Automobile brakes have two pads that are rubbed together to generate friction that slows the car. While these work well, they do have the disadvantage that small bits of pad material are strewn about the roads during braking. This was an especially bad thing when brake pads were made of asbestos, but it’s still not a good thing.
So I’m wondering about the feasibility of having electromagnetic braking of autos. Has anyone ever tried to make them? I think the Toyota Prius has something along those lines, since its brakes generate some power which recharges the battery. Anything else? Any likely drawbacks to electromagnetic brakes?
How much brake pad residue does an automobile generate? It can’t be that much, or we would be constantly replacing brake pads.
Looking at the problem from the viewpoint of energy management, braking converts a large amount of kinetic energy into some other form of energy, like heat. You can use electrical generators to brake the automobile, but what do you do with the electricity? The simple method is to dump the electricity into a bank of ballast resistors, converting it to heat. The braking system is now considerably more expensive and complex. The end result is the same, minus a small amount of brake pad dust. I’m not aware of any practical energy storage devices that could cope with the large intermittent power surges produced by electromagnetic braking.
Looks like it’s not as common as I imagined - I saw it on an advert for Virgin trains and mistakenly thought they were just shouting about something that was common. Turns out it’s a fairly new thing.
I’ve heard, or read, of these being used on heavy trucks at some point in the past, maybe the 30’s or 40’s, but I can’t pin it down to a specific manufacturer. I don’t think that the energy was recycled though, the primary objective was finding a way to slow trucks on downgrades w/o excessive braking. The engine compression brake seems to have emerged as the winner from this experimentation. I’m guessing that an electromatic brake added too much weight, thereby reducing the truck’s payload.
One issue with electrical brakes is likely to be their size and weight. The brakes on a typical car are probably capable of dissipating energy at a rate equivalent to something like 500 hp; it’s going to take a formidable electrical device to match that.
Two nitpicks:[ul][li]You don’t have “two pads…rubbed together” but a single pad against a rotor or drum, and[/li][li]it isn’t friction between two solid continuums that causes the brakes to work, but rather viscous drag. When the brakes are actuated, some initial heat is created by friction; this causes the matrix of the brake pade to melt very slightly, creating a thin film that adheres to the rotor. This very viscious film causes a drag that slows the rotor (and therefore the wheel).[/ul][/li]
Inductive braking, as others have noted, requires generators with a high throughput and some way to convert the power generated into potential energy to be used later. Doing this at high speeds is very difficult for something compact enough to fit on a wheel hub, and if it is very heavy it will contribute to the unsprung weight of the vehicle, affecting performance.
Mangetout, regenerative brakes to generate electricity in the same way any other electric generator does; by converting rotary motion into an electrical current. That the energy comes from the kinetic motion of the car (via the axle, coupled to ground through the tire) rather than from the motion of a turbine powered by a heat engine or a water head is immaterial.
I understand that, it’s just that the article I first linked to reads as if the trains are not only moving people around, but are generating a net surplus of energy in the process.
One disadvantage is that there is no braking force at zero speed, so you still need friction brakes to keep the vehicle from rolling while stopped. As such, the technology is good for controlling speed on downgrades, but not so hot for completely stopping.
Regenerative braking has been around for many decades in railway applications. In a diesel-electric locomotive, it has the disadvantage that there is nowhere to return the electricity to in any usable way, so it is released to the atmosphere as heat. There is a further disadvantage that in a locomotive-powered train, regen braking will only retard the speed of the locomotive and not the wagons (unlike traditional air brakes which apply to the entire train), so the train will bunch up (this is a good thing sometimes, but usually not). Then, as mentioned, there is the irony of regenerative brakes needing speed to work, so if there’s nothing to regenerate, they are useless - ie. you can’t use them to come to a complete stop. They are still useful as a supplementary braking system - a 4000 ton train can need all the stopping power it can get.
In EMU (electric multiple unit), they are pretty much the modern standard, and will feed power back into the overhead. Here in Sydney, you can notice it on the newer trains -the motors will hum as you start to slow down, then at about 15kph, the humming stops, and the disc brakes take over.
I know that in diesel-electric railway use and in automotive use, there have been experiments with storing the energy in a flywheel. I’m not sure what came of these. In a locomotive, the weight shouldn’t be such an issue, as many locomotives actually carry ballast for extra adhesion anyway.
Don’t most of the hybrid cars have regenerative breaking?
I have read things online saying that new BMW 5 and soon 3 series cars will have regenerative breaking. But I cannot really determine what they do with the battery. It sounds like it augments the alternator for recharging the battery used to start the car.
