P&G Questions

[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.

[palemelanesian]

Thank YOU for the compliment. Yes, it's all a constant refinement game. The cold winter showed me some flaws in my technique, and I refined them. Now, with warmer weather, I'm doing even better. Details like starting the pulse just before an uphill, and pulsing partway up, and the coasting up and over the crest. The uphill gives you more load, and makes it easier to hit that peak BSFC point. Then the downhill is free.

And it IS fun!

The vaccuum gauge is correct if you're applying it to steady-state cruising. Which is not one of the "70% or off" options. But in it's place, it is useful.

The grill block is more for faster warmup than it is for aerodynamics. My normal driving doesn't go over 55mph speed limit, so I'm not even going that fast.

[monroe74]

dosco: "Here it is"

Thanks very much for the link. Yes, I had seen that. I think that was linked in another thread around these parts. I agree, it's very interesting, and it supports the ideas expressed in the other article you cited.

pale: "it's all a constant refinement game"

Yes. And I think it also lends itself to a steady commute, where you're driving to the same place every day (over an identical route, or a route with minor variations; also probably at the same time of day, mostly).

For most drivers, that sort of driving is probably a large percentage of total driving. But we all sometimes drive in unfamiliar circumstances (and for some of us that might be a very large percentage of our total driving). Then P&G (and related techniques, like EOC) can become more challenging.

"The grill block is more for faster warmup than it is for aerodynamics."

Makes sense. I wonder if maybe it also helps the engine maintain warmth while you're gliding. The engineers who designed your cooling system probably didn't anticipate that the engine would spend a lot of time idling (or off) in a 45 mph wind.

"The vaccuum gauge is correct if you're applying it to steady-state cruising."

True. But that really just boils down to this guideline: if you must travel at a constant speed (for whatever reason), then the best you can do is just use the highest possible gear. And if you can, choose a lower speed. I think someone being guided by a vacuum gauge is really just applying those rules. But they're being misled, because they're being encouraged to adopt a constant speed, when P&G is better.

[palemelanesian]

I occasionally take a different route for that very reason - to keep myself fresh on unfamiliar roads.