Is accelerating down hills fuel efficient?

[Ecky]

So, to put it a little differently, on low-grade hills I will generally leave my car in 5th and climb it under high load, near peak BSFC, then shut the engine off and coast down the other side. This allows me to maintain a constant speed, which is more efficient (in this particular case) than shutting off the engine at the bottom of the hill, coasting up and arriving at the top with near zero speed, then accelerating rapidly under power down the other side.

It takes the same amount of energy to push a car up the same hill (minus wind resistance) at 25mph as at 50mph, the difference being that at 50mph, you're using up that energy twice as fast. That's why you perceive it to be harder on a bicycle - you're applying twice as much force, but for only half the time, for the same net energy used.

Let's say the hill is shallow enough that you can keep the engine in an efficient BSFC range, and you want to average 40mph:

1) You can climb at 40mph under power, then shut off the engine and coast at 40mph down the other side, and average 40mph.

2) Alternately, you could come at a hill at 80mph, shut off the engine at the bottom and coast up, hitting ~0mph at the top, then accelerate under power down the other side of the hill to reach the bottom again at 80mph, ready for the next hill. In this case you would also be averaging 40mph.

In both cases you use the same amount of energy to lift the mass of the car to the top of the hill, but in the second case you have to overcome a lot more total wind resistance, because it goes up exponentially with speed. For this reason, pulse and glide over gently rolling hills can often be even more efficient than pulse and glide on level ground.

Problems do arise when a hill is steep enough that you need to shift your engine into lower BSFC ranges. Although maintaining a constant speed is more efficient in terms of total energy needed, you're right in that you often more than offset those savings if you have to downshift several times. I'd say in a majority of those cases, it's best to leave the car in its efficient range while accelerating up the hill and allow yourself to bleed speed, rather than rev it up and lose efficiency. For a hill this steep, you're almost certainly going to need brakes going down the other side to avoid speeding though, and any time you have to use your brakes, you're turning potentially useful energy into brake dust.

You should almost never be accelerating under power down a hill.

[JockoT]

Originally Posted by Ecky

For a hill this steep, you're almost certainly going to need brakes going down the other side to avoid speeding

If a hill was this steep on the descent I would take advantage of no throttle and DFCO. Very seldom round these parts that hills are equally steep both directions though. As I have said elsewhere, what I would do depends on the terrain as well as the traffic at the time.

[D15r]

I get what you guys are saying, but I still think it is situational in the real world to get the best fe numbers.

As an example sometimes i coast down a hill on to a flat then reach another down hill, could be a tiny gradient, at which point ive dropped below an acceptable speed so i accelerate to a speed which will suit the next coming climb...speed limits, traffic etc. SO your saying i should be waiting for the up hill to gain speed? But Many hill where I am will barely allow me to accelerate from my efficient climb speed without a downshift or 2.

Please note that I am saying that I prefer to maintain my speed up a 'hill' this would mean I reach the top without losing momenteum.

I know that in rolling country side, uphill down hill I can get better mpg in my car, and by that theory given the inefficiencies of the car engine then there should be a wieght which is optimum for the car which won't be the lowest right?

Please note I get what is being said about efficiencies although obviously my so i'm not arguing the theory just discussing the practicality.

[Panther140]

Originally Posted by cr45

This is incorrect.
There are 3 energy components involved in climbing a hill at constant speed.
1/ Gain in Potential Energy.
2/ Parasitic loss due to tyre rolling resistance.
3/ Parasitic loss due to aerodynamic drag.

As the aerodynamic drag is proportional to the square of the velocity then as the velocity increases the total energy required to overcome aerodynamic drag whilst climbing the hill also increases.

What I said was that climbing the hill requires the same amount of energy regardless of how fast you do it. Overcoming wind drag is a separate topic. That wind drag would exist regardless of the hill. Wind resistance and grade resistance are different factors and I was focusing on the topic of overcoming grade resistance.

Obviously if you encounter wind resistence while climbing the hill, you are going to burn more fuel. But the hill itself did not take more energy from the vehicle. That was the wind. Not the hill.

Originally Posted by cr45

Small engine cars often have very little excess power when travelling at highway speeds up a hill. Any acceleration will cause fuel enrichment beyond stoich AFR and lead to poor mpg. Your argument is only applicable to vehicles with large engines relative to their mass.

Small engine cars.

Brake specific fuel consumption is still more efficient under heavy load, even with fuel enrichment. My "argument" is actually a fact that applies to all quantity-regulated spark-ignited gasoline engines. Do you know what BSFC is? It basically indicates how much work the engine will get done if you give it a certain amount of fuel.

If you supply an engine with one liter of gasoline, and set the engine to peak torque near WOT, it will do more work before running out of fuel than if you give the same engine the same amount of fuel and run it with low load.

For a while, I worked as a fuel systems engineer for a well known company that manufactures internal combustion engines. I'm still very much involved in engineering projects which have the sole intent of increasing the thermal efficiency of engines. Not everything regarding efficient operation is intuitive. I'm not saying to blindly accept what I'm saying, but I recommend you withhold conclusion on this topic and research more.

[Panther140]

Originally Posted by D15r

I get what you guys are saying, but I still think it is situational in the real world to get the best fe numbers.

As an example sometimes i coast down a hill on to a flat then reach another down hill, could be a tiny gradient, at which point ive dropped below an acceptable speed so i accelerate to a speed which will suit the next coming climb...speed limits, traffic etc. SO your saying i should be waiting for the up hill to gain speed? But Many hill where I am will barely allow me to accelerate from my efficient climb speed without a downshift or 2.

Please note that I am saying that I prefer to maintain my speed up a 'hill' this would mean I reach the top without losing momenteum.

I know that in rolling country side, uphill down hill I can get better mpg in my car, and by that theory given the inefficiencies of the car engine then there should be a wieght which is optimum for the car which won't be the lowest right?

Please note I get what is being said about efficiencies although obviously my so i'm not arguing the theory just discussing the practicality.

Let it downshift on the way up the hill. Thats okay. The point is to get as much of the work done with the engine near WOT as possible. If you have a manual transmission, pick a gear that keeps you near peak torque, then open the throttle all of the way. If you have an automatic, it will still run more efficiently with the throttle at WOT. If the RPMs rise too high, let off of the throttle quickly to let it upshift. Then resume WOT operation.

If there is no hill around, you have to accelerate on the flat ground. If the grade isn't steep enough to have a significant impact on the amount of potential energy instantaneously aiding or being acquired by the vehicle, then you will most likely only see negligible gains from leveraging them with strategic pulsing and gliding.

[Ecky]

Originally Posted by Panther140

Let it downshift on the way up the hill. Thats okay. The point is to get as much of the work done with the engine near WOT as possible. If you have a manual transmission, pick a gear that keeps you near peak torque, then open the throttle all of the way. If you have an automatic, it will still run more efficiently with the throttle at WOT. If the RPMs rise too high, let off of the throttle quickly to let it upshift. Then resume WOT operation.

If there is no hill around, you have to accelerate on the flat ground. If the grade isn't steep enough to have a significant impact on the amount of potential energy instantaneously aiding or being acquired by the vehicle, then you will most likely only see negligible gains from leveraging them with strategic pulsing and gliding.

So, although I've been arguing for accelerating up hills, my real-world experience shows (after a certain point) a huge FE penalty from downshifting. My Insight has peak torque (from gasoline alone) at 5000rpm, but accelerating in low gear, high RPM reliably murders economy with this engine, despite BSFC charts not showing a cliff.




I drive the same routes frequently (hundreds of times over the last few years) so I've had a chance to do a lot of apples-to-apples comparisons. Let's say I have a 10 mile trip on the highway. I reset the fuel economy gauge before taking the on ramp, and check it again after 10 miles.

Pedal to the floor during acceleration but shifting at ~2000rpm and taking my time getting up to speed, I may arrive at my destination with 100-110mpg on the gauge, after 11-12 minutes of steady-state cruising at 50-55mph.

Shifting at ~3500rpm, I might arrive around 90mpg.

Taking 2nd gear up to highway speeds and shifting at 5000rpm, I'll be lucky to get my average up to 75mpg.

Redline it and it will take a long time to climb out of even the 30's with a result of the end of trip reading being in the low 60's.

I figure 10 miles is probably long enough that the greater rate of acceleration probably makes very little impact on average trip speed, so all I can think of to account for this is a serious drop in efficiency, despite what I can make of BSFC charts.

YMMV

[JockoT]

Originally Posted by Ecky

Pedal to the floor during acceleration but shifting at ~2000rpm and taking my time getting up to speed, I may arrive at my destination with 100-110mpg on the gauge, after 11-12 minutes of steady-state cruising at 50-55mph.

Shifting at ~3500rpm, I might arrive around 90mpg.

Taking 2nd gear up to highway speeds and shifting at 5000rpm, I'll be lucky to get my average up to 75mpg.

Redline it and it will take a long time to climb out of even the 30's with a result of the end of trip reading being in the low 60's.

This is very similar to my findings, though I can only wish my numbers were as good as yours!

[cr45]

Originally Posted by Panther140

What I said was that climbing the hill requires the same amount of energy regardless of how fast you do it. Overcoming wind drag is a separate topic. That wind drag would exist regardless of the hill. Wind resistance and grade resistance are different factors and I was focusing on the topic of overcoming grade resistance.


Actually, this is what you said.

As far as the amount of energy it takes to climb the hill, it doesn't matter how fast or slow you do it. It takes X amount of energy to move your vehicle up to the top of that hill, whether you're coasting or not.
Obviously if you encounter wind resistence while climbing the hill, you are going to burn more fuel. But the hill itself did not take more energy from the vehicle. That was the wind. Not the hill.

There is no indication that you are discounting aerodynamic drag!



Brake specific fuel consumption is still more efficient under heavy load, even with fuel enrichment. My "argument" is actually a fact that applies to all quantity-regulated spark-ignited gasoline engines. Do you know what BSFC is?
This is an incredibly facetious statement!

It basically indicates how much work the engine will get done if you give it a certain amount of fuel.

If you supply an engine with one liter of gasoline, and set the engine to peak torque near WOT, it will do more work before running out of fuel than if you give the same engine the same amount of fuel and run it with low load.

For a while, I worked as a fuel systems engineer for a well known company that manufactures internal combustion engines. I'm still very much involved in engineering projects which have the sole intent of increasing the thermal efficiency of engines. Not everything regarding efficient operation is intuitive. I'm not saying to blindly accept what I'm saying, but I recommend you withhold conclusion on this topic and research more.

Small engine cars are already near full throttle whilst climbing hills at highway speed. Several posters have indicated as much to you yet you keep repeating what is only applicable to large engine cars.

[Panther140]

Originally Posted by Ecky

So, although I've been arguing for accelerating up hills, my real-world experience shows (after a certain point) a huge FE penalty from downshifting. My Insight has peak torque (from gasoline alone) at 5000rpm, but accelerating in low gear, high RPM reliably murders economy with this engine, despite BSFC charts not showing a cliff.




I drive the same routes frequently (hundreds of times over the last few years) so I've had a chance to do a lot of apples-to-apples comparisons. Let's say I have a 10 mile trip on the highway. I reset the fuel economy gauge before taking the on ramp, and check it again after 10 miles.

Pedal to the floor during acceleration but shifting at ~2000rpm and taking my time getting up to speed, I may arrive at my destination with 100-110mpg on the gauge, after 11-12 minutes of steady-state cruising at 50-55mph.

Shifting at ~3500rpm, I might arrive around 90mpg.

Taking 2nd gear up to highway speeds and shifting at 5000rpm, I'll be lucky to get my average up to 75mpg.

Redline it and it will take a long time to climb out of even the 30's with a result of the end of trip reading being in the low 60's.

I figure 10 miles is probably long enough that the greater rate of acceleration probably makes very little impact on average trip speed, so all I can think of to account for this is a serious drop in efficiency, despite what I can make of BSFC charts.

YMMV

That makes sense. 5000RPMs is too high of an RPM for that car. Gotta upshift, we're on the same page there.

[Panther140]

Originally Posted by cr45

Small engine cars are already near full throttle whilst climbing hills at highway speed.

Good. Keep them at WOT and near/at peak torque while climbing. Coast down the other side of the hill.

Originally Posted by cr45

Several posters have indicated as much to you yet you keep repeating what is only applicable to large engine cars.

The sizing of the engine has nothing to do with it. Regardless of which car you are in, drive like Ecky and I are saying. It is the most efficient way. Get the work done with the engine at peak torque and WOT. Coast down the other side. Avoid wind drag.

Originally Posted by cr45

Actually, this is what you said.
As far as the amount of energy it takes to climb the hill, it doesn't matter how fast or slow you do it. It takes X amount of energy to move your vehicle up to the top of that hill, whether you're coasting or not.
Obviously if you encounter wind resistence while climbing the hill, you are going to burn more fuel. But the hill itself did not take more energy from the vehicle. That was the wind. Not the hill.

There is no indication that you are discounting aerodynamic drag!

I see. You assumed extra conditions when I specifically addressed the factor of grade resistance. Grade resistance, rolling resistance, wind resistance, and vehicle parasitic losses are all separate factors.

We are talking about grade resistance and how to leverage it most efficiently.