Gas driving Strategy for hills


#1

Consider a path formed along a two mile flat stretch of road at sea level followed by a 6% grade that rises 300 feet above sea level, followed by a 1% grade that takes you back down to sea level, followed by a two mile flat stretch of road. Suppose a car at 45 mph is on cruise control and follows the path that first goes up the steep side and then down the gentler incline. At the end of the path, MPG is calculated. Then the car returns on the path at 45 mph, up the gentle incline and then down the steep incline to the end of the path. AT the end of the return trip MPG is calculated again. The speed limit on the stretch is 60 mph.

Do the trips get equal MPG, or is one direction providing better MPG than the other. If so which one and why? Would the answer change if driving at the speed limit both ways?

This question came to me while driving through hills in my 2010 Prius, but I see no reason why it should be limited to that particular car.


#2

I can’t answer that for a Prius, or any hybrid for that matter as they operate under a different set of parameters than a direct driven car. But for direct drive cars, the outbound trip would yield better gas mileage than the return trip.

Hypermilers have found that hard acceleration followed by a long coasting period yields the best gas mileage, and the outbound trip more closely follows that profile.


#3

@keith: yeah, but that’s provided one is willing to shut down the engine for the “glide,” or at least take it out of gear.

Gasoline engines get their best “power-specific fuel consumption” at around 80% output. Fuel economy contests are generally won using “pulse and glide” techniques: the engine is either running at 80%, or shut down. The hilly scenario best approximates P+G (though 1% gradient is really too flat unless car is VERY aerodynamic).

In the real world, though, you have concerns like safety and not impeding traffic, which limits the applicability of the theoretical optimum solution.


#4

Don’t take it out of gear, that uses more gas. FI cuts off fuel flow when the manifold vacuum is higher than the normal idle vacuum.

The OP specified using cruise control so this is simply a comparison of the outbound vs return trip, not a quest for the “theoretical optimum solution”.


#5

Yah,
The best way to maximize mpg going up and down hills is…
the wrong way to drive amongst other traffic…!

gaining lots of speed going down via gravity and very little accelerator input so as to carry that momentum up the next hill hopefully needing no, to very little, accelerator input as the speed gradually drops just as you crest the hill to begin the next downward momentum gain.
Doesn’t work if the other traffic isn’t of the same mind.
you’ll catch up to and pass on the down hill,
then end up a snail near the top of the up hill.


#6

I would stop doing what you are doing; you would be endangering traffic and your own safety.


#7

Considering the fact that you’re driving a Prius, I’d take it for granted that it has a computer that is smart enough to figure this out for you. I’d set the cruise control and forget about it, especially since, no matter how you drive, the Prius will use your momentum going downhill to recharge the hybrid battery pack. Also, your Prius probably has a CVT transmission, so it’s always in the perfect gear for the situation.

Just set it and forget it.

If I was driving a big rig, or something hauling a large load that isn’t a hybrid, I’d gently accelerate before climbing the hill, gently decelerate as I climb the hill, and gently accelerate going down the hill. Accelerating going uphill uses more fuel than just about anything else.


#8

There are three easy ways to save gas:

  1. Keep the car maintenance up to date, tire pressures checked
  2. Slow down, drive smoothly
  3. Drive less

Everything else isn’t really worth the trouble, to me.


#9

@ keith: “don’t take it out of gear, that uses more gas.”

No, it doesn’t. We’ve been over this before.

The energy required to freewheel an engine at typical crusie RPMs EXCEEEDS the fuel required to idle an engine at a considerably lower RPM. Besides, the optimum solution is “in neutral and shut off.” So, the most economical method on a shallow descent like this is: engine off, in neutral…followed by idle, in neutral…followed by in gear. On top of which, the car’s ECU has very proscribed conditions under which it will actually shut off fuel; if you haven’t personally verified that a car in question cuts fuel in a given situation, you’re just guessing.

Note that the OPs car is a Prius. Ford uses Toyota patents in its hybrids, and I can personally attest that…in coasting situations, under 40 mph…it shuts down the ICE and leaves it in neutral all by itself.

[Leading to the interesting question: is it technically illegal to operate a hybrid in locales that have laws against driving in neutral?)


#10

Meanjoe - the Prius is not in neutral when coasting, the drive train is full engaged, the coasting results in the motors turning into generators, recharging the battery.


#11

meanjoe, we have been over it before, but we do not agree and never have. But again, the question was about a specific set of conditions, not on strategies to get the most fuel efficiency, which everyone seems to be jumping on.

DrCrispy, to answer your second question, going the speed limit would use more fuel in both directions, but for direct drive cars, the answer is still the same, the outbound trip would use less fuel. I do not know if that applies to a Prius though.


#12

@texases, ok, I’ve never driven a Prius. During EV-only mode, does the engine freewheel…or go to 0 rpm like the Fusion hybrid does? If the latter, then it is, in fact, shutting the ICE down, and putting it in neutral.

@keith: you’re free to deny that in-gear uses up more energy than idling. You’re free to deny gravity, too, for that matter…doesn’t change the fact that it exists.

It obviously takes more energy to spin an engine at 2200 rpm that it does to spin it at 500. With fuel cut-off, you don’t IMMEDIATELY burn fuel…you scrub speed as you trade momentum for engine freewheel. When you eventually need to bring the vehicle up to speed, THEN you burn more fuel than if you left it in neutal the whole time.


#13

meanjoe, I don’t deny gravity, but in gear on a fuel injected engine coasting downhill uses less fuel than out of gear, in all conditions when the engine is left running, period. But again, the OP is using the cruise control so in gear is a given.


#14

Oh, and I’ve “best practices” hypermiled a '98 Contour from State College, PA to Cleveland, OH. The flat Ohio portion got worse MPG than the hilly PA portion.

Basically, pulse and glide gives the best FE, and hills make it possible to P+G while maintaining a near-constant speed.


#15

Did that mean engine off when gliding? AT?


#16

Meanjoe - I have a Ford (MKZ) hybrid, which operates just like the Prius. The engine shuts down, but there’s no ‘neutral’ to the system in these conditions - you might want to look at this description of how the system works:
eahart.com/prius/psd/

At no time is anything disconnected (put in neutral) when the ICE is shut off.


#17

sigh…

Ok, keith, I’m done trying to convince you. I guess everybody that’s entered and won a mileage competition is using the incorrect strategy. You should enter and win as many as you can, as you are apparently the only one using a “correct” strategy.

And, again, OP is in a HYBRID, so the engine can and will be taken out of gear according to the whims of the computer. Apparently, the computer is smart enough to realize that the “pumping losses” of spinning an ICE at several thousand RPM consumes copius amounts of energy that will eventually have to be repaid with gasoline, even if certain people aren’t.


#18

What the computer is smart enough to realize is that coasting is an opportunity to charge the battery, an opportunity that doesn’t exist on non-hybrid cars (except for a very few new ones).


#19

@texases; that link you gave me confirms that the prius system puts the system in neutral. It says ICE can=0 rpm if vehicle speed =<42 mph. That’s neutral! (ancillary generators and motors notwithstanding.)

@keith: yes, P+G always means engine off. The procedure I used was:

  1. Climb hill, at legal(ish) speed, in top gear and at greatest throttle opening that prevents open loop mode.
  2. At crest, shift to N, turn car off, then back to run after engine stops spinning.
  3. Maintain legal(ish) speed during descent, using brakes and restart if absolutely necessary (rotue planning helps).
  4. At bottom, bump start in 5th. Rinse, lather, repeat as necessary.

#20

Meanjoe - no, ‘neutral’ is when an engine is disconnected from the rest of the drive train. In this hybrid, the engine is never disconnected. The rpms are zero, and the wheel-driven movement is absorbed by one or both of the electric motor/generators.

So yes, the engine is shut down, but no, it’s not in ‘neutral’.