So disappointed in my 1st hypermiled tank - Page 2 - Fuelly Forums

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Old 04-17-2008, 02:31 PM   #11
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Actually it's better to give it more gas pedal on the uphill part, and let it coast downhill, while trying to maintain a relatively constant speed. Best engine fuel consumption (per unit of horsepower) is at low engine speed and about 70% throttle. It's also more efficient to P&G within the narrowest range of speed, which is much lower (range of speed) when you climb the hill and coast (engine off if you please), because it avoids exponential increases in drag. I think I read that Wayne Gerdes stated that they actually got better mileage on hills than on flat ground, but probably not steep hills, kind of like a roller coaster effect.

Most of the hills around here are not that large and you dont have to worry about too much speed downhill. If you end up going too fast downhill (due to slope) use high gear and release all pressure on the pedal to utilize fuel shutoff, which uses no gas.

65 MPG at 65 MPH on hills between Richmond and DC using that exact procedure with a little aero help from the truckers going 65-67 to conserve fuel.

I dont know why they used to think constant high engine vacuum was an indication of good economy. In reality the more air you get in each cylinder for every combustion pulse the more efficient the engine produces power. In reality your P&G should be in the highest practical gear at 75% of max throttle, ideally between 1200 and 2500 RPM. On a vacuum guage that reading should be as close to zero as possible with the least throttle opening that produces the lowest vacuum reading.

Anytime you are not filling the cylinders completely you are reducing the effective compression when the explosion occurs, which can only reduce the energy extracted from your fuel consumed.

Cars with better aerodynamics can P&G at higher speeds. Consider when I am travelling at 45 MPH with the engine idling, which cosumes .2 gal per hour, I ma getting 180 MPG even with the engine idling, whcih will offset the increased fuel consumed in the pulse, because you are using only half as much more fuel to apply the 50% greater power you use in the pulse phase.

regards
gary
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Old 04-30-2008, 07:36 AM   #12
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Originally Posted by R.I.D.E. View Post
In reality the more air you get in each cylinder for every combustion pulse the more efficient the engine produces power.
Isn't this a very simplistic explanation?

Could one make the (similarly simplistic) argument that if more air is going to the engine that more fuel is being consumed by virtue of the fuel-air ratio?
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Old 04-30-2008, 08:56 AM   #13
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By all means, DO shut off the engine at stoplights. Anything more than about 10 seconds is worth it.

Pulse & Glide works wonders on the highway, especially if your steady-state rpm is above 2,000. That's 45mph in my case, and maybe Jesse's as well. I've done 75mpg on the highway using 45-60mph P&G, and 60mpg using 55-70 P&G.

And I do pulse the uphills and glide the downhills. Higher load = more efficient engine operation.
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Old 04-30-2008, 09:13 AM   #14
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Well, my 2nd week of hypermiling was dissapointing too - 44.314 mpg tank, when I had several 50+ mpg drives but one bad 30 mpg drive with heavy A/C no hypermiling.

Hey Pale - I'm 99HXCivic on Cleanmpg too!
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Old 04-30-2008, 10:43 AM   #15
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dosco:

Simplistic is the easiest to understand, sorry if I erred in that direction.

The effective compression is the potential compression minus the restriction. If you have 18 inches of vacuum you are only allowing about 1/3 of the air the engine is capable of ingesting.

The result is lower compression which makes the engine less efficient, the difference being you can get 50 hp using only 50% more fuel than 20 hp, by adding only 50% more fuel with the throttle open 70-80%. Your manifold vacuum reading would be close to 0.

Pulse and glide means you are using the vehicles own mass as storage by adding the extra 30 hp into acceleration and then allowing that stored energy to replace the sustained low efficiency demand of continuous operation. The extra 30 hp costs you only half as much as the first 20. You are using fewer total revolutions while increasing the air for each revolution at the same rpm.

Of course cylinder filling combustion events use more fuel than restricted events.

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Old 04-30-2008, 10:59 AM   #16
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Simplistic is the easiest to understand, sorry if I erred in that direction.
It's not a problem, and sorry if I was being a prick (I was born with that innate talent).

I'm grappling with the trade-off between reduced mass flow through the engine (e.g. at idle RPM) versus operating at "most efficient RPM."

I'm interested in a study (like a bunch of calculations using the governing equations) between the two to determine where and when it would be optimal to let the engine idle versus driving with the engine at max efficiency RPM.

It seems to me that there is some difference of opinion regarding whether it is more better to let the engine idle (i.e. P-G) or run it at load.

Quote:
Pulse and glide means you are using the vehicles own mass as storage by adding the extra 30 hp into acceleration and then allowing that stored energy to replace the sustained low efficiency demand of continuous operation.
I understand the concept of P-G. As I indicated above, where I'm going wrong is the theoretical understanding when to idle versus when to maintain constant load. For example, why is it more efficient to P-G than to operate the car at a steady state at the "most efficient" RPM?

Another thing that comes to mind about P-G....In a perfect frictionless world, using P-G wouldn't get me much because the amount of work I'm applying to the car during the pulse would be the same as I would get out during the coast.

Of course, the real world has losses due to friction, so I have losses during the pulse and losses during the glide. Theoretically speaking, won't the losses be greater during the pulse due to higher speed (thus higher aero drag), windage losses in the engine at higher RPM, and more fuel mass flow during higher RPMs?

I've only begun experimenting with real-world P-G driving. Palemelanesian indicated the "best" P-G is using a wide speed range ... wider than I've yet tried. So I'll soon see what the deal is with my own data/own car.
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Old 04-30-2008, 11:21 AM   #17
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The deal with P&G vs steady state at the same rpm is load. There's an ideal rpm/load combination. Often it's something like 2000 rpm and 75% load, but it varies with the engine. If you try to maintain the load, pretty soon you're going 120mph. So you use the high-load for a pulse, then glide down on the energy you built up.

See this article: http://www.autospeed.com/cms/A_110216/article.html especially the 5th color-coded chart. You want to be in the red zone. The fuel consumption per power output (unsure what the units are) is 0.42. If you keep that same rpm, but go to steady-state at maybe 20% load, you're in the purple zone at .6 or .7 or even higher. See how quickly it builds up closer to no-throttle.
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Old 04-30-2008, 12:02 PM   #18
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The deal with P&G vs steady state at the same rpm is load. There's an ideal rpm/load combination. Often it's something like 2000 rpm and 75% load, but it varies with the engine. If you try to maintain the load, pretty soon you're going 120mph. So you use the high-load for a pulse, then glide down on the energy you built up.

See this article: http://www.autospeed.com/cms/A_110216/article.html especially the 5th color-coded chart. You want to be in the red zone. The fuel consumption per power output (unsure what the units are) is 0.42. If you keep that same rpm, but go to steady-state at maybe 20% load, you're in the purple zone at .6 or .7 or even higher. See how quickly it builds up closer to no-throttle.
OK, exactly the info I was interested in!

Thanks for the link.
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