That’s one reason.
The other is this - the batteries and electric motor provide a substantial percentage of the total horsepower of the drivetrain. If the batteries were unable to provide any power, the car would perform really poorly, and owners would get upset.
Sure I know what question I’m asking. Gas powered cars recharge the 12 volt batteries they use to start the car with a gear attached to one of the belts the engine powers. I was asking why not use that to recharge the much larger battery on hybrid cars. That was the question I was asking.
No I’m not making a classic mistake. I realize the engine will use some energy to recharge the battery, just didnt know how much. The engine already uses a belt to drive other parts of the car not immediately engaged in powering the car, so was wondering why not use it to recharge a hybrid battery. I do not know how much energy that takes. It has already been stated that some hybrids do just that.
Yes, that’s exactly why I asked about using the gas engine to recharge the battery, because I did not understand that a battery could be recharged.
No, I just figured that braking, especially on highway driving, does not occur all that frequently. So not all that plentiful a source of energy for charging, free though it be.
No the question in my mind is how much harder the engine would have to work. It does not have to charge the battery all at once, and it does not have to charge the battery all the way to create more fuel economy.
OK, here is the thing. The small battery in a regular car draws a small portion of its charge to start the engine. It usually takes around and hour of driving to get a zonked battery back up to charge, usually quite a bit less than that to replace the starting charge. Meanwhile, the engine is using some of the juice from the alternator to fire the plugs, run the computer, power the lights, radio, etc. Whatever is drawing power is effectively putting a load on the alternator, which adds some drag to the engine. When the battery is up to charge, it is no longer placing a load on the alternator.
With a hybrid like the Prius, all that juice is coming out of the transmission back-load generator, which also participates in regenerative braking. If you are on the highway, even if you have cruise on, your speed will vary a bit, and it is in those periods where the car is either decelerating (just a little) or descending a grade (even a nearly imperceptible one) that a hybrid will capture some of the braking/coasting energy and return it to the battery.
The point of a hybrid is to make the gas engine as minimal as possible, just enough to keep the car going at a decent clip on the flat, and make up for it with the electrics. If you add an alternator to charge the battery, you might well end up tipping the scales and taking more energy from the small engine than the system can tolerate.
Is that so bad? Then it’s effectively like an electric car that has the option of running on gas from time to time when you have to go farther. Why didn’t they just design all hybrids to be that way from the start?
It’s not so bad, it’s just not so good.
The standard Prius comes with a 1.3 kWh NiMH battery, of which, as pointed out you could not fiil up too much of. The manufacturer produced plug-in Prius, with a 4.4 kWh Li-ion battery only gets 6 miles all-electric, so you can figure that these folks’ self-modified versions are likely only gaining them a mile or two max of all-electric - not quite an electric car with an option of running on gas from time to time unless you only drive more than two miles time to time.
Other plug-in hybrids (from Ford’s Energi line to the Volt and others) come with bigger batteries, which depending on your driving habits/needs may indeed be enough to have a car that functions as an electric daily commmuter and as a hybrid when you need to go farther. But each increase in battery size comes at the cost of weight, space taken up, and dollars.
I’ve never understood exactly how regenerated braking works, but am now learning.
OK… When you are slowing or braking some power is generated by a generator which slows the car down somewhat. And gives some amount of free power that would normally be wasted.
I assume you still need real brakes too, which will still produce wasted heat.
Is there any other way to turn that wasted heat into energy?
Hmm no you don’t understand the question you are asking.
As has been said several times in this thread the Prius recovers free energy with regenerative braking AND uses the gas engine to charge the hybrid battery. It is not an either or situation, the car does both.
By using regenerative braking the car has to run the gas engine less. The less the gas engine is on the better the fuel mileage.
Yes exactly. Turn an electric motor with external power and it becomes a generator.
Heat is energy. You aren’t going to turn heat into energy, it is energy. I assume you are asking if you can do any useful work with that energy. Hybrid regen brake systems are pretty good. Depending on driving style the regular brakes might be used rarely. Light pedal application is all regen braking. Only when you step down harder do the friction brakes come into play. On a Sonata hybrid I can feel when the friction brakes come into play.
I supposed if you collected the waste heat it you could cook a grilled cheese sandwich with it.
I was going to say that “clueless salesman” was tautological. Now I am beginning to see some of the problem he must have had explaining the concept.
It’s collected in the brakes. Some of the heat is carried away by bits of brake lining as they get hot, like ablative heat shields. The rest of the heat gets the pads and disk hot (or shoes and cylinders). Brake a heavy vehicle as it goes downhill and you might be able to turn the car on it’s side and fry an egg on the brake disc (probably easier to fry that egg on the engine, but what fun would that be?).
Yes. The regen brakes have limited power capacity; the electric generator and battery setup can only absorb energy at a limited rate, so if you want to stop quickly, the conventional friction brakes kick in.
Once you’ve dumped the vehicle’s kinetic energy into the brake rotors as heat, it’s not really practical to convert that back to mechanical work again.
However, there are other ways to capture/store that kinetic energy during a braking event. Instead of electric generators and batteries like the Prius, you can use hydraulic pump-motors and pressurized storage tanks. This is how hybrid-hydraulic vehicles work. The pressurized hydraulic fluid storage tanks have limited energy capacity, basically enough to get the vehicle from a standstill back up to cruising speed, but the regenerative braking system has very high power capacity: the friction brakes are very rarely needed. This system works great for vehicles that travel at low speeds and make lots of stops: UPS trucks, garbage trucks, mail trucks, urban buses, and so on.
I don’t know much about these, but can’t you normally short the output to create maximum resistance in generators, and decouple them from the batteries to keep from overloading them? Or is that something that can only be done with an alternator for some reason?
Obviously a mechanical brake may still be required, for safety if nothing else.
ALL current hybrids do that, using the motor/generator in the transaxle. There’s no need for something belt driven to do the same job.
the lone exception is the GM “eAssist” (mild hybrid) system which uses an integrated starter/generator on the accessory belt drive. But those cars just have a conventional automatic transaxle.
problem there is that as the vehicle slows, the generator is turning more slowly and regenerative braking force drops off precipitously. further, it can’t keep a stopped vehicle from moving (e.g. if it’s on a hill.)
I’m not sure what you’re asking about when you mention alternators.
Just trying to figure out if something is different about the design that allows or prevents shorting to create a maximum load on the devices. I assume the car motors are basically DC devices of some variety, probably brushless, so there shouldn’t be much difference between them and simple AC motors.
You wouldn’t “short it”; you’d connect the gen outputs to a power-dissipating resistor, something with substantial surface area and temperature tolerance. This is an important feature of railroad locomotives. When you’re descending a miles-long mountain grade with 100+ rail cars behind you, it’s not possible to dump all that energy into friction brakes without overheating them; instead, you generate electrical power and then dump it into a bank of power-dissipating resistors with a big-ass cooling fan.
You could do something similar on a hybrid passenger car, but what would be the point? You’ll still need friction brakes for coming to a complete stop, and they already provide the capability for rapid decel from high speeds without overheating (unless you’re turning in hot laps at Road America). A system for electrical power dissipation on a hybrid-electric passenger car would add weight, cost, complexity, and provide no particular benefit.
They do.
Regenerative braking is essentially getting something for nothing, in conventional automotive terms. Rather than clamp the brake pads to the rotor and convert all the kinetic energy of the car into heat like normal brakes do, regenerative braking converts a lot of that kinetic energy back into electricity, which goes into the batteries.
So in drastically simplified and hypothetical terms, if you used 100 units of energy to get your Prius up to 50 mph, and 1 unit per 5 minutes to keep it at that speed, and then slowed to 0 mph using friction brakes, you’d have used 100 units + however much it took to get where you’re going.
Regenerative brakes might get you back 40 units on braking down to 0, so you only used 60 + the energy used to cruise.