Got my Scangauge-pretty eye opening

[bkrell]

As I was just posting somewhere else, it's amazing how insightful this little doo-dad is. Oh man, all I can say is I'll never use cruise on anything but level ground again. Watching it on the scangauge, the difference was when it was on, going uphill I'd drop to around 9 mpg even though I was still maintaining vac and not slipping to turbo boost. Off, just letting the car lose a few mph, I was able to keep the instant miles in the high 20's/low 30's. Unreal.

[monroe74]

That's very interesting. Is your car automatic or a stick? If the former, did it switch to a higher gear when you eased off the throttle? Just curious.

[fumesucker]

Originally Posted by bkrell

Watching it on the scangauge, the difference was when it was on, going uphill I'd drop to around 9 mpg even though I was still maintaining vac and not slipping to turbo boost. Off, just letting the car lose a few mph, I was able to keep the instant miles in the high 20's/low 30's. Unreal.

I've noticed that effect too.. Efficiency plummets when you try to keep a constant speed going uphill. I've found that gradually slowing going uphill, a slight speedup when starting downhill then coast until near the bottom and an acceleration near the very bottom or on the flat between hills seem to boost overall efficiency the most.

Some kind of "adaptive cruise control" that would read engine load and efficiency as well as speed would be very useful.

[RoadWarrior]

Beware the instantaneous MPG leading you astray, it's not as efficient to accelerate at 23mpg for 2 miles when you could accelerate at 15mpg for 1/4 mile to get up to your 28mpg cruise....

.01666g +.0625g < .0869g

[bkrell]

Originally Posted by monroe74

That's very interesting. Is your car automatic or a stick? If the former, did it switch to a higher gear when you eased off the throttle? Just curious.

auto. Stayed in 4th (O/D) the whole time.

[bkrell]

Originally Posted by RoadWarrior

Beware the instantaneous MPG leading you astray, it's not as efficient to accelerate at 23mpg for 2 miles when you could accelerate at 15mpg for 1/4 mile to get up to your 28mpg cruise....

.01666g +.0625g < .0869g

Right, but I was just talking about hills and momentum. Good point, though

[monroe74]

Originally Posted by bkrell

auto. Stayed in 4th (O/D) the whole time.

Thanks for answering the question.

Road is making a good point. Reading instantaneous mpg can be fundamentally misleading. That's because the SG (or similar instrument) is taking into account how much distance the car is covering, but it's not taking into account work that's being added to the vehicle, in the form of potential and kinetic energy. That energy will pay you back later. And it's also not taking into account the duration of the energy use (which is basically what Road was saying, I think).

That's why it's important to make comparisons based on an entire trip, and not just a series of instantaneous readings.

And these issues are separate from other accuracy issues that come up with something like the SG, associated with the fact that it's not directly monitoring the injectors.

It's still a great tool, but it helps to understand the limitations.

[bkrell]

Right, but I'm not talking about prolonged situations. I'm talking about dropping a few mph on the upslope instead of maintaining constant speed. Doesn't that make sense or am I wrong?

[monroe74]

Originally Posted by bkrell

Doesn't that make sense or am I wrong?

Good question. I don't know. What I'm saying is that the math is deceptively complicated.

Here's an example of what I'm talking about. Consider this comparison:

A) Travel up the hill at a steady 25 mph
B) Travel up the hill at a steady 30 mph

Contrast that this with comparison:

C) Travel up the hill at about 24-26 mph, but with the speed slowly dropping
D) Travel up the hill at a steady 30 mph

When an instrument like the SG looks at C, it's going to give you a deceptively optimistic result. Why? Because it's calculating mpg based on fuel use and distance, but it's not taking into account the fact that you're losing speed. The momentum you're losing was paid for previously, by gas you burned a little earlier. And now you're relying on that momentum, instead of fuel, to propel the car. That's why the car is slowing down.

The reverse happens when you're accelerating. The SG will give you a deceptively pessimistic result. It's going to tell you that you're burning a lot of fuel relative to the distance you're traveling, but it's failing to take into account that some of the fuel you're burning is being used to create momentum in the vehicle. It's like putting money in the bank. In a few moments, you'll be relying on that momentum.

When you add speed, you're adding kinetic energy, and when you climb a hill, you're adding potential energy. Either way, you get this energy back later. But the SG is not taking this into account, if you're just looking at an instantaneous reading. That's why you have look at a broader interval of analysis, like a trip.

[bkrell]

Gotcha. I already have a resettable trip gauge on the car's computer. This is what I've always relied on. I understand what you're saying, but my question is this...as long as I am staying in OD gear, wouldn't it still make better sense to use momentum even if you get deceleration that you have to make up somewhat on the downslope? I mean, aren't you loading the engine more by keeping steady speed on the upslope?

I barely passed physics. That's why I am an archaeologist.

[samandw]

But to look at it another way. Assume you have a hill to climb. Our test starts and ends at the same elevation. For the sake of argument, let's examine two scenarios:

#1: Constant speed 60 mph, we make 17 mpg going up the hill and 38 mpg going down.
#2: Constant TPS, say we slow to 55 mph at the top of the hill, and accelerate back to 60 mph by the bottom. Let's say we can maintain 26 mpg going up, and get 35 mpg going down.

In scenario #1 we average (17+38)/2 or 27.5 mpg at 60 mph.
In scenario #2 we average (26 + 35)/2 or 30.5 mpg at say 58 avg mph.

These are roughly the numbers I was getting on a trip I took in a rented Pontiac G6 GT, experimenting with constant speed vs constant TPS. I was able to get the total average for the trip up to 32.5 mpg by the time I finished my trip, using the constant TPS approach. It's rated at 18/29.

[monroe74]

Originally Posted by bkrell

as long as I am staying in OD gear, wouldn't it still make better sense to use momentum even if you get deceleration that you have to make up somewhat on the downslope?

Not necessarily.

aren't you loading the engine more by keeping steady speed on the upslope [compared with letting the car slow down]?

Yes, but that can be a good thing. Assuming you're not downshifting, i.e., you're using low revs, then a gas engine is typically more efficient when operating with a larger throttle setting. That doesn't mean it's using less gas, as measured absolutely. It just means it's using less gas per unit of work produced.

More on this subject: http://autospeed.com/cms/A_110216/article.html

I barely passed physics. That's why I am an archaeologist.

I have a lot to learn about both subjects, so you're probably ahead of me, on balance.

[bkrell]

Right, for me, the numbers were silly-like 9 uphill then 55 mpg or so downhill. The downhill didn't change much when I dropped the speed and got close to 26 mpg uphill. So then would it just boil down to total trip time?

[bkrell]

That was a response to the previous post by samandw

[bkrell]

Originally Posted by monroe74

Yes, but that can be a good thing. Assuming you're not downshifting, i.e., you're using low revs, then a gas engine is typically more efficient when operating with a larger throttle setting. That doesn't mean it's using less gas, as measured absolutely. It just means it's using less gas per unit of work produced.

Well, see, I had figured on this too. Does the fact that I have a turbo car make a difference? B/c the less vac I make the more fuel I seem to use....

[R.I.D.E.]

Here you go.

Your mileage only reflects your distance, not your elevation.

An 8% grade 8 feet up per hundred, you are travelling 100 feet per second horizontally.

You are lifting your car using the road as a lever, exactly 8 feet per second.
2200 pounds (car weight) times 8=17600 pounds (one foot every second)divided by 550=32 horsepower

That is what you are storing per second for as long as you climb.

Whats really sweet about it is you can get it back, when you go over the top of the hill, but it only cost you 50% more fuel than driving level (better place in the BFSC map). Use a gear that keeps you closest to staying in the sweet spot of the map.

Lets say it takes 12 hp to maintain your level speed but 32 more to climb the 8feet per second. you are lifting your car up at a rate of 480 feet per minute like it was an airplane, both gaining altitude.

Now follow me closely, if the grade is less than one too steep to maintain speed in any gear, the gear the gives you a mileage equal to two thirds of you level mileage is best minimum speed. The choice is the gear that gives you the best mileage as long as it doesnt take a lot longer to climb the grade.

The total amount of power it takes to climb the grade (vertically) is the same regardless of how long it takes, but your engine wants to get the job done quickly, and rewards you with better overall mileage when you get the most of those HP for the absolute least amount of fuel. Up to about 75% of full throttle is OK, past that you migh try another gear.

Use your mileage difference to find the minimum peak mileage, then gradually increase your speed to the best balance between what it cost you per second and what the total cost would be. Your mileage will go higher at lower speeds, but it takes you longer to finish the climb, offsetting penalties so to spesk.

It may be more efficient to use a lower gear on steep grades. shallow grades are a hypermilers dream.

I use my local knowledge to always accelerate up a shallow grade, then cost down the shallow downslope. If the grade is fairly steep and sustained I woud actually reach my highest pulse speed at the bottom then gradually lift the pedal as I try to maintain the same speed regardless of the fact I am going uphill or downhill. Constant speed will give you the lowest overall drag, by avoiding the higher speeds exponential increase. Vertical climb no more than 60 feet in this example.

If I could design a hypermiling road, the approach grade would be 3 times as steep as the downslope, which would allow you to pulse up the hill and maintain a constant speed downhill with your perfect slope.

Then your 9 MPG would be 36 average up one side and down the other., if you shut the engine off on your coast. Then the downhill mileage would be infinite.

regards
gary

[samandw]

Originally Posted by bkrell

Right, for me, the numbers were silly-like 9 uphill then 55 mpg or so downhill. The downhill didn't change much when I dropped the speed and got close to 26 mpg uphill. So then would it just boil down to total trip time?

Yes, you'd want to compare total trip mpg to give you an accurate idea of which method yields better mpg.

[samandw]

Originally Posted by R.I.D.E.

I use my local knowledge to always accelerate up a shallow grade, then cost down the shallow downslope. If the grade is fairly steep and sustained I woud actually reach my highest pulse speed at the bottom then gradually lift the pedal as I try to maintain the same speed regardless of the fact I am going uphill or downhill. Constant speed will give you the lowest overall drag, by avoiding the higher speeds exponential increase. Vertical climb no more than 60 feet in this example.

That's an extremely interesting idea. I think a series of tests is in order over moderately hilly freeway.

Test 1: constant speed
Test 2: decelerate up hills, accelerate down
Test 3: accelerate up hills, EOC down.

I wonder if the real answer depends on the steepness of the upslope vs the downslope.

Maybe I'll make an excel spreadsheet to find an answer. Anybody know of a car I can find weight, CdA, and BSFC vs. hp?

Imagine in the future your cruise control syncing with your GPS to optimize fuel economy while maintaining a target average mpg. That would be sweet!

[bkrell]

Okay R.I.D.E., here's the thing. I live in Louisiana, so these are TINY hills in comparison to what most folks think of. If I start at the bottom of the hill at 55 mph, the lowest I let it drop to is maybe 48-50 mph. It takes maybe 7-10 seconds to go from base to crest of one of these Everest-sized monsters. Maybe that will clear up my circumstances a tad.

[R.I.D.E.]

bkrell, always used the slightest upgrade to apply a decent load to your engine. On very shallow grades you want to hit your peak speed at the top, then coast as far as you can to your minimum speed, downhill, then repeat.

Regardless of everything else your highest speed at the top gives you the longest glide.

If the grades are far enough apart you may have to pulse again between them (or more than once), but develop your timing for peak speed at the peak. Highest sustainable load in highest gear without downshifting, neutral coast.

The hill I would reach top speed at the bottom would be one that can catch me below my power band and peak efficiency, then I would have to downshift. That would never happen on your slight grade.

I had a Valiant that would only go 45 up a certain steep grade, downshifted to second floored it and shifted to third, still floored, and watched it slowly loose speed to 30, back to second and repeat. I could have left it in second floored for 8 minutes it took to climb 2500 feet, but it was better to average 40 mph by peaking in second and then going to third even thoug it woud not maintain speed. A couple more gears than a 3 speed would have been nice.

Same as the truckers in parts of West Virginia.

regards
gary

[fumesucker]

Originally Posted by R.I.D.E.

If the grade is fairly steep and sustained I woud actually reach my highest pulse speed at the bottom then gradually lift the pedal as I try to maintain the same speed regardless of the fact I am going uphill or downhill. Constant speed will give you the lowest overall drag, by avoiding the higher speeds exponential increase. Vertical climb no more than 60 feet in this example.

That's my experience, we have few shallow grades here, I have over 70 feet of vertical climb on two hills just to get out of my small subdivision. About 230 feet vertical climb to get to the closest shopping.. 2.6 miles away.

With a racecar on a 16' tandem axle trailer behind an Expedition @ 8800 lbs total weight I can get 17 mpg at 60 mph average speed using this technique.. Do most of the accelerating at the bottom of the valley, gradually slow going uphill and coast downhill.

Put the cruise control on and go the same average speed and it's more like 14 mpg.

[R.I.D.E.]

Cruise control can only react not anticipate. If they had any brains they would put a level in the system to determine the immediate sope of incline or decline and modify the program.

They could also put in an auto hypermile program.

I thought once I would put a bungee cord on my Toyota automatic to pull it out of drive unless I forced it into drive. It spent most of its time in neutral anyway .

regards
gary

[fumesucker]

The Expedition is not mine so I hesitate to put it in neutral for coasting, I could probably squeeze another mpg or two out of it if I did that..

[R.I.D.E.]

Dont think it will hurt as long as you leave engine running, then primary pump is working which controls shifts as well as lubrication.

I would make darn sure it was OK. Most owners manuals tackle the issue when you look up towing recommendations.

Some auto trannys will flat disintegrate if you tow them without the engine riunning.

regards
gary

[monroe74]

Originally Posted by samandw

In scenario #1 we average (17+38)/2 or 27.5 mpg at 60 mph.
In scenario #2 we average (26 + 35)/2 or 30.5 mpg at say 58 avg mph.

It's possible you did better simply (or at least partly) because you slowed down. Slowing down is pretty much a sure-fire way to enhance mpg. But eventually you reach a limit, with this tactic. The trick is to figure out how to operate more efficiently, which means enhancing mpg without sacrificing distance or average speed. Efficient operation generally means large throttle openings, which is inherently counterintuitive.

Test 2: decelerate up hills, accelerate down

Depending on the circumstances, this tends to happen naturally. But if you take this too far, you end up with a large variance between your min speed and your max speed. A moderate variance is OK, but when it's too large it tends to create various problems. The extra aero drag was mentioned. It's also more likely to annoy other drivers, and more likely to create a problem with speed limits.
-----------------------------

Originally Posted by bkrell

the numbers were silly-like 9 uphill then 55 mpg or so downhill.

That's typical of what happens with an instrument that reports instantaneous mpg. There are wild swings. The implied strategy is obvious: never drive uphill, always drive downhill. Kind of like the classic stock market advice to buy low, sell high. Easier said than done.

The downhill didn't change much when I dropped the speed and got close to 26 mpg uphill.

From 9 to 26 is a big jump, and I can't explain that. But maybe it's real, so keep doing it and see what the results are when you fill.

The instantaneous information is helpful, but it becomes more valuable when you put it in the context of a broader interval, like a 'trip' (however you define that).

Does the fact that I have a turbo car make a difference?

Good question. I don't know.

[bkrell]

I'm beginning to hate having a turbo. It throws so much incertainty into the picture.

[R.I.D.E.]

Dont let the turbo bother you too much, it just makes it possible to extract more power from less engine.

A big plus is the boost guage which is basically a vacuum guage, with ability to show pressure, in positive or negative values.

This allows a greater sustained load on a smaller engine which should be more efficient.

The negative is it can also consume a lot of fuel quickly.

The best example I can think of was the 1.5 liter Alfa grand prix engine of 1950. 90 cubic inches 2 stage supercharging and 390 horsepower, at 2 MPG!.

Your turbo should only be used to sustain steep grades (or emergencies) if mileage is your priority. Other than that use the boost guage to try to maintain the lowest manifold pressure (close to 0) when you are pulsing in the highest gear you can use for your pulse. Gradually depress the gas pedal until you just reach 0 on your boost guage, then you will get a feel for your best pulse acceleration.

If you have to choose a lower gear or some boost use the boost. This would be up significant grades like those the big rigs can not maintain higher speeds.
For hypermiling avoid the turbo.

Use engine braking instead of friction braking because it shuts off all the fuel to your engine when the engine braking keeps the engine above about 1000 rpm. The savings can be significant if you can eliminate as much idling as practical without making everyone around you mad.

regards
gary

[fumesucker]

Originally Posted by bkrell

I'm beginning to hate having a turbo. It throws so much incertainty into the picture.

Considering that a turbo reclaims lost heat energy from the exhaust gases when it is making boost it can be an overall efficiency enhancer, this is certainly true with a diesel.

Try making moderate boost at low rpm if you can without going open loop and rich. According to theory you should actually gain efficiency..

But then, in theory, theory and practice are the same while in practice they are not.

Zero a trip meter and then drive a predefined test loop with one technique..

Zero the meter again and try another technique..

You should be able to tell in a five mile loop if you are gaining or losing substantially with one technique or another. The longer you make the loop of course, the more accurate your results.

[bkrell]

But see, that's the other thing I don't understand. Are you saying on acceleration? That's usually my guide. I avoid boost if at all possible, which I can usually do very well. But are you saying that if I have a choice of accelerating with high vac verses close to none, I would get better FE? I somewhat understand pumping losses but on bal;ance, I seem to still be doing better at full vac.... Maybe it's just b/c keeping high vac gives me room to exceed my target and still stay on the negative side verses when I try to maintain no pressure I'm dipping into boost???

[bkrell]

fumesucker, to be honest, that's another thing I don't understand, closed/open loop? Can someone clear that up for me??? But otherwise, see what I said in my previous response, econ just seems to dip for me when I go into boost....