The regen braking isn't the sole source of efficiency gain -- aerodynamic drag, gearing and engine optimizing are responsible for the "true" gains -- but regen braking makes it more possible to realize the "true" gains because you're no longer throwing away all of the true gains on braking.
The above statement is false. Regenerative braking in the Tesla (which only has regenerative braking on the rear wheels) only recovers about 20% of the energy lost in stopping the car. Accelerating again requires 100% of the car's final kinetic energy to be provided by the battery. Therefore city driving is always less efficient than driving at a steady speed unless you're planning to drive at 90 miles per hour.
All regenerative braking can do is minimize the losses caused by stop-and-go driving, it is never an improvement over driving at a steady speed at the same average speed or, for that matter, most higher speeds.
I wish this myth would go away -- by way of some physics education.
> ... you're no longer throwing away all of the true gains on braking.
No, only 80% of them. Which means you would have to drive very fast to equal the losses of stop-and-go driving.
According to this, the tesla regenerative braking is just as efficient as the motor is at acceleration. Result: up to 64% of the original battery energy returned to the battery.
http://www.teslamotors.com/blog/magic-tesla-roadster-regener...
And this page about the Volt suggests that the efficiency might get up to 70% in some situations.
http://www.designnews.com/document.asp?doc_id=235313&dfp...
This is confirmed by measurements which show pretty much all hybrid vehicles get better mileage in city driving than in highway driving.
That's true but misleading. If the motor's combined electrical and mechanical losses cause only 64% of the battery's energy to propel the car forward, then by that reasoning, only 64% of the braking energy is made available for charging.
Let's look at all the factors that influence the outcome:
1. Both electrical and mechanical losses must be accounted for.
2. The electrical loss creates a factor of 0.8, the mechanical loss also create a factor of 0.8. Result 0.64.
3. On the Tesla, only the rear wheels have regenerative braking, the front wheels have disc brakes. Factor 0.5, so we are now at 0.32.
4. During a stop, the car pitches forward onto the front wheels and unloads the rear wheels, this needs to be taken into account. Factor 0.8 for a total of 0.2: 26%.
So I stand corrected -- this analysis concludes that the regenerative braking scheme recovers 26%, not 20%, of the original kinetic energy.
A literature search reveals that the best regenerative braking systems that involve all four wheels and in which every effort is made to maximize efficiency, can produce about a 40% energy recovery:
http://www2.hesston.edu/physics/201112/regenerativeenergy_cw...
A quote: "Since regenerative brakes can only recover about 40% of the energy we take the recoverable energy and multiply by .4 (Everett, Michael)."
The linked paper goes on to show that 40% is actually overly optimistic for various practical reasons. But notwithstanding that, we can take the original 40% figure and apply it to the Tesla. By taking into account that only two of four of the Tesla's wheels have regenerative braking, we arrive at a figure of 20%.
> And this page about the Volt suggests that the efficiency might get up to 70% in some situations.
No reputable source makes claims this high for a full round trip from battery stored energy to battery stored energy (the linked article doesn't try to do this). Also, the linked chart reports energy recovered in a four-wheel test and measured before being applied to the battery, not energy delivered to the battery as stored energy.
> This is confirmed by measurements which show pretty much all hybrid vehicles get better mileage in city driving than in highway driving.
That's true, but that doesn't result from regenerative braking. It results from lower average speeds and reduced wind and mechanical resistance.
Remember that regenerative braking cannot possibly improve on driving at a steady speed. That would violate some very basic physical principles.
I would hope that the on board software wouldn't use those front brakes during light deceleration. Anybody know more about this?
No, this isn't safe. A car that only allowed one set of wheels to brake would in some cases spin around dangerously. Safe brakes must apply the same force on all four (or more) wheels simultaneously.
Remember that the computer can't possibly know if the car is sliding across black ice, a situation in which the application of even force is an absolute requirement for a safe stop.
You're arguing against something I was careful not to say.
That may be true, but it's not the topic. The topic is how efficient regenerative braking is on the Tesla Model S.
I suspect that in both the NYT article and in this CR article, the author is mistaken about the role of regenerative braking on fuel efficiency, but they are not incorrect to point to regenerative braking as the enabler of improved fuel efficiency.
Some made up numbers to illustrate the point. I do not know if these apply to the Model S specifically but do apply to other vehicles with regen braking, familiarity with which is likely to have influenced the authors' explanations.
City driving vs. highway driving:
Lower drag: +20% efficiency
Lower engine RPM: +20% efficiency
More stop and go with normal braking: -50% efficiency
More stop and go with regen braking: -50%(.6) = -30% efficiency
= Net city driving with normal braking: -10% efficiency
= Net city driving with regen braking: +10% efficiency
But regenerative braking cannot produce net positive energy compared to not braking at all -- that violates basic physical principles.
Regenerative braking is better than no regenerative braking, but regenerative braking is not better than not braking at all. That has been the point all along.
> = Net city driving with regen braking: +10% efficiency
No, not compared to driving at a steady pace. If the scenario you describe were so, it would represent a gain in energy greater than 100%, and that is not possible.
Stop-and-go driving is always less efficient than driving at a steady pace at the same average speed. Regenerative braking can only minimize the loss caused by stop-and-go driving, it can't reverse it -- it can't produce a net gain as you are implying.
> More stop and go with normal braking: -50% efficiency
Wrong value. Apart from the fact that the number is invented, because each stop turns all the car's kinetic energy into heat, the number should be -100%. Either that or perform a full analysis, with a listing of all the factors.
After you recover 20% of the energy with regenerative braking, you end up with -80% instead of -100%.
I ride a low-power motorcycle. On the freeway, I get about 50 mpg. On back roads, with braking and generally higher rpm, I get 60+ mpg. For my low-power bike, the drag at freeway speeds takes more effort to overcome than does the varying speed and braking of back road riding.
And that is without regenerative braking. If I had regen braking, I'd get even better economy on back roads.
This is two articles now where the authors, or technicians they're talking to, seem to think the model S has found away around the laws of thermodynamics. It has not. Regenerative breaking will not charge your battery any more than you drained it getting up to that speed in the first place. In fact, by definition, it will charge the battery less. So anyone who thinks the key to getting more range out of an electric vehicle is to abandon smooth and steady driving and find a way to slam on the brakes is in for a rude surprise.