Is Regenerative Braking Real? Does it work as we thought?

[NeilBlanchard]

Edison2 is owned by Oliver Kuttner, and I know him -- he invited me to be on the Edison2 X-Prize team, so I could be there at the X-Prize. Their cars are not electric (though they may be putting an electric drive in one in the future). He is correct in that it takes less energy to accelerate a low weight car, and the regen would be proportional to that. So, it takes less energy to accelerate it, and you get less back, as well.

But that doesn't mean that regen is worthless; because even if you only get back a little -- it is worth just as much to you, because you don't need as much. If they put an electric drive and battery in their VLC, it would be heavier than it is with the ICE they use now.

If you need brakes to slow down (and what car doesn't?), then regen offers you braking where you can recover some of the energy.

[dcb]

but I think the case can be made that it does not work as well as people think, generally.

And if you are building a lightweight fuel car, useable regen would be extra weight.

[RobertSmalls]

Originally Posted by dcb

but I think the case can be made that it does not work as well as people think, generally.

And if you are building a lightweight fuel car, useable regen would be extra weight.

Depends on what people think. But I think the best post on the subject so far is your post with the graph showing a 22% improvement regardless of weight on the EPA cycle. Everyone here can beat EPA by doing a little DWB and anticipation, so expect an ecodriver to get <22%. Maybe 10%?

Edison2 has obviously determined that the weight penalty for making their VLC a hybrid erases their fuel savings.

DIY EV'ers may find that for the $ cost of going AC, they could instead buy more batteries to achieve greater range.

Folks who buy one-ton or heavier vehicles will probably say regen is well worth it, because it's a real boon on hills, and it's the best way to keep a hybrid's 12V battery charged.

[ShadeTreeMech]

one thought I have is what is the net energy profit? If it takes x amount of extra equipment to use regen braking, how much is the cost to move that equipment down the road? If x=0, then 1% recovery of the energy otherwise lost is infinitely larger than 0%

[Patrick]

Originally Posted by ShadeTreeMech

one thought I have is what is the net energy profit? If it takes x amount of extra equipment to use regen braking, how much is the cost to move that equipment down the road? If x=0, then 1% recovery of the energy otherwise lost is infinitely larger than 0%

A 2010 Prius weighs about 165 lbs more than a 2010 Corolla.

[user removed]

If you have to use the brakes in heavy urban traffic situations like my 40 mile 46 traffic light round trip daily drive then regen is helpful. When my battery was fully charged I could pulse with the battery and coast in neutral to increase my mileage in my CVT Insight.

I could manage low to mid 70s MPG, while averaging about 38 MPH for the two legs of my drive. If I got unlucky and hit more than a couple of lights my just replaced by Honda battery would be just below 20 bars on the gauge and I would pulse and coast to use some of the capacity.

Out in the country where IO might not have to stop for many miles, I would still use the same technique on grades when the battery was full.

I probably can't quantify the percentage benefit but it was most likely in the range of 10% under those circumstances. I never tried the daily trip without battery assist but I would guess it was 20% under those circumstances.

regards
Mech

[dcb]

Originally Posted by RobertSmalls

...the graph showing a 22% improvement regardless of weight on the EPA cycle....

Lemme clarify that, the percentage returns were approaching zero as the vehicle got lighter. The 5000 lb car had ~22% return, at 2000lbs it "looks" like about a 9% return.

But I have no clue how they made the graph, it fits nicely with their philosophy though. and at 750 lbs the return looks really tiny if at all.

"Our analysis of efficiency showed that the energy gained from regenerative braking simply was not worth the cost in added battery weight. One 6 lb gallon of gasoline contains the energy of 500 lbs of batteries; at least 1000 lbs of batteries would be needed if our 750 lb VLC were electric. That is a lot of weight to push around for 200 miles."

[Patrick]

Originally Posted by dcb

But I have no clue how they made the graph, it fits nicely with their philosophy though. and at 750 lbs the return looks really tiny if at all.

They said that the graph was made by holding the aero drag constant while reducing the weight. So aero drag would assume a higher proportion of the total energy use as weight was reduced. Presumably this could apply to a Prius that had been "lightened" by removing unneeded (for the competition) interior items, changing windows to lightweight materials, etc. But I'm not sure how applicable it would be to purpose-built car. Surely the aero drag could be reduced also and then the regen would take on a higher proportion of the total savings.

[dcb]

well it does get a bit complicated down the aero path, the aero has much less impact at lower speeds, where I have to do most of my braking.

[ShadeTreeMech]

Another thing to consider is the amount of braking power given by regen brakes. Seems like I read about Ben Nelson's friend with the AC powered electric car who hardly used his service brakes due to the regen braking strength of his motor.