P&G Questions

[dosco]

Originally Posted by monroe74

Thanks for reminding me where to find that article. I think it's very helpful.

It is. I found another link to a very long paper that expounded on the graphs at the speed site. I'll dredge it up again, if interested ... but it basically says the same thing as the speed article.

The graph doesn't say that 3200 RPM is highly efficient "regardless of load."

Perhaps my choice of words wasn't so great, but the graph clearly shows that at the various load conditions (25%, 75%, 100%), the most efficient rev range is 3,000 to 3,500 or so.

I agree with you, though, so no argument.

For almost all modern cars (even with low-power engines), if we assume a flat road and a vehicle carrying only one or two people, then full throttle (or 70%, if we want to take the open-loop issue into account) is enough to produce acceleration, even in top gear.

Indeed. The very essence of P&G driving. I think we're in agreement.

Life is much more complicated with an automatic. It's probably brisk because your tranny decided to downshift.

I used to drive a stick, and am very conscious of what the auto trans is doing - thus my focus on both RPM and throttle angle.

I think the "brisk" part is b/c I've been driving with small throttle openings and low accelerations for so long that it seems very odd to be "flooring" it (not literally) and accelerating briskly.

Theoretically, we want full throttle 100% of the time, except when we're coasting.

This is where a CVT for a gasoline+spark engine comes into play... ...

[GasSavers_RoadWarrior]

Originally Posted by dosco

This is where a CVT for a gasoline+spark engine comes into play... ...

Or should... unlike the shining example of the dodge caliber

'95 Neon using venerable torqueflite hydraulic 3 speed transmission. 26mpg combined.
'05 porked out final year Neon, with electronic 4 speed ultradrive, 25mpg combined
'08 Caliber, CVT transmission, 24mpg combined.

On that evidence I should be grateful I've got the old 3 speed torqueflite technology auto in my minivan, and not go wishing for CVTs or a 4th gear.

[dosco]

Originally Posted by RoadWarrior

On that evidence I should be grateful I've got the old 3 speed torqueflite technology auto in my minivan, and not go wishing for CVTs or a 4th gear.

Oh, I don't wish for one either. It reminds me of the SCCA F400 (?) class that used snowmobile CVTs. I can't say that I cared for those things at all.

[monroe74]

dosco: "I found another link to a very long paper"

If you found that link on this site, I think maybe I saw it too. I vaguely remember some other similar paper, that I found via a link cited in this forum. If you found the link on another site, I think I'd like to see it, if you have it handy.

"the graph clearly shows that at the various load conditions (25%, 75%, 100%), the most efficient rev range is 3,000 to 3,500 or so"

Yes, the graph shows that high revs (about 3000) are more efficient than lower revs. But this is easily misunderstood, because the graph also shows that at any given engine speed, 100% throttle is always more efficient than 25% throttle. Following the graph, we would seek to be at 3200 RPM and 100% throttle. Trouble is, even a low-powered car is typically too powerful to maintain this state, unless it's climbing a steep hill.

The essential message of the graph is this: the engine is most efficient when the throttle is fully open. It's very counter-intuitive, but the key to saving gas is to open the throttle! (Almost fully, except for the issues I cited, about open-loop and about how an auto trans likes to downshift.) And then coast.

"I think the 'brisk' part is b/c I've been driving with small throttle openings and low accelerations for so long that it seems very odd to be 'flooring' it (not literally) and accelerating briskly."

Good point. I see what you mean. P&G is a challenge because it violates our old habits and expectations. It also has to be done skillfully, and with planning. I'm wasting gas if I accelerate and then have to turn that energy into brake heat, instead of coasting distance.

"This is where a CVT for a gasoline+spark engine comes into play"

Exactly. Grasping those BSFC charts makes it easier for me to understand why CVT is important. But I think we can see that it's possible to get good results without one, if the driver understands the principles we've been discussing.

[monroe74]

Pale, thanks for the compliment. I greatly respect your opinion. A lot of what I've learned is from you. I've read 100% of your comments. Your comments and your gaslog are a big part of what convinced me that I should try really hard to thoroughly grasp the theory and practice of P&G.

I've looked very closely at many gaslogs. There are two things I find very distinctive about yours. The first thing is that you're very much (95%) above EPA. This is virtually unheard of. Out of almost 400 vehicles currently in the garage, only 3 or 4 currently exceed you in this measure. And none by very much. This is especially impressive since your car is almost 100% stock. The four vehicles ahead of you all have fairly extensive mods, most including aero mods (beyond just a simple grill block, which is your only aero mod).

The other thing I find distinctive about your gaslog is that it shows a slow, steady improvement. Obviously FE is a combination of car factors (e.g., tire pressure), environmental factors (e.g., ambient temperature) and driver factors (e.g., skillful application of techniques like P&G). It looks to me like your steady improvements are mostly based on driver factors. I think the nature of P&G (and related techniques, like EOC) is that it can be done in a moderately skillful way, or in an exceptionally skillful way. It lends itself to constant refinement. So you seem to be on a learning curve, and perhaps not yet done refining your technique (but nevertheless way ahead of the rest of us).

Anyway, I had a lot of trouble grasping P&G until I saw those BSFC articles. They're very technical, and my previous posts are long, so I'll put the key insight in a nutshell:

There's one throttle setting (probably about 70%) that's more efficient than any other. ADJUST YOUR DRIVING SO YOU'RE NEVER USING ANY OTHER THROTTLE SETTING (except, of course, when you're coasting, and your foot is off the pedal). You should also be using the highest possible gear, most of the time. It also helps to understand the three kinds of coasting.

(Getting all this right with an automatic is a special challenge.)

It seems to me that this is what P&G is really all about: always use 70% throttle, or no throttle at all.

With P&G, you're operating the engine at high efficiency (70% throttle, during the pulse phase). This creates more power than you really need, so the car accelerates. The extra power is being converted into kinetic energy (momentum in the car). Then you coast, and rely on that kinetic energy. Gas consumption is very low (or even zero) while you're coasting. You save gas because you approach the following ideal: the engine is either operating at top efficiency, or not operating at all. This is much better than a steady speed, at partial throttle, where a lot of energy is wasted on pumping losses.

It's OK if you (i.e., non-technical readers) don't understand the concept of pumping losses. Just apply this simple rule: the only acceptable throttle setting is 70%.

[monroe74]

What I find interesting about P&G is that it requires me to unlearn years of driving habits. I was always taught to strive for moderate throttle openings ('drive like there's an egg under your foot'). WRONG.

Likewise for those old vacuum gauges which embodied the idea that high vacuum meant high FE. Also wrong. The idea only persisted because high vacuum often meant 'going really slowly.'

Those old vacuum gauges were sometimes called an 'economy meter' or a 'motor minder.' Some cars came with them as original equipment. Stewart-Warner still makes it, I think (see the left item in this photo).

What's glaringly remarkable is that it's OK to monitor vacuum, but the scale is WRONG. It's marked (in red) to indicate that low vacuum is bad. That's wrong. Low vacuum is good, provided I use it alternately with coasting (and provided that I also strive for high gear). Low vacuum is only bad when I'm persisting too long in a low gear, and when it means I'm creating excess kinetic energy that will ultimately be consumed by the brakes, instead of by coasting.

People are still very confused about this. I notice with great amusement a bunch of Camaro owners discussing this recently, saying things like this: "It's pretty well accepted that the greater vacuum reading you can get while cruising around the better your economy will be." Wrong. (BTW, I used to drive a '68 GTO, so I have no ill will toward Camaros, or other Detroit muscle.)

P&G driving can be FUN because it involves accelerating. Accelerating is fun, and it's a good thing, as long as the excess kinetic energy will ultimately be used in coasting, rather than consumed in braking.

[dosco]

Originally Posted by monroe74

If you found the link on another site, I think I'd like to see it, if you have it handy.

Here it is: http://www.tc.gc.ca/programs/environ...nal_Report.htm

It's rather long, but I found it very very interesting that they remapped the contour charts that were in the speed article, accounting for 4-valve heads, turbos, etc.

Good stuff there.

Good point. I see what you mean. P&G is a challenge because it violates our old habits and expectations. It also has to be done skillfully, and with planning.

Exactly.