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

[PaleMelanesian]

Originally Posted by MetroMPG

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)

I agree with this.

I think we need to specify for manual or automatic. The answer may be very different in these two cases. I would do more DWL with an auto, but pulse up / glide down in a manual.

[Duffman]

Originally Posted by 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.

#1, you caught me editing my post.
#2, I did some searching and learned a bit about wide band sensors myself, here are 2 good links:
http://www.airfuelmeter.com/german/lsu4_ge_sensor.htm
http://www.gmtcny.com/WBO2S.htm
Pull the map off the second link.

I thought you were trying to optimize a P&G scheme to reduce pumping losses? Regardless if you look at a BSFC map of nearly every gasoline engine FE is never at a maximum at WOT, the 80% that Johnny told you would be a good place to try. You do want to be lean of stoich, but if you engine behaves anything like a non-lean burn car there will be a point that it will move into the rich region, that should be after the 80% throttle mark. At stoich there is enough O2 to burn all the fuel if it mixes 100% perfectly (which it doesn’t), once you move into the rich region it is physically impossible to burn all the fuel without even considering mixing and burn characteristics.

[monroe74]

duffman: "Pull the map off the second link."

Those are interesting links, thanks.

I think I can't really use that map directly, because it's showing current, not voltage. I'm reading voltage. I suppose if I was less electrically illiterate I could measure the resistance of something and use Ohm's Law to do the conversion. But I think I don't really need to know the precise AFR, expressed as a ratio. I think it's enough to know if I'm lean or rich, and I can precisely monitor (at least in a relative sense) the gradual change from one extreme to the other.

"I thought you were trying to optimize a P&G scheme to reduce pumping losses?"

I'm trying to better understand the tradeoffs between light throttle (which has the benefit of lean burn) and WOT (which has the benefit of minimal pumping losses). I'm also very intrigued by the possibility that WOT is optimal, and better than 70-80%, despite what I've heard about the penalties of open loop. I'm generally intrigued by the possible benefits of WOT, especially because I don't see it mentioned much as a FE technique. It seems so counterintuitive.

"Regardless if you look at a BSFC map of nearly every gasoline engine FE is never at a maximum at WOT"

Metro made a very good point, that optimal BSFC is not quite the same thing as lowest overall fuel consumption. But aside from that, I don't know your basis for claiming that WOT does not optimize BSFC. The article I cited earlier has a helpful graph:

http://www.autospeed.com.au/cms/gall...0&a=110216&i=6

That graph indicates that WOT does indeed optimize BSFC. That's why the text says this: "At 100 percent load (ie wide open throttle) this engine has a minimum SFC."

But then I start to wonder if open-loop is still an issue, somehow. But consider the following observation. I'm climbing a grade at a steady 30 mph. Top gear, about 1200 rpm. WOT. The injector duty cycle is about 10%. That seems to be not very high, and the change, as I open the throttle more and more, was quite linear and gradual. So I'm getting the sense that I never hit open loop, even at WOT.

"the 80% that Johnny told you would be a good place to try"

I'm having a hard time finding objective proof that 80% is better than 100%, especially given what's stated in that article.

"there will be a point that it will move into the rich region, that should be after the 80% throttle mark"

Actually, the transition from lean to rich comes much sooner than 80%. It happens at about 25%. So I'm basically comparing the following two scenarios:

A) Cruise at a steady 55. RPM is about 2000. Throttle is about 10%. AFR is quite lean. Definitely in lean-burn mode. I haven't checked the injector duty cycle in this scenario, but I figure it's about 3-5%.

B) Use P&G, from about 45 mph to 60 mph. WOT, whenever the throttle is used. During WOT, AFR is very rich. Injector duty cycle is about 14-17%. (That's also kind of a guess; I haven't exactly measured that yet.)

I guess what I'm realizing is that I can basically evaluate A vs B by comparing the two injector duty cycles, and seeing how that ratio compares to the P:G ratio (i.e., the amount of time I spend in WOT, as compared with EOC, or neutral-coasting).

But I'm becoming convinced that P&G wins, and that WOT is optimal, and that lean burn should be given a chance to happen only when for some reason P&G is simply not an option.

"once you move into the rich region it is physically impossible to burn all the fuel"

Right. But I think the pumping losses offset this. In other words, WOT does such a good job of cutting pumping losses that it's worth using a rich mixture that doesn't burn all the fuel. Or at least that's my naive theory, at this point.

[PaleMelanesian]

This image: http://www.autospeed.com.au/cms/gall...0&a=110216&i=9
is from the same article. I follow that one, and keep my throttle to 80% or so maximum. I don't have lean-burn, but in my experience, P&G absolutely kills anything else for mileage.

[MetroMPG]

Originally Posted by monroe74

I'm also very intrigued by the possibility that WOT is optimal, and better than 70-80%, despite what I've heard about the penalties of open loop.

I'll just chime in an say that I also have doubts that the "rule" against going into open loop during P&G applies to all cars.

I've never spent much time watching loop status on the ScanGauge, except when I was initially curious to see how much time it took to switch over after a cold start. (Answer: in the depths of winter, no more than about 30 seconds).

But a couple of weeks ago, I called up that gauge while I was doing my usual P&G routine, and to my surprise, when accelerating I was in open loop most of the time! The transition point was much shallower than I had assumed.

I haven't tried to compare the difference between P&G with open vs. closed loop, but I will say that for my car at least, it doesn't appear to be a massive penalty. Consider the 104 mpg fill-up I saw last summer was using mostly open loop pulses (based on what I saw recently, using similar technique).

It definitely bears looking into.

[PaleMelanesian]

Interesting. Mine also goes into closed loop almost immediately. It won't go Open again unless Lod is above 95%, sometimes higher. So I'm almost always in closed loop. Again I say, interesting.

[Duffman]

Originally Posted by monroe74

"the 80% that Johnny told you would be a good place to try"

I'm having a hard time finding objective proof that 80% is better than 100%, especially given what's stated in that article.

"there will be a point that it will move into the rich region, that should be after the 80% throttle mark"
I guess what I'm realizing is that I can basically evaluate A vs B by comparing the two injector duty cycles, and seeing how that ratio compares to the P:G ratio (i.e., the amount of time I spend in WOT, as compared with EOC, or neutral-coasting).

The link that PaleMalasian posted was exactly what I needed:
http://www.autospeed.com.au/cms/gall...0&a=110216&i=9
The red region of 0.42 @2000 rpm is the optimum island here but more power is available above 120 BMEP up to 140 BMEP. What is happening here could be a couple of things: Richening of the mixture or maybe a reduction of timing but I place my bets on richening. What I am trying to get you to do is find the transition out of the red region. Obviously it is not as black and white (this case red & yellow) as a step, but plotting some data might show a bend in the mixture rate.

[monroe74]

pale: "This image ... is from the same article"

Good point. And that image suggests that something like 80% beats WOT. But maybe that image is based on an engine that goes into open-loop, past a certain threshold.

metro: "in the depths of winter, no more than about 30 seconds"

I don't have an SG, and the weather is warm now. But I'm using a DMM to monitor what the O2 sensor is doing, and it seems that upon a cold start it only takes about 20 seconds to warm up and start operating. Similar to your observation.

"Consider the 104 mpg fill-up I saw last summer was using mostly open loop pulses"

My guess is that the penalty for open-loop can be small or non-existent, because it depends on the values in the table the ECU is using. So it will depend on conditions, and it will depend on the kind of car you're driving.

I think it's interesting to make these discoveries which test conventional wisdom.

Conventional wisdom says lean burn is a very important part of how the VX does so well. I think lean burn in the VX can be very valuable, to certain drivers under certain conditions. But I also suspect that the optimal way to drive the VX is WOT, which means minimizing lean burn.

pale: "I'm almost always in closed loop"

You and metro both have SG. One of the many nice things about it is you have an easy way to know if you're in open-loop, or not.

With my OBD1 vehicle, it seems to be much harder to get an objective definition of that state, and how to detect it. But maybe it doesn't matter much.

[MetroMPG]

Originally Posted by monroe74

Conventional wisdom says lean burn is a very important part of how the VX does so well. I think lean burn in the VX can be very valuable, to certain drivers under certain conditions. But I also suspect that the optimal way to drive the VX is WOT, which means minimizing lean burn.

Here's my guess: at higher speeds, lean burn permits a VX to get better MPG than you could at the same average speed doing P&G. 60 mph, for example.

At medium/lower speeds, where the aero penalty during the glide is less, you're likely right: P&G in your VX is probably where the most extreme MPG numbers can be had.

But the beauty of lean burn is really good fuel economy without having to do the work. If I had a VX or Insight, I would probably just DWL at highway speeds for the convenience.

[PaleMelanesian]

P&G in the VX means you have a regular, non-lean-burn civic, only lighter weight and with better gear ratios. Good stuff. But you also have the option to use lean-burn sometimes. Even better stuff!

And Metro, you're right about convenience. P&G is a lot of work for longer distances.