Regenerative Braking

Yes, it is possible but, not advised.

 

Wouldn't the "B" mode accomplish this as well to some degree, particularly when your foot is lifted off the pedal?

 

There are some misunderstandings here still.

B mode is actually an "engine braking" mode, similar to downshifting a car with an automatic or manual transmission. Since the Prius uses an electric continuously variable transmission, there are no gears to downshift to in order to slow the vehicle down. What B mode does is simulate this by applying a regenerative current in MG1. Since MG1, MG2, and the gas engine are all constantly engaged with each other, and since MG2 is directly engaged to the drive wheels, applying a braking force to MG1 causes the gas engine to rev up, and then the computer adjusts the valve timing to maximize engine vacuum braking which aids in slowing the car down without using the friction brakes. This is actually designed to be used on long downhill stretches where the hybrid (HV, or traction) battery would normally fill up completely leaving no room for regenerative braking and causing all braking to be done by the friction brakes. This could eventually cause the brakes to overheat resulting in brake failure, so B mode is there to help mitigate this and prevent brake overheating when regenerative braking can no longer be done because the HV battery is fully charged.

B mode does not charge the HV battery more than D mode will, it actually charges it less for two reasons. One is that the HV battery in the Prius is relatively small and doesn't hold all that much energy (compared to what a full-sized car would need to be a useful all electric car), which means that it can only accept so much current at once-- beyond that current, and the car has to blend in the friction brakes to make up the difference. B mode has the same electrical current limits because these are battery limits, not MG limits, and the additional braking is done by spinning the gas engine with the braking force applied to MG1 rather than that energy being stored into the battery. And that is the second reason-- the extra energy isn't stored in the battery, it's used to spin the gas engine up so it can use the engine vacuum to aid in slowing the car down. When you're driving a given car at some arbitrary speed, there is only a fixed amount of energy available by slowing the car down. So anytime you direct any of that energy to somewhere other than the battery or other energy storage system, it's energy that's lost and can't be stored in the battery later-- you don't get that energy back. You'll save more energy by using D mode than B mode-- the only time B mode is a benefit is when you need to slow down faster than regen by itself can capture, or when you're going down a hill long enough that it will fully charge the HV battery in which case the excess energy is lost anyway, but B mode will bleed the excess energy off without eating up your friction brake pads and rotors.

As for pressing on the brake pedal while stepping on the gas to regenerate electricity to store in the HV battery, yes this can be done but it's not efficient and wastes gas.

What happens here is that a lot of people assume that regenerative braking is "free" energy. It's not free energy, it's energy that you've already burned to get the car moving in the first place. Regenerative braking recaptures some of that energy, but not all of it. There are losses at various points along the way that you can't get around because that's the way physics works.

What happens is that you convert either electrical energy or gasoline into vehicle motion. Gas engines in cars are only something like 40% efficient-- meaning 60% of the energy created by burning the gas is lost to internal friction in the engine and drive train itself, and to heat lost to the atmosphere, and only 40% of it actually makes it to the wheels to make the car move. Electric motors I think are more efficient than that, but there are still losses due to electrical resistance in the wiring and the motor itself, losses to heat from moving electricity into and out of the battery, and losses in the voltage/current conversion in the inverter that converts the electricity the motors need to the electricity the battery needs and vice versa.

I've read threads on here by people a lot smarter than me who've worked out the math on these losses, and if I remember correctly, by the time all is said and done, regenerative braking only captures about 35% of the energy used to create that energy in the first place. That means that if you burn let's say 1 gallon of gas and use it all for regen to charge up the HV battery (assuming it would hold anywhere near that much energy in the first place, which it won't), and then used that electricity to drive the car without using the gas engine, you'd only be getting .35 gallons of gas worth of energy back to move the car with-- and this is AFTER losing 60% of that energy by running the gas engine in the first place.

All in all, regen braking is only recapturing a very small fraction of the energy used to drive the Prius and using it to drive the Prius with the battery. This makes it sound like it's a waste of time if it's all so inefficient, but the reality is that 100% of the energy that the Prius captures with regen braking is energy that any other car just throws away completely and doesn't use in any way at all. That's why a Prius can get 50-60 MPG when any other similar car would probably only get about 35-40 MPG doing exactly the same thing but without the hybrid system.

What you can do-- and I do this myself when I'm going to automated car washes where you have to leave the car in neutral and the car wash pulls the car through-- is step firmly on the brake pedal with one foot while pressing the gas pedal firmly with the other foot in order to force-charge the HV battery so that it will last long enough to get through the car wash in neutral since it's not able to charge itself in neutral (you should also always turn off your accessories-- particularly the climate control-- in this type of situation because the A/C can draw about 2,000 watts at full blast which can drain the HV battery completely in a matter of minutes depending on the charge level when you first shift into neutral). This is one of the only times when there's a legitimate use for doing that, but it's still a very inefficient use of your gas.

 

Very nice explanation. The car wash related paragraph was particulary interesting for me, as I had to figure all that by myself the hard way. Not long after purchasing my Prius I almost got trapped in one such car wash. It was down to one red bar, the warning message (Pretty please, shift out of neutral!!) beeping out loud in my face. I was not far from screaming Let me out! ... Well, my Precious survived that, I hope he is no longer angry with me. In return, I keep my eye on the charge level before entering the car wash trap.