Auto FE - It's all about driving style

[Scott_in_Tulsa]

I forgot to mention that this technique mimicks that of a roller coaster where you are trying to build enough momentum going down hill to carry you through to the top of the next hill.

As with roller coasters, sometimes you don't have enough momentum to carry you through the top of a higher hill or steeper slope and you have to add some additional energy i.e. fuel to carry you through to the top of the peak.

[dosco]

Originally Posted by theholycow

Scott, I'm curious why you chose pulsing downhill and gliding uphill. Is it because you can get a more useful pulse without causing a downshift?

In my truck I've been pulsing uphill and gliding downhill, just like I do in my manual transmission car. The theory is that the engine produces power more fuel-efficiently at higher loads, and that the wider throttle opening reduces pumping losses too. (Additionally, your speed is more steady, which helps with the difficulties of P&G.) However, with an automatic it's tough to achieve those advantages because it will downshift. My strategy has been working for me, but maybe I'll try yours too, to see if it works even better.

holycow - not sure what your background is, but in HS physics you may have learned about vectors. When a block is on an incline, the weight vector has to be broken down so that one component is in the same direction as the incline.

So if you're going up a hill, this vector is working against you, requiring the engine to perform more work to maintain a particular speed.

If you're going down a hill, this vector is working with you, requiring the engine to perform less work to maintain the same speed.

Extend this a little further to P&G. If you are pulsing up a hill, you are asking the engine to perform even more work that it would normally do during a pulse on level ground. Regardless of the BSFC stuff we've been discussing, the bottom line is that more work now needs to be done by the engine (esp compared to level ground), requiring more fuel, thereby impacting FE.

On the downhill, you're getting some work for "free" from gravity. That same pulse on level ground will now get you a higher road speed, hopefully netting a better glide.

BSFC in an automatic...I'm not convinced yet. I'll say it again: it seems more about minimizing RPM and fuel flow rate, and less about hitting the BSFC.

[theholycow]

Oh, no HS physics class is necessary to know that it takes more work to move a load uphill than downhill. However, regardless of when you pulse and when you glide, it still takes the same amount of energy to move the same weight over the same hills.

Pulsing downhill means a larger difference between your high and low speeds. Your high speed will be higher, and common knowledge is that waste from aerodynamic drag is not merely equal to the increase in speed but is exponential or some such. So, that, in addition to BSFC and pumping loss minimization (which in an automatic are difficult at best), are why I would assume that uphill pulse and downhill glide are the best.

Still, I can't argue with results, and Scott got results. Therefore I'll have to try it on my next tank.

[dosco]

Originally Posted by theholycow

Oh, no HS physics class is necessary to know that it takes more work to move a load uphill than downhill.

Sorry, wasn't trying to be a prick or anything, just never know about who knows what.

However, regardless of when you pulse and when you glide, it still takes the same amount of energy to move the same weight over the same hills.

Pulsing downhill means a larger difference between your high and low speeds. Your high speed will be higher, and common knowledge is that waste from aerodynamic drag is not merely equal to the increase in speed but is exponential or some such. So, that, in addition to BSFC and pumping loss minimization (which in an automatic are difficult at best), are why I would assume that uphill pulse and downhill glide are the best.

However increasing speed (say pulsing from 60 to 70 mph) when traveling uphill requires more energy than maintaining a constant speed uphill - correct? So why does it make sense to pulse uphill if the car has to expend more energy (i.e. use more fuel) to accelerate uphill?


Pulsing from 60 to 70 mph downhill would require less energy due to the conversion of potential to kinetic energy - correct?

Furthermore, the energy wasted to aerodynamic drag is going to be the same regardless of going uphill or downhill, right? So why not take advantage of the free work from gravity to offset the energy wasted to drag, rather than use more fuel on the uphill to work against both the incline and aero drag?

[theholycow]

Originally Posted by dosco

Sorry, wasn't trying to be a prick or anything, just never know about who knows what.

NP, I didn't think you were trying to be a prick.

However increasing speed (say pulsing from 60 to 70 mph) when traveling uphill requires more energy than maintaining a constant speed uphill - correct?

That's fine if you're only going up the hill, but as I said it takes the same amount of energy to move the vehicle through the same length of road whether you power up and coast down or power down and coast up. The same amount of work is being done; that's a basic and unalterable physics concept.

Pulsing from 60 to 70 mph downhill would require less energy due to the conversion of potential to kinetic energy - correct?

Yes, but you're going to gain the same amount of free work from gravity boost whether you're pulsing or gliding. In the meantime, you're going to lose the same amount of work going up the hill whether you're pulsing or gliding.

Furthermore, the energy wasted to aerodynamic drag is going to be the same regardless of going uphill or downhill, right?

Not in the strategy we're discussing. If your desired average speed is 60, and you have to do 75 downhill to maintain your average, that means you spend a lot of time doing 70 or 75 mph. If you pulse uphill and glide downhill, you can pretty much keep a steady 60mph. Since aerodynamic drag increases with the square of speed, it's unlikely that you make up for it when you get down to 50mph up the hill.

I can sit around and theorize all I want, applying knowledge that I don't entirely posess and just guessing at the math, but it doesn't change the results that Scott (and presumably you) get -- and no amount of knowledge beats experience, so I'll just have to try it.

[palemelanesian]

The idea of pulsing uphill is exactly about using the best BSFC. If you aim for the peak point (usually around 2000 rpm and 3/4 throttle) on level ground, you'll be going waaaaay fast in a short time. So you pulse and glide. The engine puts out the needed energy efficiently, and then drops to idle level, which is nearly zero.

If you have hills, you can drive up the hill at high load and then coast down, while maintaining fairly even speed. You're still pulsing and gliding, but without the speed variation. As stated above, this does also help with aerodynamics. You're using the engine at its most efficient point and building up potential energy in the form of elevation. Then you can use it up on the way down for (nearly) free. It's the same as P&G on the flat only without the extreme speed variations.

I think the pulse-uphill style works better with manual transmission, honestly. However, I took a 600-mile highway trip last week in the Odyssey (rated 18/25 old epa) and used a mild pulse-uphill coast downhill style. 28.5 mpg fully loaded with 6 people and camping gear and AC.

[dosco]

Originally Posted by PaleMelanesian

I think the pulse-uphill style works better with manual transmission, honestly. However, I took a 600-mile highway trip last week in the Odyssey (rated 18/25 old epa) and used a mild pulse-uphill coast downhill style. 28.5 mpg fully loaded with 6 people and camping gear and AC.

Could be splitting hairs, yes.

On my recent drive to NY (400+ mi), the "hills" that worried my were the Catskill Mountains. Pulsing uphill didn't seem to make much sense, rather, maintaining constant rpm was what I was concentrating on. I would pulse near the top to get back up to 65 mph or so and then glide downhill. The big downhills made for very long glides that undoubtedly boosted mileage.

[dosco]

Originally Posted by theholycow

Not in the strategy we're discussing. If your desired average speed is 60, and you have to do 75 downhill to maintain your average, that means you spend a lot of time doing 70 or 75 mph. If you pulse uphill and glide downhill, you can pretty much keep a steady 60mph. Since aerodynamic drag increases with the square of speed, it's unlikely that you make up for it when you get down to 50mph up the hill.

Yes, I agree that it depends on the speed range you're trying to work within.

I can sit around and theorize all I want, applying knowledge that I don't entirely posess and just guessing at the math, but it doesn't change the results that Scott (and presumably you) get -- and no amount of knowledge beats experience, so I'll just have to try it.

As I indicated to Pale, we could be splitting hairs here.

[theholycow]

Well, I filled up today and then attempted to try pulsing down and gliding up. Unfortunately, I won't be able to find out if it's more effective. It is not practical for me for two reasons:
- It can't be done if there's other traffic on the road, due to the buffer distance required
- I just don't have the patience even when there's not other traffic around

I did get over 1mpg improvement on my previous tank, doing nothing different than I did for that tank, and I can't confidently identify why. Take a look at my truck's gaslog...

[jcp123]

Originally Posted by philp100

jcp123- A newer tranny with OD and lockup would probably give your Mustang great highway mileage.

I don't doubt it. 2500rpm @ 60mph isn't a recipe for huge MPG. Maybe someday...getting the crossmembers and other hardware along with a shortened driveshaft is a bit much to contemplate at the moment...especially when the trans I have now only has 4800 miles on it.