High acceleration reduces range. but Why?

[Sven]

It is a commonly known fact that a heavy foot on the accelerator pedal will reduce range, but It would be nice to know the physics behind the fact.
In a very simplified point of view, the same amount of energy is required to bring a vehicle to a certain speed regardless of the acceleration. With this in mind, the rule does not make sense.

Can anyone with more knowledge explain this?

 

[Randy]

There is definitely more wind resistance to overcome at higher speeds....

 

[johnqh]

There is definitely more wind resistance to overcome at higher speeds....

But that's about speed, not acceleration.

In physics, 0 to 60 in 5 should use the same amount of energy as 0 to 60 in 20 seconds.

 

[greenleaf]

http://en.wikipedia.org/wiki/Drag_(physics" onclick="window.open(this.href);return false#Drag_at_high_velocity

The power needed to push an object through a fluid increases as the cube of the velocity. A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7.5 kW) to overcome air drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW). With a doubling of speed the drag (force) quadruples per the formula. Exerting four times the force over a fixed distance produces four times as much work.

 

[johnqh]

http://en.wikipedia.org/wiki/Drag_(physics)#Drag_at_high_velocity

The power needed to push an object through a fluid increases as the cube of the velocity. A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7.5 kW) to overcome air drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW). With a doubling of speed the drag (force) quadruples per the formula. Exerting four times the force over a fixed distance produces four times as much work.

Again, do not confuse speed (velocity) and acceleration.

Acceleration is the delta of velocity.

 

[ebill3]

It is a commonly known fact that a heavy foot on the accelerator pedal will reduce range, but It would be nice to know the physics behind the fact.
In a very simplified point of view, the same amount of energy is required to bring a vehicle to a certain speed regardless of the acceleration. With this in mind, the rule does not make sense.

Can anyone with more knowledge explain this?

I'm not so sure it is a "commonly known fact" that rapid acceleration (drastically) reduces range. The subject was tossed around quite a bit about a year ago, and as I recall, it was not considered a major contributor to range loss. Higher IR losses at higher currents will cause some reduction, but speed is the killer, not getting to speed.

 

[RegGuheert]

It is a commonly known fact that a heavy foot on the accelerator pedal will reduce range, but It would be nice to know the physics behind the fact.
In a very simplified point of view, the same amount of energy is required to bring a vehicle to a certain speed regardless of the acceleration. With this in mind, the rule does not make sense.

As already mentioned, the faster you accelerate up to speed, the higher your AVERAGE speed during the trip, so the higher your aerodynamic losses.

But high acceleration also creates additional loses in other ways. The losses in the battery are related to the square of current flow, so twice the rate of acceleration translates into four times the losses in the battery (and wires, etc.). So if you imagine accelerating up to speed at 80kW instead of 10kW, your acceleration rate will be 8X higher, but your losses in the battery will be 64X higher. Yes, the loss at 10kW will be sustained for a longer time, but not 64X longer (assuming that is enough power to get you up to speed).

 

[apvbguy]

I might be too much of an ignoramus for this conversation but I always thought that it takes more energy to get something move than it is to maintain movement, in other words taking the car from 0-60 would consume more energy than continuing to go 60

 

[camasleaf]

Like RegGuheert said electrical losses are proportional to the square root of current. There must be something similar for mechanical losses as friction in all the gears is proportional to the force applied at the point of contact.
Also efficiency varies at different power levels, mabe not much for EVs.

But effieciency and mechanical losses affects the ICEs too.

 

[Yogi62]

First off, define "range". If that is GOM range, then it reduces because it thinks you are going to keep doing that. It's logic is hard to fully understand because we don't have the code.

As far as "lowering your miles/KW" rating, here are my observations and speculations:

1. Accelerating faster to cover a set distance means that your average speed during that distance is higher. While distance/speed is a linear function, the air resistance grows faster than linear. you spend less time at 5-10, 10-15, 15-20 etc, and more time at your final speed.

2. The motor might not be as efficient at higher KW loads as it is at lower KW loads. 10 KW might provide N G's of acceleration while 20 KW might not provide 2N G's. Heat loss, converter losses etc may be much much higher at 80KW than 10KW. Could be lower too.

3. If you are "punching it" you might back off more than you need to at the final speed and trigger some regen which costs you energy. If you are slowly reaching that speed you can gently reduce positive acceleration without getting into regen.

4. If you are traveling at a higher speed at any point, the more energy you lose if you suddenly have to slow down. If those events occur randomly**, on average you will lose more energy from some events if you reach high speeds faster.

**I have noticed that traveling slower than 20 MPH appears to invite people to turn in front of you, enter the roadway etc, as going that speed or lower may be perceived as "ready to yield". Sometimes going a little faster leads to more constant speeds.

 

[RegGuheert]

Also efficiency varies at different power levels, mabe not much for EVs.

This is true and it complicates the discussion greatly. For reference, here is a plot of the 2011 LEAF drivetrain efficiency (from the February 2011 issue of Vehicle Electrification, page 17):



There are other losses in the drivetrain that are somewhat constant whenever you are applying any power, so there is a certain amount of power (well above zero power) that gets you peak efficiency. The simple result of this fact is that if you ONLY ever accelerate at peak drivetrain efficiency and then coast, you will achieve the highest efficiency and longest range. Simply using low power levels for acceleration does not get you there.

 

[johnqh]

Also efficiency varies at different power levels, mabe not much for EVs.

This is true and it complicates the discussion greatly. For reference, here is a plot of the 2011 LEAF drivetrain efficiency (from the February 2011 issue of Vehicle Electrification, page 17):



There are other losses in the drivetrain that are somewhat constant whenever you are applying any power, so there is a certain amount of power (well above zero power) that gets you peak efficiency. The simple result of this fact is that if you ONLY ever accelerate at peak drivetrain efficiency and then coast, you will achieve the highest efficiency and longest range. Simply using low power levels for acceleration does not get you there.

Since Leaf's transmission is single speed, the RPM is directly linked to speed.

Thus, I still don't know how pushing the pedal harder would decrease the efficiency.

 

[GeekEV]

I would guess it has to do with the fact that the *distance* required to do 0-60 in 5 seconds is vastly different from 0-60 in 20 seconds. Without doing the exact math, let's say it's 1/4 the distance. That would mean you could go 4x further at the lower acceleration rate than you could at the higher rate. Thus, your range is reduced by accelerating faster.

 

[davewill]

I've always suspected the answer lies in the fact that someone who accelerates hard generally ends up decelerating hard. I think that if you tested in a perfect environment where you accelerated, stayed at a constant velocity, then braked very gently to a stop, you wouldn't see a lot of difference between accelerating hard and soft. But in city driving, the person who accelerates fast ends up having to brake more when lights and other traffic get in the way (the slower car with more time available can just modulate their acceleration).

We'll never know unless someone can whomp up a test.

 

[RegGuheert]

Since Leaf's transmission is single speed, the RPM is directly linked to speed.

That's correct.

Thus, I still don't know how pushing the pedal harder would decrease the efficiency.

As previously mentioned, the battery losses will be higher the harder you push the pedal.

As far as the drivetrain goes, it will depend what speed you are traveling whether or not pushing the pedal increases or decreases your efficiency. At 6000 RPM, pushing the pedal down improves drivetrain efficiency. I doubt that it improves overall efficiency, because the battery gets more and more inefficient as you press the pedal harder.

 

[johnqh]

First off, define "range". If that is GOM range, then it reduces because it thinks you are going to keep doing that. It's logic is hard to fully understand because we don't have the code.

As far as "lowering your miles/KW" rating, here are my observations and speculations:

1. Accelerating faster to cover a set distance means that your average speed during that distance is higher. While distance/speed is a linear function, the air resistance grows faster than linear. you spend less time at 5-10, 10-15, 15-20 etc, and more time at your final speed.

2. The motor might not be as efficient at higher KW loads as it is at lower KW loads. 10 KW might provide N G's of acceleration while 20 KW might not provide 2N G's. Heat loss, converter losses etc may be much much higher at 80KW than 10KW. Could be lower too.

3. If you are "punching it" you might back off more than you need to at the final speed and trigger some regen which costs you energy. If you are slowly reaching that speed you can gently reduce positive acceleration without getting into regen.

4. If you are traveling at a higher speed at any point, the more energy you lose if you suddenly have to slow down. If those events occur randomly**, on average you will lose more energy from some events if you reach high speeds faster.

**I have noticed that traveling slower than 20 MPH appears to invite people to turn in front of you, enter the roadway etc, as going that speed or lower may be perceived as "ready to yield". Sometimes going a little faster leads to more constant speeds.

1. Accelerating faster to cover a set distance will increase the average speed, thus increase the average drag. However, increasing the average speed also decreases the travel time.

2. Agree. But how much difference do we have? The Leaf vs Volt thread has some discussion on the efficiency at high RPM above 70mph etc to improve efficiency by 10% to 20%. So, at city speed (0 ~ 45), how much difference is there between the high RPM and low?

3. That's not necessarily the right assumption. The OP didn't ask about using brakes. So, let's give the benefit of doubt and assume the braking pattern stays the same.

4. Again, See #3.

 

[TomT]

No, gear train losses are more linear...

Like RegGuheert said electrical losses are proportional to the square root of current. There must be something similar for mechanical losses as friction in all the gears is proportional to the force applied at the point of contact.

 

[johnqh]

The losses in the battery are related to the square of current flow.

1kwh is 1kwh. Actually, if there is no drag and no rolling resistance, and the motor and battery is 100% efficient, 1kwh of battery lose will increase the velocity by exactly 1kwh, whether you use 1amp or 0.1map (10x longer)

I think you meant the heat loss in battery (and the whole electric system) is proportional to the square of current flow.

 

[Sublime]

While I'm sure there is some loss in mpkW due to the electrical loss from increased current and increased average speed, its no where near how acceleration hurts mpg in an ICE. ICE drastically decrease their air:fuel ratio to increase power at full throttle. This is where this thinking comes from.

There's probably also a small trade off in motor (power for efficiency) by changing the degrees of slip. Again not even in the same ballpark as ICEs though.

 

[RegGuheert]

I think you meant the heat loss in battery (and the whole electric system) is proportional to the square of current flow.

I meant what I said.

Many of the losses in the electrical system are NOT proportional to the square of current flow. Some are constant. Others increase linearly with drive current. There are likely other relationships, as well.