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Old 07-05-2007, 03:31 PM   #21
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Note - engine-on coasting is not as good as engine off-coasting.

However, engine-on coasting can be very good if you are doing P&G for a
400mile drive (as I will do tomorrow). engine-off coasting would be too stressful on the engine, clutch, and myself, in this instance.
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Old 07-05-2007, 03:40 PM   #22
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Tried playing "key-off" this afternoon also . Happened upon an open stretch of road , No vehicles in sight (5 or so miles I guess ) ... I worry too much .
Nothing but wind noise is a little odd at first , kinda like hang gliding I guess
The deal with the SG not logging mpg with key off ?
Set it to Hybrid , correct?
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Old 07-05-2007, 05:00 PM   #23
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What gives better FE. Driving at a respectable 60 mph or P&G from 70 to 60. I know that P&G from 70 to 60 is better than doing the average of 65 mph. But since 60 is at low end of the scale, Im thinking that going slower might do more for FE than pulse and glide.
All my best runs have been with (engine on) + P&G. Those are the only ones I have posted because they are hands down better than any other driving strategy I have tried. I would be using engine-off if I weren't in LA freeway traffic. At some point I need to do the "momentary fuel injector off" mod, because I know I can take advantage of that when I am forced to slow down in traffic jams.

Quote:
Another question: Does pulse and glide really help that much at slower speeds. I know that wind resistance gets worse as we increase speeds. But that FE drops significantly once we pass 55mph, depending on your car. But I think the huge drop at highway speeds may be due more to engine braking than wind resistance. Best FE for a certain speeds requires that we be in the correct gear. No one will drive at 40mph in 2nd gear. Our car CAN actually do it but that speed is more comfortably/efficiently done in 4th gear. Cars will automatically slow THEMSELVES down once you let pedal go at 40 mph in 2nd gear. The same is happening when you go 75mph in 5th gear. 75 mph is more suited to a 6th or 7th gear (which we dont have). Thus the benefits pf P&G are more noticable at higher speeds.

Which is why I wonder if P&G is even needed when going at proper 5thgear speeds of 55mph. Would I be better off doing 55-60 or pulse and gliding from 75-55??, or somewhere within that range. I cant wait till I buy a ScanGuage II
I think that the rule of thumb is that aerodynamic losses start when you go above 35 MPH. At that point, the air friction causes your engine to work harder. That extra work is lost MPG :

Drag coefficient
http://en.wikipedia.org/wiki/Drag_coefficient
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About 60% of the power required to cruise at highway speeds is taken up overcoming air drag, and this increases very quickly at high speed. Therefore, a vehicle with substantially better aerodynamics will be much more fuel efficient. Additionally, because drag does increase with the square of speed, a somewhat lower speed can significantly improve fuel economy. This was the major reason for the United States adopting a nationwide 55 mile per hour speed limit during the early 1973 oil crisis as slower traffic would save scarce petroleum.
When I am at 35 MPH on the flat in 5th gear, I see 50+ MPG in my car. I think almost all cars exhibit similar optimum (over EPA) MPG figures in this situation. My opinion is that the most in-gear "efficient" cruising speed is where the top gear meets the engine idle RPM, aka the "lug line". In my car, going below the idle RPM in top gear doesn't work because the engine starts to buck.

For MPG, Engine-Off anything always beats Engine-On anything because it is basically "inifinite MPG" during the engine-off segments.

P&G is very sensitive to aerodynamic mods. Better Cd -> Longer glide -> Better MPG.

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Old 07-05-2007, 05:02 PM   #24
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I wouldn't shut the Eng off after flooring it at high speeds. it might have excessive fuel being dumped into the hot cat , it might melt it or clog it up- or like the old carburetor cars, make a massive back fire when starting it up again. leaving it idle would be my suggestion.
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Old 07-05-2007, 07:28 PM   #25
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This is a darn good thread and discusses things I have been thinking about.

You have learned the magic of P&G. You need hybrid mode and a fuel cut off switch now.

Larry is right that technically EOC will beat P&G but there is the practical aspect of doing it at high speed in traffic. I have done both EOC and P&G on the level at 60 mph. I don't shut the engine off as much as I used too. My Saturn seems to use a little extra fuel on restart for each pulse. At slower speeds EOC is easily my choice. I think it comes down to a personal preference and percievced stresses on the car. The penalty for P&G at higher speeds is not that severe if you take the time to calculate it out. My car uses .2 gph at idle.

I have thought that an ultru cap hybrid system would work for extending the pulse cycles. The cost and techonogly hinders this. Keep your fingers crossed for the Volt.

I have done back of the envelope calculations on dual engine systems. The small engine needs to have the BSfC of a DI diesel to make this effective. Yanmar clones are starting to be made in China. The price seems right and if the fuel specifics are accurately reported one could put together an aux drive that might pay for itself in the life of the car. The rational thing is to buy a modern DI diesel car. By the way I am not always rational .

CO's suggestion of liquid filled tires is truly elegant. I like it. The viscosity and velocity difference of the liquid would make the energy transfer smooth and of long duration. Texture or ribs inside the tire could be chosen for a more gentle or more agressive energy trasfer. This change alone could make long distance P&G possible for the rest of us.
Farm tractors have used liquid in tires for years. Sometimes it can get out of balance. The thought of 3 gallons of liquid flung to one side of a tire at 60 mph is a bit intimidating. The road speed of modern tractors can lead to a spot of trouble on a cold start with powdered metal weight in the tires . I think this problem could be managed. This is an extension of the viscous torsional damper.

On my Saturn with heated intake air I am usually pulsing at about 1/3 throttle and low RPM. This is 85% LOD or so at that RPM. The warm intake air appears to be leaning out the engine. If it begins to run slightly irregular I increase the throttle and the engine smooths out and picks up torque. I think this is due to increased fuel at higher throttle. All I am saying is that optimum throttle for pulse may depend on other variables. In almost all cases I have checked less RPM is better. I can probably find an exception to that statement too.

Don't forget that Basjoos's steady state beats most of us while were doing P&G.
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Old 07-05-2007, 07:49 PM   #26
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A few more specifics on my Burn and Glide experiment. The test distance was around 10 miles. It did return great FE but it is stressful personally (hard on the nerves) and probably hard on the car what with 65 mph reengagements from idle and many into neutral and then to 5th gear shifts as opposed to just keeping it in 5th. If I destroy my transmission I probably lose out expense wise ... gas $ saved is less than $ for a new transmission.
My 34 mpg for the trip till then included some city driving but mostly highway at 65 - 70mph. Using easy gas saving tricks I usually get 34 to 38 mpg with this car.
I would expect that B&G would be even more efficient if it was done at slower speeds ... conditions on the road dictate what speeds can be played with to some extent though. The conditions around me for my test were medium traffic traveling approximately 65 mph.
I didn't conduct this experiment in an exact way. What surprised me though was the significant 50% improvement in FE. There is something to be gained here. I would hope this would inspire others to run similiar experiments and further the base of knowledge.
Thanks for the "hybrid" setting on the scangauge information ... I've just changed it.
My hypothetical flywheel car was pure "blue sky" thinking. But if my memory is correct, Chrysler Corp. was testing a LeMans racer with this technology a few years back ... although nothing came from it that I know of.
Did you ever think that the reason small cars may be getting good economy might be because they are running their small engines at closer to Wide Open Throttle (WOT) ... the more efficient area?
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Old 07-06-2007, 07:09 AM   #27
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Most of the efficiency gain can be though of as coming down to engine rotations per distance travelled. The fewer rotations per mile, the greater your overall efficiency will be.

So if you spend an entire mile at a constant speed and constant rpm of 5280 (to make this example simple), you will travel 1foot/rotation. But if you Pulse & Glide (how we typically refer to it), you could still average the same speed, but do it running the motor up to a slightly higher rpm briefly and coasting at idle.

Using our 5280 baseline rpm, let's assume that we double our top rpm and top speed. If we only need to do that for 2 miles and it provides a coasting distance of 8 miles at 850 rpm for instance, then assuming an instantaneous peak to our maximum rpm, we have 2 miles x 10560 rpm plus 8 miles x 850 rpm. That works out to 27920 total rotations for the total ten miles travelled, and 2792 rotations per mile, or 1.89 ft/rotation - cutting our parasitic drag from the motor nearly in half.

Obviously this is an oversimplification with extreme figures for rpm that ignore the variation in combustion efficiency at various rpms as well as the fact that rpms are not constant during acceleration, but it illustrates the basic premise that rpms are the enemy relative to distance travelled. It also supports the argument for taller gearing.
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Old 07-06-2007, 01:27 PM   #28
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Originally Posted by Hondaaccord98 View Post
A few more specifics on my Burn and Glide experiment. The test distance was around 10 miles. It did return great FE but it is stressful personally (hard on the nerves) and probably hard on the car what with 65 mph reengagements from idle and many into neutral and then to 5th gear shifts as opposed to just keeping it in 5th. If I destroy my transmission I probably lose out expense wise ... gas $ saved is less than $ for a new transmission.
My 34 mpg for the trip till then included some city driving but mostly highway at 65 - 70mph. Using easy gas saving tricks I usually get 34 to 38 mpg with this car.
I would expect that B&G would be even more efficient if it was done at slower speeds ... conditions on the road dictate what speeds can be played with to some extent though. The conditions around me for my test were medium traffic traveling approximately 65 mph.
I didn't conduct this experiment in an exact way. What surprised me though was the significant 50% improvement in FE. There is something to be gained here. I would hope this would inspire others to run similiar experiments and further the base of knowledge.
Thanks for the "hybrid" setting on the scangauge information ... I've just changed it.
My hypothetical flywheel car was pure "blue sky" thinking. But if my memory is correct, Chrysler Corp. was testing a LeMans racer with this technology a few years back ... although nothing came from it that I know of.
Did you ever think that the reason small cars may be getting good economy might be because they are running their small engines at closer to Wide Open Throttle (WOT) ... the more efficient area?
Most of my bump starts are between 20 and 40 MPH because those "feel" the least harsh on the drivetrain to me. On the freeway, I have used the starter to restart the car (in neutral) instead of the bump start.

You still have to re-engage from neutral into 5th, but that is a "normal" thing to do at that speed.

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Old 07-06-2007, 02:01 PM   #29
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I thought it took more fuel to return the vehicle to a speed than it does to keep it at a constant speed?
I thought the same for MONTHS with my Geo. I finally did an entire tank of pulse and glide and got the 2nd best FE I've ever got with my Geo. Obviously doing P&G is in fact better than just maintaining speed with the engine on 100% of the time.
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Old 07-06-2007, 02:01 PM   #30
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Running your engine more efficiently is precisely what you're doing. Now, if you are concerned about your transmission, maybe miss shifting and chipping gears, or a decrease in the amount of time between clutch replacement, you might want to look into some combination of transmission gearing/wheel&tires size that will allow you to see that increase in efficiency you've seen through P&G at that speed range. The thing is, from a practical standpoint, if you do screw up your trans with a miss-shift, it's not financially worthwhile.

Otoh, if you are mechanically apt it's not too hard replacing a clutch, or even changing the gearing in your transmission such that your engine will be running at a speed where you'll see that mileage when just cruising. No need for any convoluted energy storage mechanism, or even a hybrid system, if you know what speed you want to cruise at on the highway. Just find a suitable 5th gear/ring and pinion combination and you can see ~50mpg cruising at 65-70mph. The downside is that you won't be able to accelerate as quickly or to as high a speed in 5th, but if you want to accelerate and drive a manual, there's always 4th imo.
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I think if i could get that type of FE i would have no problem driving a dildo shaped car.
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