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

[Figjam74]

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.

[monroe74]

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.
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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.
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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.

[Duffman]

Originally Posted by monroe74

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.

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.

[Figjam74]

Originally Posted by monroe74

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.

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.

[Whoops]

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.

[DifferentPointofView]

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)

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

[monroe74]

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.

[monroe74]

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

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

[monroe74]

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.

[monroe74]

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.