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Ah, fire. One of humanity's oldest ways to generate energy. Technically, you could generate energy by burning brake pads, as seen in the picture above (thanks, Flickr!), but automotive engineers have managed to come up with a way to use brakes to generate energy without going up in flames. The technology is called regenerative braking and it's the subject of this week's Greenlings.

At the most basic level, regenerative braking means re-capturing the kinetic energy of the vehicle's motion and turning it into another type of energy. Commonly, this is done by converting kinetic energy into electricity and recharging the car's battery with it. Click past the jump for more for a look at the way that regen brakes work and the different types of regenerative braking systems. Oh, and we'll discuss why you would want to do this in the first place.

How standard regenerative brakes work

When a car is cruising down the road, it has kinetic energy, which is simply defined as the energy something possesses because it is in motion. When you apply the brakes, instead of traditional brake pads clamping down and letting the kinetic energy dissipate as heat, regenerative braking systems use their electric motors to slow the car and generate electrical energy (hybrids still have conventional friction brake systems that are used at higher deceleration rates). Hybrid cars, where regenerative brakes are the most common, and electric cars can reverse the flow of power through their electric motors backwards to slow the car down. In one of those convenient engineering coincidences, electric generators are the same as electric motors.

When you apply a current to a motor it turns, converting electricity into mechanical torque. When you apply a mechanical torque to the motor it induces electric current so it can be used as a generator. Thus, using kinetic energy to turn the motor generates energy. Put this collected energy into the battery and, the next time you step on the accelerator, some of the energy you just saved is used to get you moving again. Of course, friction and other energy losses mean that you don't get to use all of the energy you captured (no potential for a perpetual motion machine here, sorry), but this is one reason the Prius and the Insight, for example, get such high mpg ratings.

Other types of regenerative braking systems

Instead of a battery, it is also possible to send the energy captured from the brakes into ultracapacitors (a sort of really fast charging, fast discharging energy storage system). While some ultracap technology remains shrouded in mystery (we're looking at you, EESTOR), many companies are testing ultracapacitors today and we expect them to make their way into more vehicles in the future. Hopefully. We've heard this claim before.

A very different regenerative braking system uses hydraulics to capture the kinetic energy. We have a detailed description of how hydraulic hybrids work here. Currently, this technology is mostly being tested in large delivery vans, such as those operated by FedEx and UPS.

F1 racing teams are currently at work on another type of regenerative braking system called KERS. Still in the early stages, KERS - aka kinetic energy recovery system - uses either batteries or a flywheel to take in the braking energy. Whether or not KERS ever makes it to production vehicles depends, in part, on how the safety concerns are dealt with on the track.

Why you would want to do this in the first place?

As stated, regenerative braking systems can dramatically increase a vehicle's fuel efficiency. This is the main reason to want them in your car. The systems do add cost and complexity to the drivetrain, but many hybrid owners think regenerative brakes are worth it. In pure electric cars, regenerative brakes are vital to increasing range and getting the most out of each charge. Regen brakes are not without complications, though. Most hybrid and electric vehicles use a more advanced electronic brake control unit that blends friction and regenerative braking adding another potential failure point to a safety critical system. In the all-electric MINI E, for example, the regenerative brakes are so strong that the rear brake lights will come on even if you don't step on the brake pedal. As the technology continues to mature, regenerative brakes will get better and better and we'll be able to feel green every time we step on the brakes.

Tire photo by rockstarassi. Licensed under Creative Commons license 2.0.

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    • 1 Second Ago
      What's going on?
      • 3 Years Ago
      If you check www.goauto.com.au you will find an article "More efficiency for VW Passat CC" where the author mentions "regenerative braking" has been added to the CC giving it a few % better mileage. The car (in Australia) is available in diesel and petrol form - but not hybrid. I have a VW CC with the 3.6 litre VR-6, so I tend to read articles on the car. I don't know what this author is reading, but I can't believe somehow VW has added regenerative braking to a non-hybrid and just to WHAT would the regeneration be sent? Maybe to the battery to reduce some of the workload of the alternator if lights, wipers, a/c, radio - but how could such a complicated system possibly add any measurable economy? I will avoid any of the usual internet commentary about the wrongness of this Australian automotive journalist. Maybe being upside down with sink drains swirling the wrong way and strange star formations in the night sky makes regenerative braking possible for a non-hybrid car in Australia.
      • 3 Years Ago
      To the people doubting that non hybrid cars may have regen braking..... Think.... in stop start traffic, the battery won't last long with constant restarts.... SO for cars with stop start, it makes sense (read the literature on these cars before making uninformed comments.... the journos may be off the wall on some things, but they often do get the rundown.... Just buy a 2012 issue Passat CC and it will have regen braking... the new battery tech is more able to handle high current recharge, or the older tech with a parallel capacitor to sore the current (energy) untill it can be fed into the battery are all possible these days.... But to have an effective braking system (regen) the motor in the wheels (or on the drive line) needs to be at least as powerful as one would need for acceleration (check out the deceleration under braking to acceleration under engine power) therefore this is the hybridisation of cars by default. Also the stop start motor, integrated starter alternator, is one unit, engaged with the engine 100% of the time, so this is the unit -which can be used for the regen-braking avoiding the need for wheel motors ie. distributed regenerative braking systems as the regenerative lart is only used for slowing Not hard braking less power is needed, also, as the start function needs to be seamless, or people will hate it, drive-train hydraulics, engine oil pressure, air con, steering etc al need to be electric, and either running while the car is stopped, or ready to fire up instantly the accelerator pedal is pressed...the initial drive (before the engine is running smoothly, is likely to be provided by this integrated electric motor. As I have already said it is hybridisation by stealth and before long, all cars will really be hybrids even without the advertising and green cudos which comes from owning an electric car.
      • 5 Years Ago
      Stepping on the brake would still waste energy, just not as much.

      I can't imagine any engineer wanting regenerative braking to begin when the driver takes his foot off the accelerator. Coasting is much more efficient than regenerative breaking.
        • 5 Years Ago
        @ dave

        Actually, the right pedal can be used to modulate your acceleration and your regenerative braking. There is also a point when the pedal is 30% depressed that is the midpoint between acceleration and regenerative braking that allows the car to coast. Because of this, you never need to lift your foot off the right pedal except in cases of extreme braking.
      • 5 Years Ago
      I think they should only engage the regen braking when the driver actually presses the brake peddle, otherwise the brake lights might inadvertently be coming on and off all the time driving other drivers behind them nuts...

        • 5 Years Ago
        Yes understood... but unless the driver holds the peddle at the right setpoint between accel/decel to achieve coasting the users foot must boarder line on breaking and a slight easing of peddle pressure might inadvertenlty trigger braking.

        When I first heard of this technique years ago from AC Propulsion, I thought it made sense, but on second thought I think I would prefer coasting mode to be when no peddles are pressed, similar to an ICE, so that the driver is not required to feather the accelerator peddle in order to obtain coasting.

        My Hope would be that they develop an adjustable coast load that doesn't qualify as braking, but enough to provide an adjustable amount of drag upto a maximum similar to the drag of an ICE system, but also allowing for a no drag setting for even better coasting than an ICE system.
        • 5 Years Ago

        If you read the linked article, you'll see that the brake lights do come on. This makes sense because the car does actually slow down even without applying the brakes.
      • 5 Years Ago
      I love the idea of big vehicles making an 80% fuel savings doing stop-and-go with hydraulic hybrid systems. However for cars, something lighter should be used. That's why I must mention The Air Car. So here goes, THE AIR CAR! :)

      My humble opinion, it's the cleanest car in the world! It's also the best solution for me, EVs seem useless considering the cold long nights it will face where there's no outlet.

      • 5 Years Ago
      "we'll be able to feel green every time we step on the brakes."

      That is what it is all about.
        • 5 Years Ago

        > "we'll be able to feel green every time we step on the brakes."

        > That is what it is all about.

        Poor allegation. It's about dramatic efficiency improvement.


        In fully comparable conditions of EPA fuel efficiency tests, to cover 15.000 miles in 55% city and 45% highway split (which according to EPA transportation researchers represents average driving patterns most accurately):

        - Toyota Prius II needs 7.4 barrels of oil (0-60 in 10.7 sec)
        - Nissan Altima Hybrid needs 10.1 barrels of oil (0-60 in 7.5 sec)
        - Volkswagen Jetta TDI DSG needs 11.9 barrels of oil (0-60 in 8.9 sec)

        Energy recuperation (also allowing for more thermally efficient Atkinson cycle) allows the Prius to be 60% more efficient than the newest common rail diesel with advanced DSG automated manual. Then one class larger and significantly faster Altima Hybrid is 18% more efficient.

        In SUVs, improvements are just as dramatic. For the same distance:
        - Toyota Highlander Hybrid AWD needs 13.2 barrels of oil (0-60 in 7.5 sec)
        - Mercedes ML 320 CDI BlueTEC needs 19.7 barrels of oil (0-60 in 8.5 sec)

        So hybrids SUVs can be 50% more efficient and much faster at the same time. All thanks to energy recuperation.

        Because of it, engines in hybrid drivetrains can achieve 37% peak efficiency, which is on par with newest diesels. Only those engines do not need variable vane turbocharging, expensive direct injection, super high pressure fuel pump (10x than in gasoline injection), double mass flywheels, particulate filters and NOx traps/AdBlue injections.

        Meanwhile, instead of going for diesel hybrids, we should extend battery capacities in ordinary gasoline hybrids to have longer all electric range/electric power assist.

        Source #1: http://www.fueleconomy.gov/feg/2008selemakef.jsp?year=2009

        Source #2: Slide 10 of "Performance and Emissions of
        The Toyota Prius" presentation, Prius Data Exchange Workshop, USCAR, October 28, 1999, Argonne National Laboratory Transportation Technology R&D Center
      • 5 Years Ago
      Nicely written. Great post. I have some high hopes for KERS merely because it's not as wasteful as motor-to-generator-to-battery systems and saves wear on the motor. Jaguar, I think, is putting a lot of stock in this, and it's possible to use it on any vehicle: engine or motor.
      • 5 Years Ago
      Brake lights: should be pressure-intensity sensitive; tapping the brakes to bleed a little speed coming up to a light traffic snarl should provide a different message to drivers behind me than an emergency "crash stop" with max pedal pressure and the ABS kicking in to prevent a slide.

      This is an add-on that motorcyclists (certainly in the UK in the 90s) used to make group rides safer. An auxiliary brake light pulsed (slowly for light breaking, faster for hard stops)

      A similar feature should be integrated into all vehicle up to a near strobe-like effect for emergency braking - it would alleviate much of the concertina effect we see on Freeways (esp in the US where automatic transmissions are the norm and cars have almost no engine braking)

      So in the above regen example, it's fine to have the brake lights indicate that the vehicle is slowing more than a following car may expect, but that indication should be different than "I'm coming to a full stop"

      While I'm on the tail-lights bandwagon, can we just once and for all ditch red turn signals? They're so easily confused with cadence braking, esp when you can only see one half of the rear of a car ahead of you from the rear-three-quarter - there's really no difference between a turn signal and a flashing brake light. We have a world standard for turn signals, it's orange, just use it!
      • 1 Year Ago
      Thank you for info...actually am an Automotive Engineering student, and i would like to do my final year projects in this topic....So i feel this info will be useful for us
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