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

"Hammering it up hills is way less efficient than coasting up it and getting back to 55 going down"

Well, this is the part that really confuses me, because lots of people talk about DWL, but I'm not sure it always makes sense. Let's say I can climb a grade at a steady 30 mph, with a small, steady throttle.

Let's say I can climb the same grade, at 70% throttle, which causes my speed to go from 30 to 45, as I travel up the grade. Of course, let's assume that there's no stop sign right over the crest of the hill. In other words, let's assume that my 45 mph momentum as I crest the hill will be fully used. (And assume constant top gear in both scenarios.)

For various reasons I've become convinced that B is more efficient than A. With a large throttle opening, I do a better job of converting fuel into kinetic energy, and then I can use that kinetic energy for a nice long EOC after I crest the hill.

I've become convinced that the essence of effective P&G is to consistently avoid small throttle openings. Trouble is, this seems contrary to DWL doctrine.

I've been pondering this for a while, somebody who lives around hills may have a better Idea. Should this become a new thread?

Sure - it's good new thread material.

Pulse up and glide down makes the most sense at lower speeds or on steep hills.

At higher speeds where aero losses make a glide on the downside not worthwhile (or if the downgrade isn't steep enough), then I'd say driving with load will win out.

Driving with load doesn't always mean keep a steady throttle up a hill, it's keep a steady throttle, and if it's big enough letting off the gas and bleeding off speed until you get to the top, then gaining speed down.

If your idling up the hill and hammering down it, it'd be more efficient than the other IMO

If you know you're not going to have to brake on the downside, you're best bet is probably 80% throttle in top gear. Much more than that, you'll be in open loop mode (rich).

I've found DWL to be the most efficient for me, but I drive in mountains, not hills. I keep a constant RPM going up the hill/mtn, when possible, and don't worry about the speed dropping off. Often this opens the throttle more anyways. If you zoom up and coast down, then you win up breaking a maximum or minimum speed limit by a lot....

I live in an area that is basically a drumlin field, which means we have many many hills about 150 to 200 ft tall. The roads cut through them abit but it I think the longest perfectly flat stretch of road on my commute is about 2 miles. But most of the time you are going up and down.
So I pulse and glide with the hills which keeps me with traffic if there is any, and anyone behind me probably has no idea I'm coasting alot of the time. On a few hills I am speeding 25km/h over coasting in neutral. Also with my neon, 50% throttle will get me up the hills at 100km/h and then I coast down to 80km/h and then gas it up the next hill. For me the hills allow me to pulse and glide while maintaining speeds between 80 and 100km/h which is reasonable to do in light traffic.
I'd think slowing as you climb a hill is a bit counter productive as it keeps your engine in the inefficient low throttle opening zone for longer. The gravitational energy you have gained is equal at the top of the hill wether you idle up or use %50 throttle. In high drag vehicle I'd guess you want to always stay slow but for a streamline car probably you want to stay at 50 to 70% throttle all the way up and then coast down.
Ian

Thanks, these are very helpful comments, and they make sense to me.

What still bugs me is this statement: "... you will back off the accelerator and lose speed ... It's far more efficient than pressing the accelerator more and more to maintain speed on the way up a hill and then releasing it down the other side."

(From here, item #55: http://www.ecomodder.com/forum/EM-hy...ecodriving.php)

I think the problem with that statement is it encourages people to use a small throttle opening, in a situation when a large throttle opening might be more efficient.

I realize people have pointed out some important exceptions, where the larger throttle, and higher speed, isn't a good idea. Like if you end up having to stop prematurely, after you crest the hill. Or if you're building up so much speed (e.g., over 50 mph) that aero resistance starts to be a real problem. Or maybe there's so much speed that it's unsafe or illegal.

But if conditions like that are not present, then I think #55 is perhaps a little misleading. Unless there's some other aspect of this that I'm not grasping.

The most important exception is: if you're not going to glide, engine off, down the back side of the hill, then the least fuel will be consumed overall if you DWL up the front side.

Remember: best BSFC doesn't necessarily equal lowest overall fuel consumption, it means most power produced per unit of fuel consumed.

That statement applies to traversing hills but not valleys. That is you climb first and coast second. Steep declines will accelerate the car providing free energy. Every slope has a point where gravity = Drag at a certain MPH. If you approach the decline slow, the hill will accelerate you more MPH until you hit the equilibrium and you waste less energy on the climbing side.

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.

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.

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.

"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.

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.

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.

"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.

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.

- 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).

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.

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.

#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.

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.

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.

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.

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.

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.

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.

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.

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.

I achieved my highest trip MPG on a stretch of hills in western Ontario (Map Here if interested). Accelerate down the hills and glide up. The engine is idling up the hill, and the load on the engine is reduced when powering down hill.

duff: "What I am trying to get you to do is find the transition out of the red region."

I think that's what I'm also trying to get myself to do! Trouble is, I think I don't really know how to get all that nice data with the simple instruments at my disposal. Maybe if I think about it some more it will dawn on me. I guess I don't really have a clear grasp of the methodology used to create those graphs.
----------------------
metro: "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."

Good point. I didn't think of that. I suppose P&G starts to fall down when it gets into the aero-penalty zone.

On a recent tank when I did very well with P&G, speeds were very moderate (under 50 mph). And I've just started exploring P&G.

"the beauty of lean burn is really good fuel economy without having to do the work"

Yes, exactly. I like the idea of fully grasping the different capabilities the car has. Then I can use it in various ways, depending on my mood, and the conditions. Sometimes the desirable choice will be to relax and rely on lean burn.

pale: "P&G in the VX means you have a regular, non-lean-burn civic, only lighter weight and with better gear ratios."

Yes, well-said.

"P&G is a lot of work for longer distances"

My recent P&G tank was all short trips, local driving. I imagine P&G could be burdensome on a long highway trip.

I'm finding it interesting to try to assess different factors. For a few weeks, I had zero lean burn, because my car was the CA model. Then I converted it to Federal, and did a few fills that way. Now I'm learning about P&G.

At this point my routine doesn't require a lot of miles, so it's taking a long time to measure results, and compare the different options.
-------------------------
fig: "Accelerate down the hills and glide up"

Doesn't that create a problem with excessive speed, going downhill? I suppose maybe it wouldn't, depending on conditions.

How serious do you want to get (meaning cost & effort). There is a lot of expertise on this board, you just have to ask the right questions. Off the top of my head, the throttle position sensor (you car should have one?) gives a 0-5V reading based on the throttle opening. You already have the wide O2 sensor hooked up. Will this give you a BSFC map? No, but you can map out the A/F ratio which may help find efficient vs inefficient operating regions.

Probably any other place on earth it would be a problem. This particular stretch has some especially favourable conditions: Zero traffic, gorgeous scenery and a variety of hills and twisty turns. It was a very fun drive, which of course makes it easy to stay focused on the road. Speeds probably ranged from 140kph in the valleys, and 90kph at the crest of the hill. 44.7MPG on that stretch in my 2005 Hyundai Elantra.

You seem to have some presumptions and comments which I would like to make sure you are aware of, prior to my feedback on my experiences, in my car.

When you are commenting, you seem to presume that you are in as high a gear as possible, going up hill, that you are able to maintain your speed and with a throttle opening which is less than 70-80%. Your question is further really restricted by the fact your car has a wide band oxygen sensor, but your question is about what seems to you to be conflicting information and which you have not been able to clearly discern an absolute answer to, for your car.

Presuming my understanding of your presumptions is correct, I am offering this. I am driving a 89 Honda Wagon, 5 speed. At one time I had a voltmeter wired into my oxygen sensor, which is narrow band. What I found was that on my car it would stay in closed loop, monitoring the oxygen sensor, until about 80-85% throttle. At that point, it would go into open loop and the oxygen level would decrease substantially.

If I can keep my speed up, keep the car in 5th gear and keep the throttle below 80% or so, then I use less fuel. However, if I have to drop down a gear or increase the throttle beyond closed loop, then my mileage goes down, drastically.

Consequently, where you are looking for a hard and fast rule, their are probably not a lot. However, if you can keep it in leaner burn mode and you can keep your rpm down, you are going to get better mileage than if you can't. Whether you can, what speeds you can, whether you can keep it in 5th, how steep the hill is, how long the hill is are all subjective issues which are largely only relevant to you, your car, where you are driving and the conditions.

I would focus more on trying to keep the your engine in more of it's lean burn mode, whatever that entails. If it's leaning towards rich, then you are going to using more fuel.

If I can go up a hill, keeping it in 5th, with below 70% throttle I can get better mileage, even if I might lose a little speed. If I can coast down a hill and pick up speed, while driving safely and reasonably for the conditions, then I can use that energy to help get me up the next hill, which will save me some fuel. How much, I don't know, yet, but some.

What about gliding up the hill and pulsing down it? Not long drawn out hills, but quick, steep hills (like the ones in IL)?

If you coast up the hill, you use the same amount of fuel as when idleing, and if you get to the top and mash it down, you have gravity on your side, and less fuel is consumed than mashing it up against gravity, and then for maybe 3 seconds of EOC won't make up the fuel consumed going up the hill.

Manual transmissions are a whole different subject :thumbup:

duff: "you just have to ask the right questions"

Then maybe I should start with this question: what's the right question that you think I need to ask that I haven't already asked?

"you car should have one?"

Yes, my car has a TPS.

"you can map out the A/F ratio which may help find efficient vs inefficient operating regions."

I don't know what you mean.

The trick is to create a BSFC map, or the equivalent, without the use of a dyno. I don't know how to do that, but maybe you do.

To assess "inefficient operating regions," I have to do more than just know the throttle position, and the AFR. I also have to have the engine under a known load, so I can maintain a steady state and monitor how much work is being done per unit of fuel consumed. Dynos were invented to solve this sort of problem.

fig: "gorgeous scenery ... very fun drive"

That's exactly what I thought when I looked at the map. Sounds like fun.

whoops: "if you can keep it in leaner burn mode and you can keep your rpm down, you are going to get better mileage"

Thanks for jumping in.

Actually, I think I'm finding that I do better if I sacrifice lean burn in order to minimize pumping losses. Although the situation could shift at highway speeds, as metro astutely noticed.

"What I found was that on my car it would stay in closed loop, monitoring the oxygen sensor, until about 80-85% throttle. At that point, it would go into open loop and the oxygen level would decrease substantially."

As you pointed out, a fundamental difference between my car and yours is that I have a wideband sensor. I tend to believe that this means I don't suffer from the same kind of non-linear response that you just described. In other words, WOT doesn't trigger open-loop, or if it does, the ECU is still managing things in a moderate way, so there is no threshold where suddenly an extra penalty kicks in.

I'm more convinced of this as I monitor my injectors. As I move the throttle from 50% to 100%, the injectors respond linearly. In other words, there seems to be no threshold where they suddenly decide I need lots of extra fuel.

The injector duty cycle seems to be a direct function of throttle position and RPM (i.e., vehicle speed and other factors seem irrelevant). In any gear, WOT at very low RPM (say, 1000) yields a quite modest duty cycle of roughly 10%, which is only about 10 times greater than idling. WOT at 2000 rpm means a duty cycle of about 20% (the numbers are conveniently consistent, that way).

I have the idea that idling costs about 0.2 gph. So my rough calculation is that WOT at 1000 rpm is costing me 2 gph. In my overall P&G routine, WOT is only a portion of total elapsed time. Anyway, these rough numbers give me a framework to try to assess how I'm doing.

diff: "What about gliding up the hill and pulsing down it?"

I tend to think that the two approaches are equivalent, as far as pure, theoretical energy calculations are concerned.

As a practical matter, I think it usually makes more sense to pulse going uphill, because this helps minimize the differential between your maximum and minimum speed. A big differential could lead to issues with aero drag, laws, safety, and other drivers.