Question about hills
 

What do you think about mild acceleration down a hill to gain speed and coast up the hill immedietly following? I'll drive at about 58 but when I go down a hill I'll accelerate up to 65-70 gaining momentum so I can coast up the hill following it. My wife insists I'm wasting any gas i save by accelerating, i say it takes more gas to coast down the hill and then have to give it gas to get back up the hill. I have a 92 Daytona 5 speed 2.2 turbo DOHC and have picked up about 4 mpg around town since I started using some of the tips on this board, but my car is too old for a scan guage and I'm not ready to go crazy and figure out a different way to get instnt info on my car yet. I'm thinking about a scan guage for my winter beater 07 Town and Country, but I'll wait until winter to get that when I start driving that again. Any thoughts or ideas are apprecited.

Have you guys heard about water for gas?

KIDDING!

Daytona should be able to use a Chrysler "Traveller" or "Navigator" trip computer, provided you don't have a modified callibration in the SBEC or are using bigger injectors etc... There's a constant in the SBEC code that needs to be right for it to be accurate, many re-cals don't bother changing it.

Some people have proposed that in an automatic, accelerating downhill is more efficient. However, with a manual transmission, it's far better to coast downhill and accelerate uphill.

Either way your engine has to produce the same amount of work. If you do it downhill, you do it at a higher speed, which means higher RPM and more engine friction. If you do it uphill, lower RPM helps, as well as the increased throttle angle. A wider throttle opening reduces pumping losses (that's because of the reduced effort the engine has to make to suck air past the closed throttle).

An easy, inexpensive way to get some live data is with the DIY fuel rate monitor I have linked in my sig. It doesn't tell you MPG, but it does give you relative fuel rate.

My cars are all manual transmissions and I always coast down the hill and accelerate on the uphill. That is what has seemed to work best in my case. My wifes car and the one I got for my 17 year old son to drive are automatic, but I don't drive them enough to know what works best on them. I drive them the same way I drive mine whenever I drive them. I guess it is because I am in the habit of driving that way.

a while ago, someone was talking about this very thing and they posed a pretty important factor. if you accelerate down a hill, gravity is actually helping you along. I guess kind of like a teather ball. if it is already going that direction, you just give it a nudge and off it still goes. going up a hill is just the opposite, fighting gravity as you go.

I know this isn't the only factor involved but I figured I'd bring it up. also remember that at higher speeds, your drag increases tremendously. I have heard that the threshold is around 53MPH but also it is dependant on the cars design.

also remember that it will take quite a bit of gas savings to pay for that speeding ticket if you get caught going down a little coutry road at 70 to pick up speed for the next hill.

That's what I'm thinking Beef, a nudge when I'm going 58 up to 65-70 then coasting up the next hill keeping the engine from lugging in 5th or having to downshift to fourth, but from what I've read hear so far that may not be the case. I'm not real worried about cops around here, it's a small town and I know most of them. I sell cars so if I can't talk my way out of a ticket at 14 over then I don't have my "A" game that day or the cop is giving me a ticket regardless. I've talked the state boys out of tickets mainly by admitting I knew I was driving a little fast but didn't realize I was going that fast. I guess I really want to believe mild (stress the word "mild") acceleration down a hill would be better because otherwise my wife would be right about something car related and I'll never hear the end of it. Either way it's a chance for me to do a little speeding at very little cost!

Correction: I've talked a (one) state boy out of a (one) ticket, but that was a few years ago, I very rarely approach 80 mph anymore.

I have a very significant hill I drive over every day. It is about 1200 feet in elevation in 6 miles. There is no way I can drive over this without downshifting at least 1/3 of it. It seems that when downshifted, I have to ease up on the throttle to maintain the same speed. Lower RPM+higher gear (od)= 2/3 throttle. Higher RPM+3rd gear= 1/2 Throttle. Does this indicate in any way how much fuel is being used?

that is a good point as well. anytime you can avoid downshifting is usually an FE advantage.

I have an auto (in my car at least) and I have noticed that gunning it (no more than about half throttle) to get me up to 40mph and then letting off and easing up to speed will get me into 4th (my top gear) and keep me there and has yielded better MPG numbers. I have been trying this latelty and hopefully I will update my gas log at the beginning of next week or so.

I know that had nothing to do with the question but I figured I'd throw it out there.

It depends on RPM. Fuel rate is mainly a product of RPM and throttle. Generally, in my experience, lower RPM + wider throttle always beats higher RPM + less throttle, but that differs from one car to the next, and could differ for specific situations like yours.

Best way is to coast downhill, then maintain speed uphill.

If hill is too steep keep it in top gear and let off the throttle completely, this shuts all fuel off to the engine=DFCO.

If you have to downshift, going downhill, to keep your speed low enough to avoid a ticket then use lower gears, but keep your foot off the gas.

The ideal scenario is to coast downhill and uphill until you need to keep your speed up to the desired level.

regards
gary

I tried the acceleration downhill techniquie and what I've found out is this...

You use gas going down the hill/risk getting caught by cops/and if you don't judge the next hill right - you have to shift it back into drive and continue up the hill.

I've found it's better to just coast down the hill and TRY to get to a fast speed and coast up until I HAVE to shift in drive and maintain a speed above the point of downshifting. I've noticed gains that way and it's so much more common sense and easier on the stress level when you're unsure if you'll make it from the cops or shifting in drive again...

There have been some good comments here, but I wanted to make a comment regarding the "gravity multiplier" idea:

The laws of physics disagree with this idea. Regardless of whether you are coasting in neutral or accelerating in gear, gravity will pull on your car with the exact same amount of force. You do not get any kind of "multiplier" effect by accelerating downhill--you just get the same extra acceleration you would have gotten by coasting. Likewise, you are going to have to fight the same amount of drag for going up the hill whether you do it with leftover momentum from coasting or with the accelerator pedal.

I'll try to summarize that effectively:

It will take a certain amount of power to travel up a hill and down the other side (or down into a valley and back out), and, in terms of physics, it makes no difference when you apply that power (i.e. accelerate to 100MPH at the beginning and coast, or coast the beginning until you must press the gas pedal, or anything in between).

Here's the only possible difference that I know of (and I do not have perfect knowledge, so there could be something I'm missing): If you coast down a hill in gear, then you are effectively "engine braking", and you would be better off in neutral (in terms of FE -- safety and local laws may differ on this).

Oh, and whatever you do, don't use your brake pedal on a hill unless you absolutely must. It's a pet peeve of mine that I run into on my commute on a regular basis--people brake to keep from going 5MPH over the speed limit, and then have to hammer the gas to get up the other side. There's one spot where, if no one is in front of me, I can coast for almost a mile (up to a stop sign), but that only happens about once a month.

Anyone here gone over the continental divide? 6393 ft, start at about 5300. That's one of 4 passes I cross to get to Coeur De' Alene. With 5 passengers in my GP I managed 34mpg. I held it in 4th (no lock) as long as possible going up, maintaining about 50-55mph. Going down, I put in N and coasted, no braking. That isn't safe, nor do I advocate doing 97mph down a pass. But I did manage 34mpg with 800 extra lbs in the car than I normally have crossing those passes.

I usually can hold the throttle at about 2/3-3/4 without it hopping outa lockup. With that I can eek up to 80-85 before the pass, and just slowly ease off the throttle till I want to backshift (tapping the brakes with the left foot while maintaining throttle) has worked quite well for me.

I know this works because my friend drove back and didn't do any of the things I do, he just drove. 29.4mpg. Yeah. He won't be driving my car again.

I know what you mean about people braking down a hill only to accelerate to get back up it. They have effectivly wasted the gas in their car as well as everyone that is behind them. I usually will try to figure out how fast the car in front of me is going and back off .5 to 1 mph, let anyone who wants to pass me, and most of the time I'm free of other drivers costing me fuel/money because of their poor driving habits.

Your total energy use, will be lowest if you avoid speeds above your desired average. This is becasue of the exponentially greater aerodynamic drag at higher speeds.

Of course if your speed increases as you coast down the hill that additional speed costs you no additional fuel. The question is the risk of a traffic violation as well as your personal safety.

Note (coasting by my definition is always in neutral)

Most of the hills here are very small grades, so there is no possibility of dangerous speeds.

In a perfect scenario, your hill would be the perfect grade and you could maintain the same speed regardless of whether you were going uphill or downhill.

Remember aero drag is by far the greatest source of energy losses at high speeds, close to 2/3 at 65 MPH.

That is why my strategy is to try to maintain the same speed.

regards
gary

I can't remember whether it was on this site or somewhere else on the Internet, but what I was reading was suggesting coasting down the grade and then on the uphill grade rather than maintaining your previous speed let it drop a few mph to save fuel. I think they were talking about decreasing speed by 5 mph or less, although it is something that I rarely do.

I thankfully was able to follow an unloaded dumptruck up the grade this week and was able to keep the car in OD without downshifting for the entire grade at 60 mph! Shows me the drag loss is much higher than I thought.

I usually just keep my right foot steady and let the car pick up speed down the hill then let it slow slightly on the up hill side.The speeds are kept within reason though and there aren't that many big hills around here anyway.I try to keep it less than 10 m.p.h over the limit and 5 m.p.h. under.

After installing a vacuum gauge in my '88 Escort I have found that it is much better to let completely off the gas going down hill than to maintain the same or less acceleration.

Are you accounting for the greater fuel required to go up the other side because you were going slower? Or are we only talking about lone hills (which are always up first then down, without another up after the down)?

I'd be curious to see some multi-run testing done by someone with one of those computer data-logging setups, testing various methods of going up & down the same hills over and over again. Of course, it would be ever better to get several sets of data (for example, DFCO behavior plays a big role in what the "best" method is).

I aim for speed limit + 9 at the bottom of the hill and hold the LOD on the ScanGauge as high as I can. Last night I saw it hold at 99 before it finally downshifted. When it downshifts, instantaneous mileage is cut almost in half. Everything about driving the Escape around town is getting it into the highest gear possible.

The hard part is starting off uphill. There's nothing to to do but punch it to get enough speed to let it upshift. Otherwise I'm spinning uphill with mileage in single digits.

It would probably also suffice to use average mileage over a whole tank of fuel, one tank with one strategy, one tank with the other...

OK I'll step up on this one ....
We are dealing with a turbocharged car here. (boost=lots of fuel) the idea here is to keep out of the boost I think you have been doing things right if you can accelerate down the hill staying out of boost (good) and if this keeps you out of the boost going up the other side then this works. Again BOOST= bad FE

He needs a electronic boost controller, then he can turn the boost off when he wants and drive with out it. On any grade i try to put it in neutral or eoc and get as far up as i can then kick it in to the top if i slow to much. Lately thought the traffic has been so sparse i can at times slow to 40mph while p&g on the freeways. My how my thinking has changed, i look for trucks now to draft. In the past they just annoyed me to now end. This works well for overpasses and bridges. Eoc with a truck in front is the best.

This is refered to as driving with load (DWL). Keeping the engine at a set load or throttle position is more efficient than varying it, outside of pulse and glide that is.

What about cruise control
 

Talking about hills, what about cruise control? Should I disengage the cruise control? We don't really have "hills" in coastal Texas, unless you call the overpass "hills".

I've done a search trying to find the exponential dependence of drag with speed but I didn't find it. The only formula I know is

https://www.fuelly.com/attachments/fo...9c4459ebf3.png

Can you tell the formula of the exponential dependence please?

It's the V squared part. Drag is proportional to velocity squared.

On flat roads, using cruise control is DWL. My parents live in such an area of NC, and using cruise will usually net above EPA. It won't be the numbers using extreme hypermiling methods, but it's easy and reliable. On hilly terrain the cruise control can hurt milage. It tends to be more aggressive trying to maintain speed, and won't take advantage of downhill coasts.

Go ahead and use the cruise for the flat areas. When you come to a hill, click it off and use your foot to drive up and over and down. Then turn it back on.

I find that the CC wants to downshift one or even two gears for the smallest hills. You don't want that. I can maintain speed AND keep it in top gear with my foot. It's just a matter of seeing what's coming and anticipating. The computer can't do that - it can only react to what's already happening.

It's a power dependence then.

Uhm...no. It's speed dependent.

Aerodynamic drag is proportional to the square of the velocity. Double the speed, whether pushing up a hill or coasting down, drag will increase by a factor of four.

However, the issue of hills is way more complicated than just looking at drag because you have the non-proportional effects of gearing and its effect on engine efficiency (keep it in high gear or bust), speed limits and law enforcement (which tend to screw up otherwise pristinely pure physics), road conditions (best fuel economy will be achieved just before you hit the bottom of the cliff, at which point it goes to zero), etc.

So you need a strategy. Mine is to hit the bottom of the hill just fast enough not to aggrevate the local constibalry, hold as much power as possible without downshifting for as long as possible and let the speed bleed off. The inefficiency of my Escape going up a hil in a lower gear completely outweighs the drag issue. And that's on a car affectionately called The Brick due to its complete lack of aerodynamics.

Hills.

I do an easy 25% acceleration about the first 1/4 to 1/3 down, coast the rest of the way until I am near the bottom, then "accelerate" with about 25% throttle, but gradually backing off as I am going up the hill. I don't try to maintain speed up the hill, but I do slowly decrease speed on the way up.

Cruise control will give you the worst fuel economy with even the small hills or overpasses. Cruise control wants to maintain speed by accelerating up the hills, whereas you really want to back off the accelerator on the hills, then maintain that slower speed over the top, then light acceleration on the downside to get back up to speed.

IMHO

lowbridescape and 99 Metro, we share the same hill philosophy. IMHO it is much easier on the engine to get a little burst of speed using gravity to assist you and let inertia carry you over or assist in getting you up the hill.

Gravity, not just a good idea, it's the law.

I didn't realize this would get this deep when I asked the question. It's cool to talk to people as interested in fuel economy as I am. According to my wife we are special group with a special name, "dorks" is what she calls us! But I tell her we're dorks with more money in our pockets.

I call us "kooks". :D

The number "2" you see near "v" is called a power, that's what meant.

We are debating hill strategy, but the strategy depends on the hill.

Most hills around here are actually ancient drainage pathways, they start downhill and then you go uphill. Elevation changes are rarely more than a typical interstate overpass, but in reverse.

I approach then at my normal speed, coast in neutral down the hill and gain some speed, then let it coast uphill until its about 5 below my desired speed, then use top gear to get the rest of the way uphill.

One one particular hill it takes .6 mile from initial coast to final climbing of the upslope, with only .1 mile of the .6 where I actually have any engine power applied to the drivetrain. I don't EOC so for 80% of the distance I am idling at .2 gal per hour. Thats at 50 MPH average speed.

Hill country strategy totally depends on the grade you are dealing with. The slight grades I have here are actually beneficial to fuel economy, meaning I can actually get better mileage than I can on flat ground.

Hill climbing will get your brake specific fuel consumption at a much better point in your Map, the energy required to climb the hill (when it is like the ones I deal with) allows me to not have to downshift.

In fact coasting on almost every downhill slope on the shallow grades here is one of my basic strategies for better mileage, combined with drafting, and DFCO. those 3 tactics are almost all of my hypermiling efforts. The only additional one is some P&G on roads with speed limits of 45 MPH and below.

regards
gary

Sorry. Misunderstood. I think of it as an exponent.

My wife calls me all kinds of names.

You are a moron because you sold your business and aren't getting a paycheck!

When we sold the house that I built, for $165,000 more than it cost to build, I let her take the check to the bank. At one point it was appreciating $525 a week.

You paid $3000 for a wrecked car, you havent even looked at in person!

It will be painted tomorrow, 7000 miles at 58.83 MPG, almost 35,000 on the odometer today. I have a total of $5000 cash in the car and some sweat equity, about the cost of a hybrid battery for 200,000 miles of transportation.

You bought another lot!

The house sitting on that lot is now appraised at $140,000 more than it cost me to build, and we owe no one a dime, except for monthly bills.

Every once in a while she tells her two daughters she is thinking about moving out. They tell her to take a pill and have a glass of wine...............


..........It goes on Judah...................................

regards
gary