What is regenerative braking and why don't we use it?

Question

I know that many electric cars use regenerative braking to enable the car to become more efficient by converting what would otherwise be lost heat energy in to a more useful, reusable form of energy.

Surely, there must be some way of engineering a new brake design that would allow for the use of this technology on a more conventional car powered by a normal internal combustion engine.

What I don't understand is:

• What exactly is regenerative braking?
• Why can we only use regenerative braking on electric and hybrid cars?
• Is there any reason why we can't use this clever braking technology in order to increase power to the crankshaft or reduce fuel consumption?

Image Source: http://sjam4uphysics.pbworks.com/w/page/38936885/Regenerative%20Braking

Answer 1

tl;dr: We do. It's just expensive.

One of the differences between electric and chemical energy motors is that the electric systems make it much more convenient to capture and retain energy (e.g., a battery). All you need to do is use induction to create an electric current right there at the wheel while braking. Point that current at that battery and you've retained energy that would otherwise be wasted.

With a chemical energy motor, it's a lot harder to store excess energy. We can't use the brakes to generate more gasoline, for example. However, it is possible to retain kinetic energy in something like a KERS. This will use the energy in braking to spin up a flywheel. That flywheel can then be used for additional motive power when needed.

Unfortunately, KERS units are expensive and require a bunch of additional engineering (how, exactly, are you going to get that spinning flywheel to actually push the car forward?). They're also not common so they don't benefit from economies of scale.

All that said, they do work. Formula One has used them to great effect as a passing boost system. I wouldn't use that sort of capability on the road but I'd be happy to experiment with anything that helps me get more oomph out of a gallon of gas.

Answer 2

In answer to the question "What is regenerative braking and why don't we use it?", we do. With normal braking, forward momentum of the car is scrubbed off by the brakes by turning them into heat in the brake disc with subsequently dissipates and is lost. With regenerative braking, instead of the movement of the car being lost to the atmosphere as heat, it is converted to electrical energy and stored in the vehicles battery.

The Volkswagen Bluemotion range of cars (and I'm fairly confident other manufacturers do this but I'm sure Volkswagen do) have a very sophisticated alternator. When the car detects is it being slowed by pressure being applied to the brakes and the car is still in gear and that clutch is up, the alternator steps up into a mode where it draws significantly more power from the drive. This drive would normally come from the engine but in a braking situation, it is provided solely through the transmission and is effectively running against the drivetrain.

This system is fitted on both petrol and diesel powered Bluemotion equipped Volkswagens. More information is available here and here

I do appreciate that this isn't quite on the same scale as the F1 cars use but it is a form of regenerative braking and it is in use on the public roads today.

Answer 3

Regenerative braking is a system that stores the energy that instead would be lost to heat in the brakes. While these systems sound great they come with a whole host of their own problems.

1. Before even getting into the energy storage and generation the actual service brakes become very complicated. For regenerative braking to work properly the brakes can't come on while regenerative brakes are doing their thing. Further regenerative brakes only work at high speed and loose their effectiveness as you slow down. This necessitates a transition where at high speed the regenerative brakes slow you down and the regular brakes don't and as you slow down the regenerative brakes do less and less and the regular brakes will finally bring you to a stop. The system that makes the brake pedal feel normal while the regular brakes do nothing and then transition you into regular braking smoothly with you noticing that a switch happened is very complicated and very difficult to get perfect. Toyota had problems with this system in the Prius where breaking was lost for a half second during the transition making people feel very uncomfortable.
2. Energy storage is a huge problem. Everything sounds great in hybrids, just charge the batteries, where in reality the amount of energy that the car generates during a stop is way more than the batteries can actually absorb. If the stop is rapid then the problem is further amplified. A system that @BobCross mentioned is the KERS. By combining a motor/generator to the flywheel the extra regenerative energy that can't go directly into the batteries is used to spin up the flywheel. Then over time the energy is then converted to electricity to charge the batteries. These systems are very heavy and expensive. Another systems is using super capacitors. The extra energy is dumped into the super caps. The problem is that when the energy is stored in the capacitors it is not super useful without conditioning. Voltage in capacitors drops very rapidly. If storing a high voltage then a system is needed to step the voltage down to something useful. If storing no more voltage than the battery then a system is needed to step the voltage up to something useful. Super capacitors are also rather expensive.

Hybrids and electric cars have built in electric systems to take advantage of the regenerative braking. All of these problems are further amplified with straight gasoline engines. A new system needs installed, can't leverage something existing, to provide regenerative braking adding to the expense. It becomes hard to sell someone on a system that adds $5k to the price of a car that gets better fuel economy in the city but suffers on the highway.

Ford is developing a system where a positive displacement pump charges a hydraulic accumulator as you come to a stop. When taking off again the fluid pressure is sent is reverse through the pump to turn it into a motor to assist starting from a stop. The system is meant for large trucks where the size of the system is not too much of a burden. Also the system is loud and prone to leaks.

Answer 4

Diesel-electric railroad locomotives do use regenerative braking, but they call it "dynamic braking". Also, they don't store the energy, they blow it off through resistor grids and fans at roof level.

Full electric locomotives and some streetcars can generally feed the regenerated power back into the wire or third rail; this is much simpler in DC systems.

But the traction motors (generally one per axle) are driven by the train's momentum and so run as generators.

Answer 5

Regenerative braking is only really useful on electric or hybrid cars for the simple reason that they have a way to make use of the energy stored. Also once you have an electric motor as part of the drive train regenerative braking is pretty much built in (at least in terms of hardware) as you simply use the existing motor(s) a a generator and divert the power generated to the battery for later use.

In essence there is no point as all in using regenerative braking in anything other than a hybrid car, indeed having regenerative braking is a good definition of a hybrid car.

While there is no reason why you couldn't fit a convention IC engine car with a regenerative braking system, unless you are going to use that power to drive the car then you haven't really achieved much as you are adding a lot of extra weight and complexity to store electrical energy that you can't use.

There is also the issue that on long highway/motorway journeys you may not do enough braking to meet even the small needs of the auxiliary electrical systems (lights, ignition, radio, charging the battery for starting etc) so you may not even get the advantage of not needing an alternator.

Answer 6

We can do the next best thing.

1/2 mile (800m) ahead, I see a light turn yellow; at 60 mph (100kph) that's 30 seconds. And I know that light has about a 35 second cycle, plus 10 seconds for the stack of stopped cars to get moving.

I've been applying continuous power to keep speed. I will continue doing so for another 25 seconds, then 5 seconds of firm braking and stop. 15 seconds playing with my phone, then I'm on my way.

Or... I go to idle power immediately. My car coasts. With the engine still in gear, my momentum is pushing (spinning) the engine, for a mild braking effect, so the fuel injectors shut off entirely. My speed gradually slows... 55... 50... 45... Little bit of power... 40 (65kph)... flip the light turns green while I'm still 1/8 mile (200m) back. The stopped cars unpack and I gauge exactly when to apply power. And I never brake.

In the first scenario, there are two energy exchanges. First the fuel applied for that 3/8 of a mile (600m) to stay in cruise. Second, the braking energy shed in the last 1/8 mile. Not only are these nearly equal, they are the same energy. If we were doing proper regen, we could read them off the ammeter and derive the actual joules out and in.

In the second scenario, there are none. Fuel is not expended and brake energy is not shed. The effect of regenerative braking is achieved, but without any conversion losses.

Granted, this is an education approach rather than a technology approach, but it works. Even on an EV capable of regen, it still works better than regen - in fact it'll work even better on an EV because you get better "coast" - no need to use drag to spin the engine merely to provide brake and steer assist.