The Model 3 already uses the Bosch iBooster setup which can (and does in the case of my Volt) support blending regen and friction brakes on the brake pedal. So the hardware is already there. Just a matter of software. Doesn't seem to be a huge demand for it though.
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Model 3's awesome Neutral coasting / Is it more cost effective than regenerative braking?
- Thread starter stretchymantis
- Start date
The Model 3 already uses the Bosch iBooster setup which can (and does in the case of my Volt) support blending regen and friction brakes on the brake pedal. So the hardware is already there. Just a matter of software. Doesn't seem to be a huge demand for it though.
joebruin77
Active Member
Some pertinent reading from the EVobsession.com website:
Another mythical idea I have come across is that you can increase your range by coasting. By “coasting,” people mean putting the vehicle into neutral and simply rolling down hills. In electric vehicles, although there is a selector, which is similar in appearance to an automatic gearbox, there is no gearbox or clutch of any kind in an electric vehicle. The motor is permanently connected to the driving wheels and the only gear is a permanent reduction gear to provide the required level of torque at the driving wheels. In an ICE car, the engine has to be started, and so, cannot be permanently connected to the driving wheels. The neutral position actually disconnects the engine from the driveshafts, and the “reverse” position actually uses a gear to turn the driveshafts in the opposite direction, while the engine, of course, always spins in the same direction. In an electric vehicle, the “reverse” position operates an electrical switch to reverse the polarity of the motor to make it spin backwards. It is, I have found from my own experience, an alarming feature of electric vehicles that they accelerate just as quickly in reverse as in drive — though, thankfully, the top speed is generally software limited to about 30 mph in reverse.
In an electric vehicle, putting the selector into neutral does not disconnect the motor, but is, again, an electrical device, which isolates the motor from the control system so that the motor just spins freely, with neither drive nor regenerative braking. So, putting your electric vehicle into neutral and coasting does not allow the vehicle to roll more freely, as mechanical losses from the drivetrain remain unaltered. If you are going downhill faster than you would if coasting, then you are using some current from the battery. If you are going at exactly the same speed as you would if you were coasting, then even though your selector is in drive, you will not be using any current. And if you’re going slower than you would be if coasting, then regenerative braking will come into play, and you will be recharging your battery while going downhill. That being the case, there is no advantage in putting your car into neutral, and you may even be missing out on some much-needed recharging. If you actually use the brake to control your speed while coasting, then you are converting your kinetic energy into wasted heat rather than converting it into electrical energy for charging your battery, so this is actually less efficient.
The only time it would be more efficient to select “neutral” rather than “drive” is when stopped with your handbrake on, as even with your foot off the accelerator pedal, a low current is generally fed to the motor. This would be taking power from your battery with no movement, and so reducing your range. The same does not apply to using the brake pedal, as the motor management system cuts out any current to the motor when the brake is applied.
Another mythical idea I have come across is that you can increase your range by coasting. By “coasting,” people mean putting the vehicle into neutral and simply rolling down hills. In electric vehicles, although there is a selector, which is similar in appearance to an automatic gearbox, there is no gearbox or clutch of any kind in an electric vehicle. The motor is permanently connected to the driving wheels and the only gear is a permanent reduction gear to provide the required level of torque at the driving wheels. In an ICE car, the engine has to be started, and so, cannot be permanently connected to the driving wheels. The neutral position actually disconnects the engine from the driveshafts, and the “reverse” position actually uses a gear to turn the driveshafts in the opposite direction, while the engine, of course, always spins in the same direction. In an electric vehicle, the “reverse” position operates an electrical switch to reverse the polarity of the motor to make it spin backwards. It is, I have found from my own experience, an alarming feature of electric vehicles that they accelerate just as quickly in reverse as in drive — though, thankfully, the top speed is generally software limited to about 30 mph in reverse.
In an electric vehicle, putting the selector into neutral does not disconnect the motor, but is, again, an electrical device, which isolates the motor from the control system so that the motor just spins freely, with neither drive nor regenerative braking. So, putting your electric vehicle into neutral and coasting does not allow the vehicle to roll more freely, as mechanical losses from the drivetrain remain unaltered. If you are going downhill faster than you would if coasting, then you are using some current from the battery. If you are going at exactly the same speed as you would if you were coasting, then even though your selector is in drive, you will not be using any current. And if you’re going slower than you would be if coasting, then regenerative braking will come into play, and you will be recharging your battery while going downhill. That being the case, there is no advantage in putting your car into neutral, and you may even be missing out on some much-needed recharging. If you actually use the brake to control your speed while coasting, then you are converting your kinetic energy into wasted heat rather than converting it into electrical energy for charging your battery, so this is actually less efficient.
The only time it would be more efficient to select “neutral” rather than “drive” is when stopped with your handbrake on, as even with your foot off the accelerator pedal, a low current is generally fed to the motor. This would be taking power from your battery with no movement, and so reducing your range. The same does not apply to using the brake pedal, as the motor management system cuts out any current to the motor when the brake is applied.
destructure00
Active Member
Seems to me like there's really no benefit (other than just "feeling" like it's different) to actually shifting to neutral vs just managing the accelerator pedal position. No physical change in the car/drivetrain, and managing the accelerator can reduce/eliminate current to the motors just like going to neutral does.
That's not what I mean. Tesla already blends regen and mechanical brakes. You can drive in low regen and if you press brakes - it still going to use full regen. But with any pedal pressure you always use mechanical brakes.
To avoid that system have to be not boosting but isolated. Complete disconnection of pedal and hydraulics. You put an actuator that gives feedback to the pedal and use ESP with booster to do what system feels is right. It's very expensive to do it reliable and fault tolerant. On a positive side you can completely redefine brake behavior.
Technically even now Tesla can make software adjustable brake bite curve. But it will hurt an economy. Track Mode 3? And for a city driving one pedal drive is anyway more convenient where it's already using mechanical brakes only when it's needed.
In the scenario where regen is set to "Low" instead of "Standard", the max regen level is limited on pedal lift. If the iBooster was fully supported, the car could blend in that extra regen potential when the driver uses the brake pedal. It is the way my Volt, and many other cars, work.
eladts
Member
Not to mention that illegal in many jurisdictions to coast in neutral, for this exact same reason. Your life and the lives of others on the road are worth much more than the few cents you save by coasting.
At the top of a hill you have Potential energy. The mass of the vehicle pulls you down the hill and converts potential energy into Kinetic energy. The car will continue to accelerate until drag and rolling friction equal the available energy provided by the hill. All this with no losses. If the vehicle is in drive and you fail to keep the accelerator exactly neutral, energy will be transferred either by acceleration or regeneration. As the transmission is not 100% efficient, this results in losses. QED the use of neutral would be more efficient, if you can be bothered.
Also I don’t understand why this would be more dangerous, the brakes still work in neutral!
Also I don’t understand why this would be more dangerous, the brakes still work in neutral!
Msjulie
Active Member
I *think* the reasoning is that in neutral, you can't accelerate if you have a sudden need to get moving again and in an ICE car, particularly? a manual gearbox, the engine goes to idle so popping it back into whatever gear to get moving is another few moments/throttle blip/etc on top of just having to re-engage the drivetrain.
So how big a danger, good question. Shifting to neutral hardly seems worth it to me though
Mass matters because aero effects are proportional to frontal area/shape and kinetic energy is proportional to velocity/mass. Any time we're talking about aerodynamic drag, we're really talking about the aerodynamic drag bleeding off kinetic energy, which depends on mass. It's a similar concept to how a bowling ball will accelerate much faster than a bowling ball sized foam sphere if you drop it. They both have the same aerodynamoc drag, but one has a higher potential energy which gets turned into kinetic by the force of gravity.
That being said, it sounds like the Model 3 isn't much heavier than similar sedans, which surprises me a bit! I always assumed the batteries must outweigh ICE powertrains but I guess that's not really the case?
You get no regen in N so I don't see how that's more efficient than lightly stepping on the accelerator and coasting down hill and still getting regen. Plus driving in N is dangerous, if you suddenly need to accelerate you can't and you already said, in the Tesla braking is compromised in N.
Not a big danger. There really aren't any practical scenarios where someone would be coasting (say, up to a light) where emergency accelerating is the answer. To be fair, there are few if any actual emergency situations where accelerating is the answer--anytime you hear of someone throttling out of any "emergency" situation (like, stuck between those two trucks), they're almost always talking about a convenient escape, not an emergency escape and certainly not the safest escape.
We're not Cole Trickle blasting through the high line here; pretty much any true emergency other than oversteer (and maybe a rear-ended-while-stopped scenario) is best mitigated by braking. Rarely is adding kinetic energy the most effective way to reduce vehicular risk.
/endrant
Regarding efficiency of regen vs coast, its case specific and definitely more of a spectrum than a one-or-the-other. At-speed scenarios would tend toward burning energy through regen vs coasting, but that can be more uncomfortable for the occupants and annoying for surrounding motorists. Low speed scenarios would tend toward the opposite, though with an equivalent potential for occupant/motorist annoyance.
Given the basic motor skills required to modulate the pedal vs the general annoyance of flipping between Duh and Nuh on the stalk, the case-specific efficiency bias for each situation, and the micro-efficiencies we're talking about here, I'd be surprised if the long term ∆ efficiency was anything but a wash.
fsch
Member
Hi guys,
This answer comes a bit late but, in fact, in most situations (i.e. when the slope is not too steep and not too long), you can save a quite small amount of energy by coasting rather than regen (because you store energy in the form of kinetic energy rather than in the not-100%-efficient battery, but this increases drag losses). So there is an optimal distance from the end of the slope to go in neutral, which depends on the mass m and drag force F = cv²+k of the car. Assuming a straight slope on highway without anything forcing to slowdown (other cars, speed limits, whatever), that distance is given by:
where sin(θ) is the slope, ε is the battery charge-discharge cycle efficiency (typically 60-65%), and v0 is the cruising speed in the regen case. A is the ratio of the gragitational force to the drag force at speed v0. L_opt is typically more than half a mile for slopes <6% when v0 is 60 mph (= 96 km/h = 27 m/s). But it will only save a few Wh compared to regen.
See more details in this thread and in this article.
This answer comes a bit late but, in fact, in most situations (i.e. when the slope is not too steep and not too long), you can save a quite small amount of energy by coasting rather than regen (because you store energy in the form of kinetic energy rather than in the not-100%-efficient battery, but this increases drag losses). So there is an optimal distance from the end of the slope to go in neutral, which depends on the mass m and drag force F = cv²+k of the car. Assuming a straight slope on highway without anything forcing to slowdown (other cars, speed limits, whatever), that distance is given by:
where sin(θ) is the slope, ε is the battery charge-discharge cycle efficiency (typically 60-65%), and v0 is the cruising speed in the regen case. A is the ratio of the gragitational force to the drag force at speed v0. L_opt is typically more than half a mile for slopes <6% when v0 is 60 mph (= 96 km/h = 27 m/s). But it will only save a few Wh compared to regen.
See more details in this thread and in this article.
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