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Old 03-12-2014, 12:33 PM   #1 (permalink)
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Uphill/Downhill P-and-G

Pulse and glide question:

Given a situation where there is a set of two inclines (one up and one down)-- for example, a dip, where the slope going down is the exact same distance as the slope going back up to level again-- at freeway speeds (45-70) is it better to glide down the first one, and then pulse up the second? Or is it better to pulse down the first one and glide up the second, only applying power as needed to reach the top?

My feeling is that it's better to pulse going downhill and glide on the corresponding uphill. But what about air resistance?

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Old 03-12-2014, 12:35 PM   #2 (permalink)
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Mostly driving in mountains, I have had the best success barely going up hills (slowing down, to as slow as I feel is safe) and letting gravity do the rest. Cresting at the top and kicking it into neutral to let it glide. It will pick speed back up very quickly.
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Old 03-12-2014, 12:42 PM   #3 (permalink)
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I'm asking it more as a physics/theoretical question than anything else-- I'm assuming that the road before and after is level, and there is no wind or other traffic on the road.
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Old 03-12-2014, 04:50 PM   #4 (permalink)
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It depends on the hill's grade. Generally you pulse up hill and glide downhill. This applies as long as you can maintain your target speed downhill.

Steeper hills, you want to slowly lose speed uphill and hit your target speed downhill, without exceeding the speed to the point you risk a ticket.

More gradual grades. It depends on traffic. You may pulse&glide uphill or downhill depending on your target speed. This assumes you can not maintain your target speed downhill on the shallower grade.

Think of it as a roller coaster. It gets elevation then uses that energy to glide around the track. It's almost 100 miles from my house where the grades get steep enough to maintain 60 MPH downhill consistently.

I generally do not let my peak speed go over 7MPH above the limit. Pulsing on downhills will cost you in aero drag as it increases exponentially with speed.

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Old 03-12-2014, 05:17 PM   #5 (permalink)
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I would love to see some testing on this.



If we're talking about a stretch of road like the picture above (not as steep though), in general, all other things being equal with no stop signs/stop lights/traffic/etc., I pulse downhill and glide uphill. Pulsing uphill uses much more fuel than pulsing downhill. I rarely make up fuel spent from accelerating uphill on the descent even while in DFCO sometimes, but I do make up the fuel spent pulsing downhill while coasting uphill. To be exact, I don't always coast in neutral uphill unless I can make it to the top, but I do try to decelerate all the way to the top.

For this reason, I also accelerate more on a flat stretch if I'm approaching a hill so I don't have to accelerate while on the hill.

With that said, it all depends. All hills are different with many variables thrown in the mix.

I haven't done actual testing on this, but based on my ScanGauge readings, I get better mpg doing these things.
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Old 03-12-2014, 11:17 PM   #6 (permalink)
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It's best to ignore hills, and stick to P&G.

What's more efficient, driving at a steady speed or P&G? Obviously P&G. Why does it make a difference whether you are on an incline or decline or flat ground? It's still the most efficient way to drive. So just stick to the process. That means sometimes you will pulse up hills and even have to glide and pulse again before you reach the peak.

If you only pulse up hills and glide down hills, then that means you continue to use gas up hills even though you may have reached your glide speed. For example you approach a long hill, you pulse like normal to 65. You reach 65 at the middle of the hill. Do you go into a glide, or do you just stay at 65 until the top and then glide? Even though you might have to pulse again on the same hill, remember if we are climbing that hill it's still more efficient to P&G the hill rather than steady speed it.
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Old 03-12-2014, 11:45 PM   #7 (permalink)
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As usual, everyone is making great points.

I know you are specifically asking about a hypothetical situation- but to bring more reality (variables) into the situation, look into different ways to do it.

Other than what was mentioned above, I will suggest this:
-Coast down hill (engine off)
-When vehicle is within the lowest efficient speed for circumstance, start engine (for me it's ~57 mph)
-use CC to maintain that speed with the lowest fuel use possible

And even this can be altered. I do this to maintain a decent (to me) average speed, and not be slowing down too much. One could EOC to a lower speed, or when you have that set/adjusting speed, kick the engine on but control the throttle to decelerate at a slower rate, but not use as much fuel. Personally, when climbing multiple hills, I will CC up the hill until I approach the top. This is usually indicated by the truck/slow lane ending soon (which I travel in). When I see this, it is fair that the lane crest is near. If there is little to no traffic, I will coast down to mid 20s. If there is traffic and I can do it safely, 30s. More aggressive traffic, then the minimum highway speed or 40.

Moral of story- mostly everything is more efficient than full throttle uphill, and braking downhill. What works best (or well) for you will be decided by your vehicle, circumstances on the road, and driving technique.
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Old 03-13-2014, 08:44 AM   #8 (permalink)
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There's a big dip in a road that I drive somewhat frequently. I haven't figured out the best way to tackle it. It looks like a great hill to EOC, but I barely maintain speed going down the hill. Sometimes I just accelerate down the hill to gain some momentum for the upcoming climb.
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Old 03-13-2014, 10:20 AM   #9 (permalink)
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Quote:
Originally Posted by cbaber View Post
What's more efficient, driving at a steady speed or P&G? Obviously P&G. Why does it make a difference whether you are on an incline or decline or flat ground? It's still the most efficient way to drive.
I don't believe this is an absolute. Pulse and glide is more efficient in many situations but not all of them. If the descent is steep enough, you may not have to pulse at all. If there is a stop at the bottom of a hill, DFCO is more efficient. There are many situations where P&G is not the most efficient technique, and it matters a lot exactly when/how long you pulse and when/how long you glide.
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Old 03-13-2014, 11:59 AM   #10 (permalink)
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If the descent is steep enough, it's not that you don't have to pulse, it's just a really long glide. If there's a stop at the bottom you're not in a p&g situation, you're determining the best way to approach a stop.

Every road is different, almost every time. It's what keeps driving fun, and what makes the texters so dangerous.

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