Why is 'driving with load' better than constant speed

[NachtRitter]

Originally Posted by bikenfool

Method 2. Constant speed, begin a coast before the top of the hill so the speed at the top is the same as for the DWL case. This takes the descent out of the equation since you could use the same method (glide) for either case on the down side. The speed on the flat section will be slower for this case than for the DWL case since you don't lose time on the climb.

I'm still puzzled by your use of the phrase "Constant Speed" when the sentence following seems to clearly indicate that the speed is not constant. Can't be going constant speed if you begin a coast before the top of the hill.

[bikenfool]

Originally Posted by NachtRitter

I'm still puzzled by your use of the phrase "Constant Speed" when the sentence following seems to clearly indicate that the speed is not constant. Can't be going constant speed if you begin a coast before the top of the hill.

Sorry, constant speed right up to the coast. Coast in neutral.

[NachtRitter]

When you get your script done, post up the code and the results. Be sure to allow for different BSFCs, vehicle weights, vehicle cDAs, vehicle RR, etc so we can see how those affect the overall results.

I'd be interested to see how my Diesel compares to a gasoline vehicle. However, any simple simulation that isn't calibrated to the real world wouldn't be worth the time put into it.

Alternatively, run a few "real live" tests in your own car on your own route to validate the effectiveness (or ineffectiveness) of DWL vs maintaining a steady state speed up the hill and then coasting before the top. Use the A-B-A testing methodology described in this post: http://ecomodder.com/forum/showthrea...ery-11445.html. However, rather than testing an aero mod by doing A = with mod and B = without mod, you can select A = DWL uphill and B = maintain speed uphill. Obviously, instrumentation such a ScanGauge would be required.

With so many variables, anyone else's results won't necessarily be applicable to your car, your route, or your style of driving.

[MetroMPG]

Originally Posted by bikenfool

Method 2. Constant speed, begin a coast before the top of the hill so the speed at the top is the same as for the DWL case. This takes the descent out of the equation since you could use the same method (glide) for either case on the down side.

I think I see the problem may be one of definitions.

"Method 2" isn't constant speed. Not the way I define it, anyway. As soon as you introduce gliding, it's something else.

Constant speed is the equivalent to driving with cruise control on: Power uphill and engine braking downhill. Your method 2 doesn't describe that approach; the downhill glide effectively makes it pulse & glide, and we already know P&G is more efficient than driving with load.

[t vago]

Originally Posted by bikenfool

Enough talk, lets see some analysis.

You go first.

Originally Posted by bikenfool

My brief search didn't show up any open source simulations, but it seems like there's probably something out there somewhere. Perhaps some government project. It wouldn't be trivial, but it wouldn't be a huge task to code something up in matlab or octave. If someone knows of a sim out there point me to it. It doesn't have to be very complicated. This simple case could probably be done with some fairly simple script, but I won't be able to try it for a while. I'm sure one of you guys already have the tools ready to go

Translation: I can't be bothered to prove my own assertion in the face of cogent arguments to the contrary, either with real world experience or with doing any research outside of trolling for answers in this thread, so you do the work for me!

Look - you have to add potential energy to go up a hill. That energy either comes from your kinetic energy store (i.e., you slow down), or from your store of fuel. You have to lose potential energy when you go down that same hill. That either goes back into your kinetic energy store (i.e., you speed up), or gets wasted as braking.

[bikenfool]

NachtRitter, The post by MetroMPG on testing was very good. I'm skeptical whether one can get an accurate test without quite a few runs or some really good instrumentation to measure the fuel used. I'm not sure a scanguage would give reliable MPG numbers when changing the driving style so much. It might still be work doing tho.

MetroMPG, Nice post on testing by the way. Method 2 isn't pure constant speed, but Method 1 isn't pure DWL either. Method 2 is mostly constant speed with a little pulse at the end. Then both methods glide.

t_vago, For something that is taken as gospel by the ecomodder forum I would think there would be some proof or testing to show the effectiveness of it. I have seen arguments, but but not convincing. Science and engineering is rarely accomplished with just words. You don't have to prove your case & I don't have to prove my case, you can have your beliefs and I can have my skepticism.

Perhaps you guys are right and I'm an idiot, perhaps it works for some cars, or some conditions. For now, I remain skeptical.

[t vago]

Originally Posted by bikenfool

t_vago, For something that is taken as gospel by the ecomodder forum I would think there would be some proof or testing to show the effectiveness of it. I have seen arguments, but but not convincing. Science and engineering is rarely accomplished with just words. You don't have to prove your case & I don't have to prove my case, you can have your beliefs and I can have my skepticism.

What's there to take on faith? It's simple physics! Simple conservation of energy dictates that if you keep the total amount of energy constant in a system, you'll have to slow down if you rise in position. Conversely, if you lower your position, you'd have to speed up.

[NachtRitter]

Okay... after a page and then some of back & forth, I think I understand what you're trying to say, bikenfool, and now that I understand it I think your question is valid.

The most difficult thing to get past for me was your use of the phrase "Constant Speed". But if I look at your wording here:

Originally Posted by bikenfool

Method 2. Constant speed, begin a coast before the top of the hill so the speed at the top is the same as for the DWL case.

... what I realize is that you are describing Pulse & Glide. Granted, the way P&G is typically described is that it is done on a flat/level section of the road, but the idea is the same whether on an uphill or on a flat: increase the load so that the energy conversion from fuel to HP is more efficient, then when you reach a particular target you switch to a glide (engine on or off). With traditional P&G, your target may be particular speed at which point you begin your glide; with your Method 2 approach, you maintain your speed to a point where your glide would take you to the top of the hill (and where your speed at the top of the hill would be the same as the speed you would have if you had used DWL). While your speed does not increase as you go up the hill, you are accelerating (the acceleration on the flat ground will result in greater speed, whereas overcoming the force of gravity (which for the engine is indistinguishable from acceleration on the flat ground) will maintain a steady speed on the uphill).

So I would say the correctly worded methods to compare would be:
- Method 1: DWL up the hill
- Method 2: P&G up the hill (where the "pulse" is just to maintain your speed, and your glide starts where the target speed at the crest of the hill is equal to the speed at the crest using DWL)
(both methods assume that the measuring points are from the base of the hill to the crest of the hill)

Now, I think, you have a valid comparison, or at least one where the answer is not so clear cut. I think it depends a lot on the car and on the hill. For example, with my Honda VX it was not even possible to maintain a constant speed up the ~1 mile grade that is near my house unless I went full throttle (full enrichment). DWL was definitely more efficient in that case. With my TDI, I can easily keep a constant speed up, and I will typically do the "pulse" (maintain steady speed) up the hill, begin my glide before I reach the crest, and then continue my glide down the other side of the hill. I haven't tried to do a comparison on the same hill using DWL with the TDI, but it might very well be possible that the pulse uphill plus a coast before reaching the crest might be comparable to DWL under certain conditions.

[bikenfool]

NachtRitter, Sorry if I haven't always been clear. You can call it P&G if you want. Yes, you are increasing the load on the engine on a climb, much like a pulse, but it is still constant speed. I think a correct description of method 2 is constant speed with a short glide at the end (to the top of the hill).
One important point about the difference between method 1 and 2 is that the constraint is constant travel time, so you have to include a flat section, and the DWL method will have to be at a faster speed on the flats to make up the lost time in the climb.

Minor nit: you are misusing the term acceleration, "In physics, acceleration is the rate at which the velocity of a body changes with time" https://en.wikipedia.org/wiki/Acceleration. Of course, you are correct the engine doesn't know the difference.

Originally Posted by NachtRitter

While your speed does not increase as you go up the hill, you are accelerating (the acceleration on the flat ground will result in greater speed, whereas overcoming the force of gravity (which for the engine is indistinguishable from acceleration on the flat ground) will maintain a steady speed on the uphill).

[t vago]

Originally Posted by bikenfool

Minor nit: you are misusing the term acceleration, "In physics, acceleration is the rate at which the velocity of a body changes with time" https://en.wikipedia.org/wiki/Acceleration. Of course, you are correct the engine doesn't know the difference.

In physics, speed is commonly understood to be the scalar component of the velocity vector. The acceleration vector is understood to be the derivative of the velocity vector. So, in strict terms, one is in fact experiencing acceleration along different axes, when one is going uphill while DWL. There is a positive impulse on the vertical axis as the vehicle transitions from flat to incline, whicle then becomes negative as the vehicle loses speed. The impulse is as gradual as the transition. And in physics, impulse can be thought of as a function of acceleration, as it is defined as the integral of force (mass times acceleration) over time.

Constant speed is different - There's one positive impulse of vertical acceleration as the vehicle transitions from flat to an incline, and then another negative impulse as the vehicle transitions out of the incline.