What's better on hills: DWL or Pulse up, Glide down?

[monroe74]

duffman: "steep declines will accelerate the car providing free energy"

Understood. But cresting the hill at 45 mph could still give me a much better downhill glide, as compared with cresting it at 25 mph (and consider how a roller-coaster works; the 45 mph might be exactly sufficient to send me over a subsequent hilltop). Obviously this analysis varies with the conditions. If right over the crest is a very steep downhill grade, then my extra momentum going uphill quickly is wasted, because I might need braking (regular brakes or engine braking) to avoid an unsafe speed going downhill. But my example assumes that conditions on the downhill side allow me to use 100% of the momentum I carried with me as I crested the hill.

metro: "if you're not going to glide, engine off, down the back side ... best BSFC doesn't necessarily equal lowest overall fuel consumption, it means most power produced per unit of fuel consumed"

Good point. That's very clearly said. I see exactly what you mean. It's all about planning, and strategy.

[monroe74]

johnny: "you're best bet is probably 80% throttle in top gear. Much more than that, you'll be in open loop mode (rich)."

Which brings me to a related question which might be material for yet another thread.

I recently started using a DMM to get a better understanding of how lean-burn works on my VX. I had started with the assumption that there's a certain throttle opening (say, 80%) where there's a fairly abrupt transition to open-loop, and then a lot of extra fuel gets used.

But reading the DMM, it doesn't look that way. AFR is quite lean (i.e., lean-burn mode), even at, say, 50 mph, if I'm very gentle with the throttle. But I usually avoid that, because I've come to believe that a large throttle setting gives me better efficiency than a low setting, despite the advantage of lean-burn. (Sometimes, like in traffic, this means I apply a large setting, but for a short period of time. In other words, my P&G waveform has a fairly high frequency. This can be a lot of work, but I'm finding it pays off.)

But what's interesting is that the AFR response seems linear, even at WOT. In other words, as I move from 80% throttle to WOT, I see the AFR getting richer, but in a gradual, moderate way. There's no sudden transition.

I see the same thing when I use the DMM (dwell feature) to monitor my injectors. As I go from 80% throttle to WOT (let's say in top gear, at low RPM), the injectors continue to open, but in a gradual, moderate, linear way. And even at WOT, they don't open very far, in those conditions. I can only get them to open far if I combine WOT with high RPM.

Anyway, I wonder if a wideband-sensor system like the VX doesn't go into true open-loop in the same way we see with simple O2 sensors, which essentially act more like a switch. My understanding of open-loop is that the ECU starts ignoring the O2 sensor, and uses a stored map instead, because the sensor is no longer providing useful information, because the mixture has moved outside of the narrow range the sensor is capable of reading.

In a way, my question is this. If you're using a wideband O2 sensor, how can you tell whether you're in open-loop or closed-loop mode? With a conventional low-resolution sensor, you can tell this way: "Closed loop operation is indicated by the sensor showing several cross counts per second." (From here: http://mr2.com/TEXT/O2_Sensor.html; the term "cross counts" is explained there.) But wideband sensors behave in a fundamentally different way, and aren't constantly oscillating (creating cross counts).

So I'm wondering how to monitor open-loop mode on a wideband engine like the VX, and I'm wondering if maybe open-loop simply doesn't happen. And I'm thinking about switching from 80% throttle to WOT, as part of my P&G routine.

This is fairly radical; I've never seen anyone, anywhere, advocate frequent WOT as part of any FE strategy. But I'm considering WOT virtually 100% of the time (except when the throttle is closed). In other words, I might start treating the throttle as a switch (this applies mostly to top-gear operation). So I'm very interested in comments.

[Duffman]

Originally Posted by monroe74

In a way, my question is this. If you're using a wideband O2 sensor, how can you tell whether you're in open-loop or closed-loop mode? With a conventional low-resolution sensor, you can tell this way: "Closed loop operation is indicated by the sensor showing several cross counts per second." (From here: http://mr2.com/TEXT/O2_Sensor.html; the term "cross counts" is explained there.) But wideband sensors behave in a fundamentally different way, and aren't constantly oscillating (creating cross counts).

So I'm wondering how to monitor open-loop mode on a wideband engine like the VX, and I'm wondering if maybe open-loop simply doesn't happen. And I'm thinking about switching from 80% throttle to WOT, as part of my P&G routine.

This is fairly radical; I've never seen anyone, anywhere, advocate frequent WOT as part of any FE strategy. But I'm considering WOT virtually 100% of the time (except when the throttle is closed). In other words, I might start treating the throttle as a switch (this applies mostly to top-gear operation). So I'm very interested in comments.

First part, if you have a wide band, closed vs open loop is off the mark, you should be monitoring the actual A/F ratio, whether you have a dedicated gauge or just the voltage, this is the way to go.

Second part, JohnnyGrey is bang on. You will hit a point somewhere that the A/F ratio will begin to richen with increased throttle opening. Keep that mixture as lean as possible. On my old Concorde my foot learned the pedal positioning that the automatic unlocked the torque converter and it made a huge difference in instantaneous FE climbing a big hill. My suggestion would be a throttle stop once you learn the magic position, this could be a metal stop under the hood or as simple as a piece of 2x4 under the pedal.

I would not advocate treating the throttle as a switch. I think P&G will be very hard on your driveline.

[monroe74]

"if you have a wide band, closed vs open loop is off the mark"

I'm not sure what you mean. Do you mean that a wideband system is always in closed loop?

"you should be monitoring the actual A/F ratio"

I am. But I'm wondering if open-loop mode ever happens, and how I can detect it happening.

"You will hit a point somewhere that the A/F ratio will begin to richen with increased throttle opening."

But I don't really "hit a point somewhere." What I notice is that AFR goes from lean to rich in a fairly smooth, linear manner, all the way from closed throttle to WOT.

"Keep that mixture as lean as possible."

That means using very gentle throttle settings. My experience is that this leads to pumping losses, and hurts FE. I suppose it would hurt even more if this engine did not have lean-burn.

"My suggestion would be a throttle stop"

You're suggesting the opposite of what I found, that large settings are generally preferable to small settings. That's reflected in my mpg results (and those of others who do well with P&G, I think), but I didn't grasp why until I read this: http://www.autospeed.com.au/cms/A_110216/article.html

"I think P&G will be very hard on your driveline."

I think you'd be right if used WOT in the lower gears, but I don't. I'm mostly talking about low RPM in top gear. In my car, 2500 rpm in top gear means about 69 mph.

You'd also be right if I was causing pinging. But I'm not. Assuming a level road, I can be in top gear at about 25 mph (less than 1000 rpm), and use WOT to pull smoothly from there, with no pinging or lugging. My sense is that this motor is very happy to be driven this way. Periods of low revs alternating with periods of even lower revs (idling, or engine off).

I think I'm learning that low revs pay off, even when it means high throttle and rich mixture.

[Duffman]

OK I understand a little more know, I have never driven a car with lean burn before.
On a traditional FE car, the computer holds the A/F ratio at 14.7:1 (closed loop) up to a certain throttle opening, after this point the computer goes into open loop and disregards the O2 sensor. When in closed loop it maintains the 14.7:1 by alternating rich/lean, not by actually measuring 14.7:1.
There may not be a closed loop in the traditional sence for your car. I would try to record some data on A/F ratio vs throttle opening and post it here and seek some opinion. My previous post would apply more to a non lean burn car, I would need to see more info to provide any meaningful advice to your situation.

[Duffman]

To add to my previous, find the throttle position that you achieve 14.7:1, you do not want to be richer than that, that is where a throttle stop would be best.

[monroe74]

"There probably is no closed loop for your car."

I think you've got it backwards; it's in closed-loop most of the time (maybe 100% of the time). In other words, since the sensor has a wide band, the ECU can rely on it to produce useful, high-resolution readings even under conditions that are very lean or very rich.

"I would try to record some data on A/F ratio vs throttle opening"

I don't know quite how to translate the voltage readings into AFR. The way I've got the DMM hooked up, it shows 0 volts for stoich. It goes to about -0.8v for very rich, and to about +0.8v for very lean (in other words, I'm essentially using a reference voltage, instead of measuring the L1H1 output directly; I think this makes the numbers more intelligible). I can maintain a lean AFR if I use gentle throttle. Moderate throttle moves AFR towards stoich. Heavy throttle makes it rich. This all happens in a smooth, linear way.

Similarly, when I monitor the injectors I notice that the duty cycle ranges from about 1% (idle) to about 50% (WOT, high RPM). But under low RPM (say, 30 mph in top gear), the duty cycle won't exceed, say, 15%, even with WOT. This surprised me. And the duty cycle responds in a smooth, linear way, as the throttle opening is varied.

So I'm thinking that maybe open-loop is just never happening. And I'm pondering the relative merits of different throttle settings, and thinking that WOT is a very good thing, if used properly in a P&G context.

"find the throttle position that you achieve 14.7:1, you do not want to be richer than that"

I think I've found that large throttle settings are giving me high FE, even though they entail a rich mixture. It seems that pumping losses are expensive.

Anyway, what's magical about stoich (15:1)? If that ratio is good, isn't 17:1 or 20:1 even better? And my car can maintain those ratios, at cruising speed (e.g., 50 mph). But I think I'm learning that the pumping losses overwhelm the benefit of the lean burn. It seems that lean burn is terrific, assuming you're someone who doesn't want to fuss with P&G.

[brucey]

My main trip that I do regularly with the car is across the interstates of WV..

My best tanks have been me staying roughly 60 mph, and going for as long as possible while keeping the torque converter locked in 4th gear. (Automatic) after it unlocks, I keep the throttle in the same position, and only apply more if I get more than 20 under the limit. (Only happens on the steepest of hills)

Coming back down the hills, I put it in neutral right at the crest of the hill, gravity is usually enough to get me moving back up to the speed limit, and I put it in gear again going uphill whenever I'm back to about 60 mph. I know that it should be more efficient keeping it in gear, since theres no fuel, but on a small engine it can idle for how many hours per gallon? With no load holding me back, I've found my 2 ton car can coasts for ridiculous distances, I've been in neutral for over 2 miles before and could have gone longer, but I would have been going slower than 45 mph.

All this, and I've got to 34.4 mpg on a 26 EPA car. 40 might be attainable after all.

[PaleMelanesian]

- Manual trans with short gears = high rpm.
- Rolling hills, 100-200 ft up and down, over and over and over again.

My best over 150 miles was 75 mpg, using a 45-60mph pulse & glide, pulsing on the uphills. Pulse from just before the bottom, partway up, and then coast up and over the crest and down the back side. About 75% throttle on the pulse, in top gear (rpm 2,000 to 2,800).

[MetroMPG]

I can see the more we get into the intricacies of hill driving, the more the answer shifts towards "it depends" (TM).

Maybe as a rough guide, things could be clarified if we gave an example hill, eg. a small/medium bridge/overpass, with a moderate slope, no stop on the other side, absent following traffic. Ranking hill driving technique from worst to best might look like this:

1 (worst) - constant speed, or cruise control (or mash the throttle up, lift on the descent)
2 - constant throttle
3 - constant load (DWL)
4 - pulse up / glide down (engine off)

Missing from the discussion so far: I often read about people who do a modified version of DWL where they build additional speed on the flat before the hill, and then bleed it off on the incline. I've never tried to quantify it, but my guess is that it would slot in between 2 and 3.