mph) the time for air to slip aside a 30 sqft barn door would be .0143 seconds. I allowed for a slower rate by applying a Cd of .3 to the shape. Hey, its my model.

Tomorrow I have time to do coast down tests on my car so we'll get to the bottom yet. I have a sneaking suspecion there isn't much of a gain if any no matter what we 'thought'.
Feel free to criticize my model; if I do so can you.

 

I think the 0.2 G is a tad brisk. Not sure my fit could do that. You did not give the time but unless I made a mistake....
from 40 to 60mph @ 0.2g's will take 4.53sec

wow... you sure added a lot on the 2nd edit

More details .
At some point you have to move it to a more practical way of looking (measuring this). It is the true acid test. A-B-A with your SG and real fuel used on a repeatable test run.
The Fit car is less efficient as it accelerates harder. That's the basic premise for my entire comment. The energy used to accelerate back to speed can't be total recovered in the glide back down, compared to just stead state cruse at the mean speed.

What is you fuel flow at idle? The Fit is .25gph-ish. So if you leave it run on the glide, you have to include that.

 

I absolutely agree; my Fit could only attain .2g by falling off a cliff. My model is actually based on a CRX SiR which can, only data I had for model.
And like I said, the 60 to 40 coast down is likely way too much. I know on our little trial we got better mpg but don't remember it being even twice as efficient. Worse, we didn't even record in our comp book. I went back & looked.
I have no idea what the idle flow fuel rate is, as it isn't really involved I ignored it as that is not the pointof using stored energy.ie flywheel effect. And all cars have convex accel curves; and the reason you don't recover all the stored energy from 60 to 40 mph is the reason that the coast down time has to be shorter. Maybe in a frictionless and vacuum world. Still there may be some gain. Certainly not 2 to 5 times as much.

 

Your obsession to be right for such a non-contraversial thing is so unusual. I'm not sure if as a mod I should consider you're a troll, or wait a while and see if you're just obsessive compulsive.

You still haven't responded to my accusation that you don't understand what the 1st law of thermodynamics says. What happens if you do 50 squats? When finished, you end up in the same position as you started, so therefore you performed zero work. So why are your legs fatigued, an indication that energy was lost in the system?


PS I reread your first post. You said "newton's second law". Newton's second law is about conservation of momentum, not energy. I think you meant the first law of thermodynamics (nothing to do with newton). As a matter of fact, newton never worked in thermodynamics.

BUSTED

 

"troll" "obsessive compulsive"
name calling from a moderator... wow did not see that coming.

When you come at me like that, it's clear we have nothing to exchange at all.

 

You must of missed this.... His claim was never how good the P&G did over not P&Ging. ie no back to back testing. It was a claim that he just did better by going slower and P&G. Prove me wrong, show the data !

I have a SG and have posted many help comments to Fit users on this sight with real proven (with data) methods to save gas. Perhaps you should take some time and look at my SG contributions

 

mauout, I'll get you a coast down (time) today for my Fit Sport

 

Thanks./ Good comparison with mine is appreciated. Be sure to do it both ways.

 

Just out of curiosity what makes you think a scan gage is accurate? Could be just another one of those 'miracle' devices. The scangage is very much like the mpg computers standard in two of my cars, an Azera and a Sentra SE-R. When compared to actual fillup there is often disagreement between the mpg displayed and the gas required to fill the tank to the top.
And for the short runs that are made for the P&G tests my gages aren't worth any value as data because the measurements are so indefinite. As for fuel flow measurements, until you actually manage to put precision fuel meters on each injector or differential flow meters on supply and return fuel lines any flow measurements from OBD II are suspect.
That said, I'm not in disagreement that P&G isn't worth the efforts but there needs to be a little more precision evaluation to verify or discard. But I don't think a Scangage will do it. Its only means of data is what it gets from the OBD II computer. And those aren't good enough.

 

As promised.
flat ground no A/C windows closed, 83F
1 plump driver no other load
little traffic to promote draft. In fact I turned on my 4 ways to have them steer clear of my test run lol
60 to 40mph took 38.5sec in neutral

As for accel. same conditions
Flat footed it in 5th
12.5sec

I'm sure it would of been quicker if I had shifted down to 4th than up to 5th at 60, but if this is for P&G was thinking that would just be over the top as far as steps in each cycle.

hope this helps

edit
re/ the SG.

I have been tracking the data my SG reports and pump fill / odometer for around 5000 miles so far and I feel it may be more accurate than you feel. I have posted a spread sheet a few times. The miles are within 1%. The SG can read flow in hundreds of gph. It does not measure the START UP gas, so for sure that's off. The results, tank after tank, seem to be closer to my avg then the pump odometer. I did have a issue that Rich figured out, that made the measured trip miles off, but that's resolved and over 400miles it's within 1 miles to the cars odometer.

I have verified that the mpg from pump fill has way more variability on this can and my last. I have seen significant errors in pump gal's vss delivered over the years. Had a pump last year that was off by 40% for over 6 months. The last two fills I have gone as far as taking a 1 gal verified tank and pumping the 1st gal into that to see how the pump does. Also, the auto pump cut off varies a great deal from fill to fill and pump to pump and that's what really determines the fill gals. If the gas flow is fast or slow, foaming or not, car on flat ground all seem to effect that end of the fill and gal delivered for the mpg calc.

 

]I have been thinking about this for some time and wanted to toss this out for discussion.
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 don't know thats true. Some years back we did an experiment at a test track with a Geo Metro XFi where we did the "P&G" technique for several laps and ran steady speed for an equal number. Mine and my crew chief memories said we got about 60% better mpg. This proposition has generated heated debate in our bench racer crowd and needed at least theoretical justification. Here it is:

HOLD ON TO YOUR HATS, WE MAY NOT COME THIS WAY AGAIN

The premise that acceleration and coasting could yield better mpg than steady speeds on level ground is an interesting proposition. I gathered some experimental data and got some educated estimates from knowledgeable engineers to see if it could be shown that the P&G theory has some validity. As we thought some years ago testing a 1989 Geo Metro XFi it appears we were correct in thinking coasting assisted mpg. The key is understanding that energy stored can be used to increase mpg. Here's our position:

If you use the standard equations for force, F=ma, velocity, v=at, distance d=1/2atxt+vt, and work, W=FxD with a few reasonable model conditions such as Fit weight 2500#, cross sectional area, 24 sqft, acceleration from 40 to 60 at 5 fps2, deceleration of 1 fps2, and the acceleration of an air molecule laterally on a surface with Cd of .3 and escape distance of 1.5 ft (all the mean distance air must travel to be pushed aside by the .3 Cd surface). And air weighs .075 lb/ft3

The expectation is that it requires more input energy to push a FIt steadily thru the air than it does to accelerate it to some speed faster but having the same average speed and letting it coast down to a speed slower but yet having the same average speed.

First, lets calculate the force required to accelerate a 2500 Fit from 40 to 60 mph (59 to 88fps). dV = (88-59)=(5) t thus the time is 5.8 seconds.
The distance is .5(5)(5.8x5.8) +59(5.8) = 426.3 ft
The work to accelerate the Fit from 40 to 60mph is (2500/32.16)426.3 or 33139 lb-ft
The force to push the air aside is that required to push a ‘tube’ of air the length of the acceleration for a distance of 1.5 ft to the side in .1 seconds.
24(426.3).075 = 766.8 lb of air. Pushing that 1.5 ft in .1 seconds means the air is accelerated at 1.5 = .5a(.1x.1) and a is 300 fps2
The force to push the air aside then is (766.8/32.16)300 or 7152 lb.
The work to move the air is 7152(1.5) = 10728 lb-ft

THE TOTAL INPUT ACCEL WORK IS 33139 +10.728 = 43867 lb-ft

There is no force input on coast down; that is automatically accounted for in the coast down time, which we have measured to be 30 seconds from 60 to 40 in our test CRX(it has 4.88 gears but good aero body and made mincemeat out of the 5 fps2 acceleration level chosen.

Second, to coast down from 60 mph to 40 mph in 30 seconds yields a decel a of 29/30 close enough to 1 fps2. That time of 30 seconds leaves a distance of .5(1)(30x30) +59(30) or 2230 ft.
The total distance then for a steady state speed is 426.3 + 2230 or 2656 ft
That’s a ‘tube’ of air 24x2656(,075) = 4781 lb of air; moved 1.5 ft in .1 seconds or an a of 300 fps2. The force is (4781/32.16)(300) = 44957 lb and the work = 44957(1.5) or 67435 lb ft

THE STEADY SPEED WORK IS 44957(1.5) or 67435 lb-ft , 50% greater than the P&G

Therefore it should be expected that the mpg of the P&G technique to be about 50% better than steady driving, all other things equal. And memory and others remember that’s about what we got with the Geo.
Its similar to using stored energy in flywheels so its not free just stored.

HAVE AT IT. If you find an error bring it out. This is simply a technical discussion of possibilities so disagree away. We aren't offended.
PS NASCAR drivers have used this technique for years to extend mpg.
I never did so can't verify it in racing.

 

Name calling? Do you know what those words mean? Those are words. You think those are insults?

Why are you avoiding my question? I highly doubt you understand conservation of energy, aka what you call "Newton's second law."

 

Uhh... the distances on the two test are different.

If you're going to use work = force * distance. At least use the same amount of distance so that the amount of force can be measured properly. But if the acceleration for P&G was ajusted so that the distance covering the whole 2230 (same as coasting run) rather than 426.3 ft, then there should be no argument that one force was greater than the other. MAY THE FORCE BE WITH YOU, INSTEAD!

 

Cruising while in gear burns gas while in neutral, free wheeling (gliding) burns as much gas as stopping at a stop sign. If friction didn't slow the car down, gliding yields almost infinite miles per gallon. If friction didn't exist, but the person stays in gear, the car will slow down due to engine braking and will yield a normal miles per gallon number. The thread starter puts too much emphasis on Work, but work does not measure fuel consumption. The example I just mentioned here both use the same fuel consumption but one situation the car travels a significant distance further.

That's the point I was trying to get with my 50 squats problem. There's zero work achieved, but a high amount of fuel consumption involved. Work != fuel consumption

 

The fact is, there's still no test results that show the Honda Fit can go from A to B using less fuel using P&G.
Show me some real A-B-A test data that shows these huge gains (50% lol) and I'll eat my shoe.

The point of the thread, like most of mine, are to educate on what works and what does not on a Fit. Already been many reasons shown the P&G on a gas car is not safe, but I did not even mention that.

IMHO Gliding to a stop or on a hill is a real way to save fuel but that's different.

 

I'm not sure I get your point. They measured the force required to accelerate the car, and the distance required to accelerate it. They measured the coastdown rate and the distance covered coasting. Let's call that one Pulse-and-Glide cycle. Then they added the two distances together to get the total distance covered by one cycle. That's the distance they used to calculate the steady state figures. Looks right to me.