Do-it-yourself regenerative braking?

[EOD guy]

That actually sounds very reasonable to me. The amount of power used to go from a stop, to moving at about 20 mph is HUGE. That's all I would expect to get out of a regenerative brake system. And all I would really want. This would save a lot of fuel for a gas/diesel powered car. I know you couldn't run the car off it for very long, that would be breaking one of those silly laws that states something along the lines of perpetual motion not being possible. However, you could recoup that energy, and recycle about 60 percent of the energy used to stop. That's my goal.
However, another question would be, would you save just as much energy, by keeping the vehicle lighter, and not putting capacitors, motors, and gears in it?

[Blue Bomber Man]

Im sure that as long as you size the system appropriately that it would be energy efficient. Cost and return on investment are the real questions. With lith ion batteries plan on about $1000 per kwh, probably need about 2-3 kwh (prius has 7 i think). Weight for the batteries would only be around maybe 25 lbs per kwh (their gravimetric density is usually 150-250 w/gram)

A small motor/generator /w control unit would maybe be 75-100 pounds (I am ball parking all these numbers)

The real challenge would be in integrating it to a non-hybrid vehicle.

In another note, I am hoping to begin building my website soon that will track the progress of my car that I hope to build from scratch. I hope materials and batteries come down in the next year or two for when (if) I start building it =)

[BB73]

I had a thought the other day after reading this site. People were talking about disconnecting their alternators for better mileage. But then there is the problem of what charges the battery?

I thought that it would be neat to have an alternator or generator that works to slow the vehicle down and that would charge the battery.

Where to put it? On some front wheel drive vehicles there is a jackshaft that connects the transaxle to right side (usually) of the car. The jackshaft keeps the cv axle shafts equal length and lessens torque steer.

So replace the jackshaft or tie into it with the alt/gen and it slows the car while making electricity to charge batteries.

The negatives that I can see right off is that it would slow one side more than the other depending on the differential. Potentially leading to issues of one wheel lock up or funny steering of the car.

Of course the control of braking with the system would need some electronics too.

[jamesqf]

Why not keep the existing alternator, but wire in a circuit so that it only charges when you're braking? Should be fairly simple, just a relay that's energized by the brake light circuit.

And if you want to get a little more complicated, and have a second switch that lets you turn on charging when you're going downhill or coasting to a stoplight. You could also monitor battery voltage with a Scangauge or similar, and turn on charging if the battery gets critically low...

[Bearleener]

Yeah, getting the power to the pavement is the main problem of any retrofit system.

Here's a back-o-the-envelope calculation for a compressed air / air motor regen system (all metric units here, makes it easier):

Let's assume we have a suitable air motor and the system is 100% efficient (yeah, right...). The popular LP gas conversion systems have cylindrical tanks that fit in the spare tire well. With my 185/60 R14 tires that's about 50 litres capacity to use as a compressed-air tank. The LP tanks usually run at about 8 bar (120 psi) pressure. That means I could store an energy of E= Volume*Pgage = (50/1000 m^3)*(8 bar * 101.325 kPa/bar) = 40.5 kJ.

My car weighs about m= 1100 kg. Set the air energy equal to the kinetic energy of the car: E = (1/2)*m*v^2 means I could accelerate from a stop to a speed of about v= 8.6 m/s or 31 km/h (19 mph) -- not bad! I could bump start from there.

Let's accelerate modestly and say we can live with a 10-second zero-to-31 km/h time. That's an average acceleration of a= v/t = 8.6 m/s / 10 s = 0.86 m/s^2. Newton told us that F= m*a = 1100 kg*0.86 m/s^2 = 946 N of force needed to accelerate the car. That's quite a lot. (By the way, the power is (at half of the 31 km/h terminal speed) P = F*v = 946 N * (8.6/2) m/s = 4 kW (5.5 hp).)

Suppose the air motor uses a roller to drive the tire or the pavement directly. This roller needs to achieve 946 N of traction. Using an optimistic coefficient of friction of 1.0, the roller would have to press down with a force of 946 N (about 95 kg or 210 lb) to avoid slipping. Too much! Very hard to do with a roller or tire. Back to the drawing board! For comparison, Mike's Insight (see 1st post of this thread) presses with 130 lb of force.

Okay, but maybe we can live with less power and only use it to boost acceleration over longer periods. Or figure out a way to couple it more directly to the driveline.

No wonder nobody offers such a system!

[jamesqf]

Originally Posted by Bearleener

YThe LP tanks usually run at about 8 bar (120 psi) pressure.

How about a couple of scuba tanks instead, at about 3000 PSI (IIRC from my diving days)?

[Daox]

What kind of onboard pump is gonna get pressure up to 3000 psi though? Nothing I want to pay for.

[moorecomp]

Originally Posted by jamesqf

Why not keep the existing alternator, but wire in a circuit so that it only charges when you're braking? Should be fairly simple, just a relay that's energized by the brake light circuit.

And if you want to get a little more complicated, and have a second switch that lets you turn on charging when you're going downhill or coasting to a stoplight. You could also monitor battery voltage with a Scangauge or similar, and turn on charging if the battery gets critically low...

How about using an a/c clutch pulley on the alternator, connected to a relay, controlled by the brake light circuit. Kind of like running beltless but charges when you are applying the brakes?

[dremd]

Originally Posted by Daox

What kind of onboard pump is gonna get pressure up to 3000 psi though? Nothing I want to pay for.

Potentially; a hydraulic pump would be affordable.

[Bicycle Bob]

First, let's remember that to get as much deceleration as acceleration requires a generator as powerful as the motor. A couple thousand watts would be barely enough to noticeably affect the coasting. Full electric or hybrid setups have the kind of muscle for average braking forces, but still rely on friction for quick stops.

If you only want to store enough power for a minute, you need something that will accept very high rates of input/drawdown. The trouble with air storage is the energy lost if the air is not kept hot. It could be OK in a small system. A big one gets dangerous, anyway.

Rubber is another potential storage medium, but as with all mechanical systems, you tend to want a wide-range, variable speed gearbox between the wheels and the variable torque produced at the storage levels.

Capacitors are nice because they are plenty fast, efficient, and long-lived. I'd always use them as a buffer on a battery, to save wear and tear and increase overall efficiency.

In Bicycling Science, Whitt and Wilson list lead-acid batteries at 85 watt-hr/lb. Ni-cad is somewhat better, and can provide high rates of power per pound, although run time would usually be over five minutes, and a thousand cycles per pack might seem a bit high-maintenance. W&W also list "flywheel" at 14 w-hr/lb, "compressed gas and container at 10, rubber bands at 1, [steel] springs at .06, and capacitors at .006, although they have improved considerably since then.