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Following last week's announcement by Toyota that it would recall several hundred thousand units of the 2010 Prius and Lexus HS250h to update the brake control software, there have been numerous questions about how these systems work. Vehicles with strong hybrid systems like those built by Toyota, Ford and General Motors all use an electro-hydraulic brake system that provides partial brake-by-wire control.

While there are detail differences in the implementations from each manufacturer, the basic operating principles are largely the same. The key to the efficiency advantage of hybrids is their ability to recover kinetic energy that is normally dissipated as heat when the vehicle is slowing down and then store and release that energy to provide propulsion later. This is known as regenerative braking, and we're going to discuss aspects of regenerative braking that apply equally to pure electric and extended range electric vehicles like the Nissan Leaf and Chevrolet Volt. Read on after the jump to learn more.

Non-hybrid vehicles use hydraulic pressure to apply a friction force to brake rotors or drums. That pressure is generated by the driver applying the brake pedal. The pressure generated by the driver is typically amplified by a vacuum booster. The amount of deceleration is then in proportion to how hard the driver applies the pedal. The goal of the electro-hydraulic system on hybrid vehicles is to maintain this relationship between vehicle deceleration and pedal apply.

Maintaining this relationship requires blending of the hydraulic friction braking and the regenerative braking. Therein lies the rub. Let's take a walk through these brake blending systems to see how they work.

These systems utilize a number of additional sensors to determine what the driver is asking for from the brakes and then manage regen and friction braking to achieve it. Typically, a brake pedal position and sometimes a force sensor are used to derive the driver's intent and calculate a desired level of deceleration. For example, if the driver presses the brake pedal down about half-way, that may equate to about 0.5 g of deceleration. That deceleration intent is converted into a desired amount of braking force or, more specifically, torque (rotational force).

At the same time, the system is monitoring the wheel speeds using the sensors that are also used in the anti-lock brakes. This information is used to determine the type of surface the vehicle is on, including ice, snow, gravel or dry pavement. The instantaneous behavior of the wheel speeds can provide a lot of clues about the nature of the road surface, but it isn't perfect. This "not quite perfect" reality will be an ongoing theme throughout this discussion.

If the driver's intent exceeds what the road surface can support, the total braking force is reduced to what is available. For example, if the car is on a snow-covered road, the maximum deceleration may only be 0.3 g so the total braking force applied needs to be reduced accordingly. Once the driver's intent and the road surface have been determined, the system has to figure out how much hydraulic pressure to apply to the friction brakes and how much regenerative braking energy to feed to the battery.

Because we are talking about hybrid vehicles where efficiency is a priority, the systems will always try to maximize the amount of regen, if possible. With the nickel metal hydride batteries used on current hybrids, the amount of regenerative braking is typically limited to about 0.3 g because of the rate at which the batteries can absorb energy. The amount of regeneration that is possible at any point in time is also limited by the battery state of charge. If the battery is fully charged, no regen is possible.

The amount of braking torque provided by regeneration is pretty straightforward to calculate and is directly proportional to the voltage feedback from the generator. The control system subtracts the regeneration torque from the total desired brake torque. The leftover amount of torque is then realized by the hydraulic system. This is where things get really hard.

In these electro-hydraulic systems, the amount of pressure on the friction brakes is semi-independent of the driver's demand on the pedal. In order to control this, the system must have a mathematical model of the relationship between pressure and torque for the brakes.

Hydraulic friction brakes have a lot to recommend them. They are a very effective means of providing the large quantities of brake torque needed to stop a vehicle. They are also comparatively inexpensive and generally very reliable. However, because they rely on the act of pressing a friction material against a moving surface, the physical characteristics change over time, sometimes a very short time.

Remember that the law of conservation of energy tells us that energy can neither be created or destroyed, only transformed. The whole premise of friction braking is that kinetic energy of the vehicle is converted to heat energy by the brakes. As the vehicle slows, the brake friction material heats up and as the relationship between apply pressure and braking force changes. Other factors also cause that relationship to change, including humidity and pad and rotor wear.

The control software must incorporate adaptive control algorithms to help compensate for these changes but, just like the road surface estimations, these are not perfect. Brake systems engineers spend thousands of hours testing and developing these algorithms to make them more robust and this is one of the major factors that leads to extending the time it takes to bring hybrid vehicles to market.

Once the desired friction brake force is determined, the hydraulic control unit tries to fill the gap between the total and regen force. One of the downsides of regeneration is that no braking force is available unless the wheels (and thus the motor/generator) are turning. Even at low speeds, regen is limited. As the vehicle slows to a stop (typically between five and seven miles per hour) the regenerative braking force is ramped down and the friction force is ramped up.

This is where Toyota appears to be having its problem. If the hydraulic model isn't completely accurate, it can cause changes in vehicle deceleration even if the driver is holding the brake pedal steady. If the hydraulic model is overestimating the brake torque, then it will apply too little pressure, causing a perceived loss of braking.

The control can also be influenced by rough pavement. The wheel speed sensors rely on a toothed ring moving past a magnetic or hall-effect sensor. At low speeds, the frequency of the sensor pulses decreases and with it so does the accuracy of the signal. If the car hits a bump or pothole at low speed, it can often appear as though the wheel is going to lock. If this happens, the brake pressure is reduced to compensate. This, too, can cause what appears to be a loss of braking to the driver.

Because the vast majority of braking in the real world is in the range of 0.2-0.3 g, any loss of braking tends to be quite noticeable. The actual difference in stopping distance may only be a foot or two, but in traffic that distance can be critical to avoiding a fender-bender or something worse.

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    • 1 Second Ago
      • 5 Years Ago
      A couple of points:

      For a function as safety critical as braking, the simplest option is the best option. The driver should only have control over stopping the vehicle, not choosing how to stop the vehicle. In general, you want the driver to perform as few tasks as possible for safe operation. Go, stop, turn, shift(if applicable). It is never a good idea to add complexity such as extra pedals, or toggle switches for braking. Drivers are already distracted enough with radios, cell phones, etc. Just imagine if they were distracted by using critical functions of the car!

      EcoCar Teams all over the country are addressing these same issues as we develop the best braking algorithms for the competition. As the rules state, we must have a mechanical link to the brakes at all times, but there is still some room to develop regen strategies that make sense. The competition will also be a great learning experience for the hybrid engineers of the future! Please visit www.green-garage.org for more information, or follow us at www.msuecocar.com.
      • 5 Years Ago
      The Toyota Prius braking problem – could that have been foreseen?

      Yes it could!!!

      The way the regenerative braking is applied on the Prius is unnecessary complicated and gives way to many possible errors resolving in dangerous situations. But Toyota is not the only automaker that runs this high risk to sell cars.

      read more: http://www.easyecar.com/electric-hybrid-plug-in-news/the-toyota-prius-braking-problem-could-that-have-been-foreseen/
        • 5 Years Ago
        The 2001 - 2003 model Prius didn't have that problem at all, it showed up in later models that had the braking algorithm changed to boost efficiency - and even then, only in rare and unusual events.
      • 5 Years Ago
      To Neil (and Sam), the amount of regenerative braking is also limited by the size (rated power output) of the electric motor. No matter how much energy the battery can accept, the generator can never try to shove more in than the max output it is capable of, and the max output it is roughly the same as the amount it can output as mechanical energy when operating as a motor. In other words, there is a maximum rate at which it can convert energy between electrical and mechanical form and that limits both acceleration and regeneration.
      • 4 Years Ago
      The average person would have trouble controlling two different braking systems. Even if the average person could deal with separate systems, they would probably do it inefficiently, which would defeat the purpose. Less involved systems like a lever on the steering wheel, which are good in theory, aren't practical. When such systems are incorporated into vehicles they are often set once and then ignored, which eliminates many of the benefits of regen braking.

      Using a computer to control the blend of regen and hydraulic braking can result in very effective energy recovery and still provide the braking torque requested by the driver. The main difficulty with these systems is making the interface simple for the driver. Pressing harder of the brakes should create more braking torque and the driver shouldn't even be able to tell where the braking torque came from.

      I am a member of the UVic EcoCar team, a student team working to address this and the multitude of other issues presented by hybrid vehicles. You can find out more on my team's homepage www.ecocar.uvic.ca or on the EcoCAR competitions portal
      • 4 Years Ago
      For those of you who have driven a big truck, you'll understand this idea. The rest of you will require an explanation.

      In heavy hauling, commercial trucks, they have what are called engine brakes. When you hear the truck making a loud "farting" noise like your little 2Fast2Furious rice burner with a glass pack muffler, that truck is employing engine brakes (aka Jake Brakes). These are used to slow the truck down without requiring actual friction braking. They work in a diesel engine by restricting exhaust flow, which causes the engine to "backfire" and slow down its RPMs. They're used most often downhill, but many drivers get in the habit of using them on freeway offramps and the like as well.

      An electric car (hybrid, plug-in, whatever) has the same opportunity, though with a lot less noise. When the driver lets off the gas pedal, the car should automatically start slowing down. Those of us who drive standard transmissions probably do this with gearing already, slowing down to lights and such. Electrics can use the motor weight to slow the vehicle while capturing the kinetic energy to store as energy.

      Basically, you slow down by letting off the accelerator anyway. Right? Why not use that to the advantage of the vehicle by using that as your regenerative braking. It could be mechanical or electronically-controlled, but it doesn't have to interfere or even be connected with the brake pedal and mechanical braking systems on the car at all. That should eliminate safety concerns.

      The downer would be that the regen would not be as high a return as you would get with current models, but that could also be changed through the mechanics of the drive train rather than the electronics of the car's systems. Again, creating more safety. All that's required is that the driver learn how to use the new system, which most truck drivers pick up on in their first day of driver training. It's not a hard concept to get.

      Just a thought.
      • 2 Years Ago
      "Maintaining this relationship requires blending of the hydraulic friction braking and the regenerative braking. Therein lies the rub." I see what you did there.
      • 5 Years Ago
      People are stupid, for the most part. The average person would have trouble controlling two different braking systems. Even if the average person could deal with separate systems, they would probably do it inefficiently, which would defeat the purpose. Less involved systems like a lever on the steering wheel, which are good in theory, aren't practical. When such systems are incorporated into vehicles they are often set once and then ignored, which eliminates many of the benefits of regen braking.

      Using a computer to control the blend of regen and hydraulic braking can result in very effective energy recovery and still provide the braking torque requested by the driver. The main difficulty with these systems is making the interface simple for the driver. Pressing harder of the brakes should create more braking torque and the driver shouldn't even be able to tell where the braking torque came from.
      • 5 Years Ago
      I have a proposal, that may or may not "fly" in a commercially made vehicle:

      Use 2 brakes pedals: the far left pedal (where the clutch would be) could be the regenerative brake pedal, so it could be engineered to make as much power as the system can manage. (And use a super/ultracapacitor to absorb the higher current!)

      The center pedal would be the standard hydraulic brake pedal; with ABS.

      The driver should use the regenerative brake as much as possible, and then use the hydraulic brake to stop completely, or in an emergency. The *driver* is the integrator!

      Sincerely, Neil
        • 5 Years Ago
        I like Neil's idea. For new drivers they wouldn't even need to use the left pedal, but then when they get better they can start incorporating it. In an emergency you'd still slam the right foot brake pedal.

        The thing I wonder about with the regen brakes being applied when you lift your foot off the gas, as David Martin described and I believe is the case with Tesla, is what if something happens to you and you have to take your foot off the gas on the freeway, or your foot slips off the pedal, or you want to readjust your foot or something? Isn't that going to cause problems for cars behind you? I would imagine that I would prefer the car to simply coast along when I take the foot off the gas. But then again I am not hearing any complaints from Tesla owners.
        • 5 Years Ago
        I don't think that these systems are good. 2 pedals for breaking, that's hard to manage, modulating 2 pedals at the same time with different force, and what about manual clutch cars? Then modulating the regenerative breaks by leaving the gas pedal??? another way to have an accident and/or a hard time driving experience ??? how can we just coast slowly going downhill or before a stop or a red light, all hypermiler drive like that. Don't let theory destroy cleverness like toyota, honda, gm, ford, audi, nissan, tesla, epa, doa, europeen union, u.s.a goverment, japan goverment, autoblog green, green car congress, autonet, did. A simple ride of 5 minutes is sufficient to realize all the zealous, inneficient, costly green scrap that is proposed and put in place by these chaps.

        A simple lever on the steering wheel is perfect for regenerative breaking actuation and modulation. Anyway the hybrid of today have limp regenerative breaking because the electric drive motors are not powerful contrary to an hydraulic car ( that i said to build one prototype 2 month ago) that can almost looked the wheels because it's a very high torque mecanism.
        • 5 Years Ago
        Some designs do it in a much more simple way.
        Taking your foot off the accelerator engages the regen.
        Hitting the brake pedal engages the friction brakes.

        BTW, Good, informative article - thanks.
        • 5 Years Ago

        Coasting is only 'the most efficient way to drive' if you have a very inefficient motor that only operates at its least-inefficient at high RPMs, and no way to recapture energy when slowing down. gas powered cars require a lot of attention to keep them in their most efficient RPM bands, and any use of the brakes is a waste of energy it took to get you up to speed.

        AC induction motors are both very efficient (85%+) at nearly any RPM, and able to recover up to about 50% of the energy expended to get to speed when coming to a stop. Since any speed you are driving at (over a few mph) is 'the most efficient speed for the motor' there is no need to annoy other drivers with pulsing at high revs for a short period, then steadily slowing down as you coast to your next pulse.

        I like Tesla's solution best as well. The right foot tells the car your desired speed, constant pressure = constant speed, up or down hills. less pressure = slow down, with the difference in before/after pedal positions determining how much regen to apply. If you've fully lifted off the throttle, but aren't braking fast enough, then application of friction brakes adds more braking force independent of the regen system.

        Most drivers report that this is a little strange at first, then feels completely natural and second nature after about 20 minutes. It's the simple transformation of a 'go pedal and stop pedal' mentality to a 'go faster/slower pedal and stop pedal' mentality. You can think of it like manually actuated cruise control.
        • 5 Years Ago
        I'd like an N64 style thumbstick on the steering wheel to control the electric motor with a normal manual set up for controlling the ICE, gears and brakes.
        • 5 Years Ago
        The way the Tesla allows coasting is to vary regen with accelerator pedal pressure (how much less in this case since it is based on the accelerator rather than the brake pedal). Fully off means max regen. It requires a bit more precision, but in general I think it gives the user the most intuitive way to control regen. Brake blending works, but it doesn't give the user full control of the regen.

        As for the Toyota Prius ABS issue, I think no matter what kind of regen style you use (including ones where you don't blend the regen) it can still happen. The bump in the road just disturbs the ABS system and a decrease in braking force is just the natural way of responding. The same complaints happened to the Subaru WRX a couple of years back and that doesn't have any regen. The NHTSA found no issue with the system, but Subaru offered an reflash of the system to mask the sensation of braking loss. In reality this doesn't change the braking distance at all, it just hides the sensation of braking loss.
        • 5 Years Ago
        @ Mark and Neil:

        All hybrids today have a small amount of regen braking when you take your foot off the accelerator. This is to make the car "feel" more like a normal ICE car. While it does play a bit on the fuel efficiency, I think they've decided that a 0.5mpg decrease (or whatever it is...it's small) isn't likely to impact sales.

        Many Americans are scared to buy a hybrid car because they're afraid that they'll drive differently than their regular ICE car. This system is designed to play to that. For people like you that want to get the max FE from your hybrid, drop it into N when you want to coast, and you eliminate the regen braking altogether.
        • 5 Years Ago
        The part you're forgetting is that most drivers out there are complete morons who can't figure out how to drive a manual transmission. With the system designed as you described, my guess is you'd have even more people filing suits due to insufficient braking just because they can't figure out how to use the system.

        This is exactly why vehicles these days are becoming more and more autonomous...people are becoming less and less competent while driving, and to try and avoid dumb lawsuits down the road, engineers are attempting to develop systems that basically take all the skill out of driving. Unfortunately, sometimes the engineers didn't think of every possible stupid thing a driver could do, and back to the drawing board they go.
        • 5 Years Ago
        I like the idea of being able to choose but I don't know about the safety aspects of it. I've driven armoured cars (similar to M113) with a dual brake system (manual for steering and slow stopping, hydraulic for emergency stop) and found it cumbersome.

        Wouldn't it be easier if you take the idea of 2 brakes but use them for 1 pedal. If you press it up to halfway down you use regenerative braking only. Beyond that point (ideally you feel some resistance or a click) the conventional brake kicks in or takes over. I think it's more idiot proof.
        • 5 Years Ago

        EV's and hybrids do not have manual transmissions, for the most part.

        If people totally ignored the left pedal, they would just be like driving any other automatic transmission car. Braking like they would, anyway.

        If they learn to use the regenerative brake, then they would only use the regular brake at the very end, to come to a stop -- or in an emergency; and the hydraulic brake pedal is where they expect it to be!

        Sincerely, Neil
        • 5 Years Ago
        Hi Tim,

        You're right that coasting in an ICE powered car yields a greater increase in efficiency than in an EV, but it is still the best way to move in *any* vehicle. In fact, in an EV, you'd get "infinite" efficiency, since the motor isn't even idling.

        Ecodriving is just like riding a bicycle: you only use the accelerator a lot to get going from a stop, and to go up hills -- and you coast whenever possible; downhills are "road candy"! If you have to use the brakes to slow down, then you've used too much accelerator.

        In an EV, you have the luxury of being able to use regenerative braking (all EV's should have super/ultracapacitors to absorb all the power the EV drive train can generate) and if you have to use the friction brakes (except for the last few feet!), then you have *definitely* used too much accelerator.

        Sincerely, Neil
        • 5 Years Ago
        Hi David,

        I dislike that solution, because it makes it harder to coast, which is the most economical way to drive. Feet off the pedals, and you coast.

        I also do not want artificial creep -- and this would not work with the scheme you describe, anyway; because if the regenerative braking happened when you take your foot off the accelerator, then how could it also creep?

        The system would be "fighting" itself, and this would kill the efficiency.

        Sincerely, Neil
      • 5 Years Ago
      I've experienced something similar to Toyota's "problem" with meshing together regen and regular braking in my car (first-generation Honda Insight). The regen braking in an Insight cuts out around 20 MPH, and there's bit of a lurch when it happens. I've also had it cut out driving over rough road surfaces. I imagine it's probably a lot rougher of a transition than the one in a Prius -- I can't say for sure having never driven one -- but it's predictable and it's easy to adjust your driving to compensate for it. It doesn't really strike me as recall-worthy, it seems like more of a quirk that you just learn to deal with.
        • 5 Years Ago
        I feel that people just have lost track with how to drive cars. I will admit that I am not always the best and most alert driver out there but I am always hypersensitive to the sounds, smells, and tactile feedback that the car provides. Driving is a skill that most people take way too lightly and typically ignore what the road surface/car is telling them.
      • 5 Years Ago
      With the re-ajusment that toyota will do to the prius, it will lower the efficiency of that car ??? So the main reason to buy this car , fuel economy is gone but the drivability and security will be better. That was just a zealous and costly gadjet where you pay more and drive bad to save few few fuel.

      I say to install a lever on the steering wheel to activate the regenerative breaking to the max before applying the brake pedal if more breaking is needed. And forget electric-only gas pedal, this is too dangeurous, the cable and springs works good since a long time ago. You can add-up electrics sensors but the direct control of the cable is felt by the driver, no need to re-interpret the entire engine operation by a commitee of unknown stressed toyota engineers that read epa and doa papers.
        • 5 Years Ago
        The 2001-2003 model Prius had a less aggressive regenerative braking which works fine. The "brake problem" showed up on newer models when Toyota tried to maximize regenerative braking and changed the braking algorithms. But even with less agressive regen, my 2003 Prius still manages 42 to 50 mpg, as there are a lot of other efficiency maximizing techniques besides regenerative braking.

        As for the gas pedal issue, you've got it exactly backwards, Gorr. The "drive by wire" accelerator pedals on all Toyota hybrid models have been reliable, the only problem was when poorly fitted floormats interfered with the mechanical portion of the pedal. Moreover, the computerized hybrid engine control would ignore the accelerator input when the brake was pressed, providing a reliable "safety override" in the rare cases of floormat caused pedal sticking. In contrast, the non-hybrid Toyota models used a mechanical cable throttle system, and the pedal assemblies from certain suppliers were sticking when wet, preventing the spring from pushing the pedal up when the driver let up on the gas. In short, electronic throttle controls were reliable, mechanical throttles not quite so reliable.
        • 4 Years Ago
        You hit the nail on the head. They have overcomplicated things again, and use it as a reason not to produce these vehicles. Regen is simple and works great. You don't need 23 sensors to regulate the ambient conditions of the air.

        Also, regenerative braking can apply to just about any car that has an electric motor (ie: Hybrids have an electric motor). Even home-brew EV conversions make this work very well.

        How many engineers does it take to complicate things beyond belief? Listen to Einstein: "Make everything as simple as possible, but not simpler." Thus we have EV conversions...

        Please support electric cars by voting EVERY SINGLE DAY for Project ReinCARnate on Pepsi Refresh. Together we DO make a difference.
        • 4 Years Ago

        The first-gen manual Honda Insight does something similar. With zero accelerator application, the car will regen at about 1/3 max (10-15A). This regen deceleration decreases quickly as you apply more throttle, and there is a sweet 'coast' spot where neither the engine is providing acceleration nor is the IMA system providing deceleration. To drive the car most efficiently, you want to spend most of your time in this sweet spot.

        However, the brake pedal will blend regen and friction braking as the article describes, up to a max of 50A to the battery pack.
        • 5 Years Ago
        THERE IS A SIMPLER (and more efficient) WAY TO DO THIS and Tesla currently employs it on its Roadster.

        When you let off the accelerator, the motor begins to slow the car (regen). If you let off all of the way, you get full regen -- full slowing and full recharging capability of the motor. If you want to coast or want to slow down more gently, you simply let off the accelerator pedal less. It is VERY easy, very intuitive (takes like 2 minutes to get used to IF you aren't used to a manual car engine braking, in which case you just get it immediately), and *significantly* simpler than trying to combine regen and friction brakes on the same pedal. If you need to slow more rapidly than regen is capable of, you just apply the brakes like you do in any car. Driving a Roadster feels like driving a manual transmission car that engine brakes when you let off the gas, except the Roadster's engine braking is more linear and easier to modulate.

        On top of the obvious benefits to mechanical and computer simplicity by doing it this way, there is a very obvious and real benefit to driving efficiency. While, on a hybrid or other electric that combines regen and friction brakes in the brake pedal, you cannot tell when you are using regen and when you are using the brakes, it is blatantly obvious on the Tesla. You know CLEARLY that when you use the brake pedal you are missing an opportunity to recapture that energy back into your battery. You know when you are driving in an efficient manner, bringing your car to a stop without touching the brake pedal, and you know when you aren't. If you want people to get the most mileage out of their hybrid or electric (or PHEV), the system HAS to work this way. Completely separate regen and friction braking.

        The Roadster DOES "creep" from a stop light when you let off the brake. It does a 3 mph creep. This is convenient, prevents it from rolling backwards on a hill, makes it feel more like a 'normal' car, etc. If you are touching the brake pedal, the car is not trying to creep. Therefore, it is not using electricity trying to push against the brakes. It just kicks in when you let off the pedal.

        In normal driving, you can literally one-pedal-drive a Roadster, which is awesome! You only put your foot on the brake pedal to keep yourself at a standstill, after the regen brings you down to a very slow creep on its own.

        No, it is not annoying to have to leave your foot slightly touching the accelerator in order to prevent the car from slowing down. You have to do that on a gas-powered car anyway -- it will not maintain 60 mph on the highway if you let all of the way off the gas, right? Same thing here. It's intuitive and it's nothing that feels different than what you're used to anyway. It's very seamless. Of course, there is cruise control for long trips.

        It's fun to see your range tick UP as you go down hills :-)
      • 5 Years Ago
      The prius does have a level that engages the regen braking at its max capacity. No idea what the actual brake pedal thinks/does when you press it.

      The problem becomes: most people are lazy and want their car to be simple - i.e. gas to go, brake to stop. However, some people do want control over whether they're running on electric power or braking with standard vs. regen brakes. Until the auto industry can come up with the equivalent of a "manual shift" electric or hybrid, the public will not be able to have their cake and eat it too.
      • 5 Years Ago
      Good article technicaly, especialy Neills input.
      It omits the real reason for the Prius recalls however.
      Pressure from the oil and conventional car manufacturers who do not want their companies damaged by Toyotas sales.
      Fota in F1 was the last attack that killed Kers in F1 and put back hybrid development by a decade.
      Before that GM scrapping their EV in the 80s.
      Before that GM scrapping electric buses in nearly all US Towns in the 30s.
      Some people just dont want progress.

      There is a better way to recover energy from brakes Neill and a better way to make use of it.
      I have a patented ESERU electric shift energy recovery unit, that is supported by F1 engineers for a new generation kers. It is also a new hybrid system.
      If you want a .pdf let me know.
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