Why my Wh/mi so bad?

[sleepydoc]

This seems hard to believe. Rate of acceleration does not reduce efficiency? For sure, slower starts are more efficient.

Both do - hard acceleration uses the motor(s) less efficiently. Braking takes all of the kinetic energy you've built up and wastes it on grinding away the brake pads. (this is part of the reason winter is so bad for range - when the battery is cold you can't use regenerative braking.)

 

[ZenRockGarden]

This seems hard to believe. Rate of acceleration does not reduce efficiency? For sure, slower starts are more efficient.

I didn't say it had zero impact on efficiency - just that it doesn't have nearly as much impact as other things like driving at high speed on the highway.

The motors are over 95% efficient, both in accelerating the car, and in regenerating that power back into the battery (as long as you stay out of the friction brakes)

This is fundamentally different from traditional gasoline engines which are more like 40% efficient and have no regen at all, thus hard acceleration is very wasteful. Much less so on electrics

 

[sleepydoc]

I didn't say it had zero impact on efficiency - just that it doesn't have nearly as much impact as other things like driving at high speed on the highway.

The motors are over 95% efficient, both in accelerating the car, and in regenerating that power back into the battery (as long as you stay out of the friction brakes)

This is fundamentally different from traditional gasoline engines which are more like 40% efficient and have no regen at all, thus hard acceleration is very wasteful. Much less so on electrics

If you're driving around town then you're repeatedly accelerating and repeatedly using more energy than necessary. If you accelerate hard once then sit on the highway going 80 MPH for 2 hours then the highway driving will have a greater cumulative effect on your range. The point still remains that both hard acceleration and high speeds have an effect. (along with everything else mentioned here.)

Add one other psychological factor - ICE never list their range, they list their MPG, and even then a lot of buyers barely look at the MPG rating. If an ICE car advertised its range and had a 'miles remaining' gauge instead of a Full-Empty gauge for gasoline people might be a bit more focused on it.

 

[ZenRockGarden]

If you're driving around town then you're repeatedly accelerating and repeatedly using more energy than necessary. If you accelerate hard once then sit on the highway going 80 MPH for 2 hours then the highway driving will have a greater cumulative effect on your range. The point still remains that both hard acceleration and high speeds have an effect. (along with everything else mentioned here.)

Add one other psychological factor - ICE never list their range, they list their MPG, and even then a lot of buyers barely look at the MPG rating. If an ICE car advertised its range and had a 'miles remaining' gauge instead of a Full-Empty gauge for gasoline people might be a bit more focused on it.

You're not understanding energy vs power.

If you accelerate a car from 0 to 60 mph, it takes the same total energy to get there regardless of if you do it in 5 seconds, or 10, or 100.

Teslas LOVE stop and go traffic because regen actually works. Feel free to put energy into forward momentum, 90% of it goes back into the battery as you slow down.

What DOES matter is the aerodynamic losses, which go up as a square of top speed. So get onto the highway by flooring it, or creeping down the onramp - the only thing which will really matter is what speed you run at for most of the trip. Faster burns more power. Period. The end. Accelerate rates are not interesting in that equation unless your actions cause you to dump energy out the friction brakes.

 

[dmh444]

yeah it's nonsense. Sure, the amount of kinetic energy in the car once you reach a certain speed is the same, but getting there expends all sorts of extra energy with higher acceleration (heat from friction, air resistance, losses in the power delivery system etc etc). The equations are complex of course, and a very very slow acceleration isnt the most optimal either, but just slamming on the power is certainly going to eat into your Wh/mi.

Regen is more complex, since the physical motors generate a braking force that is essentially proportional (thought not linearly) to the input torque on the shaft and the electrical load placed across the motor terminals. I'm not sure how Tesla moderate the regen braking, but I would suspect it is via a fast on/off modulation of the load in the inverter, which (if so) should be pretty efficient at most braking strengths.

Basically, the most energy-efficient way to drive any vehicle is at constant medium-to-low speed, with as little changes in speed as possible. For ICE cars the optimal speed is higher, since they waste so much energy in heat, and the time to get to the destination factors in more than with EVs (contrast an EV stuck stationary in traffic compared to an ICE).

I don't pretend to be an expert in physics, but according to ZenRockGarden, how fast you accelearate has no impact on energy or efficiency?
Maybe I'm just misunderstanding his post. So someone who guns it everytime vs someone who slowly steps on the pedal, but both using the same amount of regen brake will have the same wh/mi?

 

[ZenRockGarden]

Yes basic physics.

Accelerating an object to a given speed involves the exact same change in energy if you do it in 5 seconds or 100 seconds.

The only real-world difference is that the motors and the pack may give off a bit more heat at their highest torque/current levels so instead of 95% efficiency you might get 92% or something.

Once at speed, you have the same ability to put (most) of that energy back in the battery pack, regardless of how fast you got there.

The one great loss in the system is the SUSTAINED energy you must apply to keep pushing the car thru the air (and on the wheels) as you keep going at a given speed. That energy is 100% lost, and is why Tesla driving efficiency is mostly about top SPEED, and staying off the BRAKES, not so much about acceleration.

 

[FatherTo1]

I don't pretend to be an expert in physics, but according to ZenRockGarden, how fast you accelearate has no impact on energy or efficiency?
Maybe I'm just misunderstanding his post. So someone who guns it everytime vs someone who slowly steps on the pedal, but both using the same amount of regen brake will have the same wh/mi?


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That doesn’t make sense to me either. If you run a mile it burns more calories than if you were to walk a mile. So I would expect accelerating harder in the Y would use more energy, it sure seems to reflect it when you watch the energy meter while making a passing maneuver on the freeway. The motor efficiency is not equal across the board so there must be a difference in energy consumption. I don’t know the exact difference but I’d still expect hard acceleration to use more energy than gentle acceleration.

 

[ZenRockGarden]

That doesn’t make sense to me either. If you run a mile it burns more energy than if you were to walk a mile. So I would expect accelerating harder in the Y would use more energy, it sure seems to reflect it when you watch the energy meter while making a passing maneuver on the freeway. The motor efficiency is not equal across the board so there must be a difference in energy consumption. You may not lose a huge amount but I’d still expect hard acceleration to use more energy than gentle acceleration.

running and walking involve different SPEEDS, not different acceleration. Put yourself on a skateboard - you can push really hard to quickly get up to 10 mph on, it, or you can take your time and push very slowly to get up to the same 10 mph speed over a longer period of time - the invested energy will be identical, and your inertial energy rolling along at 10 mph (via 1/2 mv^2) will be the same no matter if you got there quickly or slowly.

 

[dmh444]

times like this, i wished i had paid more attention or not ditch physics in school.
i think someone needs to send this to elon and have him decide once and for all!

 

[thesmokingman]

You're not understanding energy vs power.

You are so far off you're into your own fantasy here.

 

[ZenRockGarden]

You are so far off you're into your own fantasy

ok, you explain the physics of power output in the form of kW from the motors, the resulting acceleration, final speed, and the energy expended.

 

[thesmokingman]

ok, you explain the physics of power output in the form of kW from the motors, the resulting acceleration, final speed, and the energy expended.

Haha, after you're made up physics and numbers? You're so deep in this rabbit hole you dug man.

 

[ZenRockGarden]

Haha, after you're made up physics and numbers? You're so deep in this rabbit hole you dug man.

I stand by my figures. You claim they are wrong. Feel free to correct any error I have made and present the right model for energy, power, acceleration, and speed.

I suggest using a high school physics book if you need help.

 

[dmh444]

let's take working out. everywhere i've read (and adding to FatherTo1's post), HIIT workouts are better because you burn more calories in a shorter amount of time. so if i do internal sprints for a quarter of a mile vs just casually walking the same distance, you're saying the body uses the same amount of energy and burns the same amount of calories?

 

[ZenRockGarden]

let's take working out. everywhere i've read (and adding to FatherTo1's post), HIIT workouts are better because you burn more calories in a shorter amount of time. so if i do internal sprints for a quarter of a mile vs just casually walking the same distance, you're saying the body uses the same amount of energy and burns the same amount of calories?

You have turned an inertial physics problem into... a human physiology and biochemistry experiment? I am hoping this is just a joke

 

[FatherTo1]

ok, you explain the physics of power output in the form of kW from the motors, the resulting acceleration, final speed, and the energy expended.

It still seems illogical to me. So then what’s the point of Chill Mode if faster acceleration uses the same amount of energy as slow acceleration?

There’s clearly a greater energy cost getting to speed quickly versus slowly. From a physic’s standpoint, as you accelerate more briskly there’s also less time for air molecules to get out of the way, placing more force you have to work against.

Think of the air molecules as a crowd of people you’re trying to get through. Walking through the crowd gives time for you to dodge and slip between people. If you try to run through a crowd you’re going to collide with someone and encounter more resistance. If you increased from a walk to a sprint as you work through a crowd then it increasingly becomes harder to push through the crowd. Same principle when accelerating the Y, it will be less efficient the more fun you have with it. That seems pretty evident in the form of more heat from the battery and motors that require additional cooling off time if you have repeated hard sprints.

 

[drtimhill]

In an ideal system, acceleration makes no difference since the final kinetic energy of the moving vehicle is the same. However, real-world cars are not ideal, and there are energy losses in all parts of the system, including friction in the moving parts (axles, tires etc) and electrical losses in the power system (batteries, inverters, motors), pretty much all of these ending up as heat. How much heat is complex, since the losses are related both to the time taken to get up to speed (which favor higher acceleration) and the instantaneous power being applied causing greater losses in the motors etc (which favor lower acceleration). This means the optimal power curve to minimize the energy needed to reach a certain speed is complex, and not necessarily as short as possible (that is, minimal energy is not automatically equated to minimum time, i.e. maximum acceleration).

I don't know what the optimal curves are, and I suspect they are very much related to the target speed. But I highly doubt they correspond to maximum acceleration.

As for regen, that also is complex and depends on the methods by which the car torques the motors to modulate the regen rate.

 

[sleepydoc]

In an ideal system, acceleration makes no difference since the final kinetic energy of the moving vehicle is the same. However, real-world cars are not ideal, and there are energy losses in all parts of the system, including friction in the moving parts (axles, tires etc) and electrical losses in the power system (batteries, inverters, motors), pretty much all of these ending up as heat. How much heat is complex, since the losses are related both to the time taken to get up to speed (which favor higher acceleration) and the instantaneous power being applied causing greater losses in the motors etc (which favor lower acceleration). This means the optimal power curve to minimize the energy needed to reach a certain speed is complex, and not necessarily as short as possible (that is, minimal energy is not automatically equated to minimum time, i.e. maximum acceleration).

I don't know what the optimal curves are, and I suspect they are very much related to the target speed. But I highly doubt they correspond to maximum acceleration.

As for regen, that also is complex and depends on the methods by which the car torques the motors to modulate the regen rate.

Thank you. I didn’t read all of the pissing match above, but @ZenRockGarden claims to know physics but really only has a rudimentary understanding. He/she assumes an ideal, lossless system where all energy is conserved and the efficiency of the batteries and motors is perfect and unrelated to power/current draw. All of these assumptions are false. Furthermore, many of the inefficiencies are nonlinear. There is likely an optimal rate of acceleration that optimizes motor load & efficiency with battery efficiency. It would be interesting to see what that is but I haven’t seen or ready anythign about it.

@ZenRockGarden - for starters, go read up on internal resistance of batteries then come back and correct your posts.

Edit - human exercise physiology is completely different from automotive power and efficiency so there’s zero point in comparing the two.

 

[father_of_6]

Accelerating an object to a given speed involves the exact same change in energy if you do it in 5 seconds or 100 seconds.

My physics knowledge is lacking, but this doesn't ring true to me.

Given a fixed speed, getting to it in a shorter amount of time should take more energy. Reducing the time to full speed isn't free.

Right?

 

[sleepydoc]

My physics knowledge is lacking, but this doesn't ring true to me.

Given a fixed speed, getting to it in a shorter amount of time should take more energy. Reducing the time to full speed isn't free.

Right?

No, the total energy required is the same, the power (energy per unit time) is greater. This is only in a strict, theoretical sense, though, as I mentioned above. In @ZenRockGarden's world, a perpetual motion machine would exist because there are no losses. The real world has losses. Good design does everything possible to minimize them, but they're not zero.

Edit:
In an ideal system, there are no internal resistances or losses in the battery and circuitry, the motor is perfectly efficient and there are no friction losses. In this case it doesn't matter how fast you accelerate or how many times you accelerate/decelerate.

In a slightly less ideal system, the internal battery resistance is static, the motor efficiency is static across its power curve and the friction losses are static. In this case it still doesn't matter how fast you accelerate but every time you accelerate and decelerate you lose energy due to friction, heating the battery, etc.

In a more realistic system, higher currents actually lead to more losses in the battery, the motor efficiency depends on the power and friction losses depend not the forces applied.