Getting back what you put in

Several weeks ago, I packed up all of my worldly possessions and moved them nearly 750 miles from Southeast Michigan to central North Carolina. Part of this process involved driving a seventeen-foot U-Haul truck, fully loaded and towing my car on a trailer, through the mountains. I found this process to be highly unsatisfying, but also thought-provoking.

For perspective, U-Haul lists this vehicle as being more than eight thousand pounds empty. Fully loaded, it weighs in at just about fourteen thousand pounds. In addition, the trailer is more than two thousand pounds empty, with a more than three thousand pound car riding on top of it. The effect is a fourteen thousand pound vehicle with a five thousand pound sea anchor hanging off the back. Driving a vehicle this size through the mountains is enough to make anyone develop a multiple-personality disorder. Going uphill, I would simply lay the accelerator flat against the floor, then listen to the engine roar even while the speedometer steadily dropped beneath the sheer mass of the vehicle behind it. Going downhill, I’d stand on the brakes and try not to think about the feeling of the massive, dead weight of the trailer cramming itself up into the small of my back.

Each time I rode the brakes down a hill and then listened to a huge, gas-guzzling engine wheezing as it struggled to heave itself back up the hill, I found myself thinking about regenerative braking. Even though it would not be the kind of thing that is even included on a vehicle of that type, such a thing felt like it would be exactly what I needed. I spent several hours thinking about the efficiency of regenerative braking systems.

Regenerative braking is a technology that has been around for many years, but has only recently been included in cars. It works by taking an electric motor attached to the wheels and, rather than using it to drive the vehicle forward, instead using it to operate as a generator, producing electricity from the force of the wheels turning. In the process, the vehicle is slowed down. In this way, some of the energy used to accelerate the vehicle from a stop is recaptured as it slows back down, and can then be reused the next time it accelerates.

As an added bonus, regenerative braking relies on electromagnetic forces, rather than on friction. As a result, the parts don’t wear as the vehicle slows down. It reduces the wear and tear on brake pads, discs, and drums. Because sometimes more braking power is needed, vehicles with regenerative brakes almost always include standard friction brakes, as well. Used properly, regenerative braking will make the standard brake system of a vehicle last much longer.

This is a technology that has been used in trains, particularly passenger trains, subways and streetcars, for decades. Being on a large grid of electrical lines or rails, trains are able to simply dump the energy they recover back onto the grid, rather than having to store it on board. With the advent of gas electric hybrid cars which need to carry a battery array with them, it suddenly became practical to recapture energy during braking, because the car had somewhere to put it.

Once I finished my move, it took me a couple of weeks to get an internet connection hooked up again. However, once I had one available, I set about trying to discover how efficient regenerative braking systems really were. To my great distress, I’m still looking. Try as I might, I can’t find any reliable source of information on the subject.

I’ve located several resources with information about the efficiency of regenerative braking on trains. However, because trains use low voltage DC current, they suffer from large transmission losses. As a result, the efficiency of a regenerative braking system is dependent on the distance on the grid between the train and the switching station. The various estimates I found listed efficiencies ranging from as low as 5% to as high as 50%. I was not able to find any reliable data on the practical efficiency of such a system, independent of the circumstances of the grid it was sitting on.

The information on regenerative braking in cars is even less helpful. Claims range all over the map. I haven’t been able to find any reliable source of information. If one is willing to take the word of random people on forums, regenerative braking is anywhere from 99% efficient to actually worse than friction braking. A little bit of simple math will show either of those claims to be well beyond the pale, but this leaves a lot of questions about more reasonable numbers in between.

Companies that make hybrid cars spend a lot of time talking about the overall efficiency of their vehicles, but I wasn’t particularly interested in the gas mileage a Prius can achieve. Rather, I wanted to know how much of the available kinetic energy its regenerative brakes could recapture and make useful again. For this specific piece of information, I haven’t been able to find any kind of authoritative answer. In a twist of slightly worrisome irony, one of the better items I did find was an old homework set from a class at M.I.T. which lists the efficiency at 64%. Then again, it was written back when gas was $1.75 a gallon, so I don’t know that it really reflects the more recent innovations in the technology.

I’m hopeful that a member of the greater geek community can point me in the right direction. If anybody can direct me to a reliable source on this matter, I’d be very happy to hear about it.

-posted by Mark


8 Responses to Getting back what you put in

  1. goshawk says:

    Toyota says that their Prius recovers through regenerative braking about 1 liter of gas in 100 km of city driving. My experience is that the Prius can get somewhere between 45 and 50 mpg in city driving. That translates into about 4.3 liters per 100km. Without the regenerative braking this would be 5.3 liters per 100 km (assuming we believe Toyota’s claim). This looks to me like the regenerative braking is recovering about 20% of the energy.

    Another approach to this question would be to collect data off of the energy display in the Prius during driving. One display screen shows a graph of the energy you saved during the last 20 or 30 minutes of driving in 50 watt increments. I tried to figure out how much gas we saved after a trip one time and it was really small – like a teaspoon. However, I don’t know that this contradicts Toyota’s claim since I didn’t record how far we had gone.

    My gut tells me that Toyota is probably in the right ballpark.

  2. goshawk says:

    Here is another article ( that says you can only recover about half the energy lost through braking and that would amount to saving 15 to 20 percent on fuel use.

  3. Mark says:


    Thanks for taking the time to try to puzzle this out by experimentation on your own vehicle, but I’m still interested in the underlying data. How much energy you save/recover while driving is dependent on a lot of factors. Because the Prius is equipped with both regenerative and friction brakes, I suspect that performance is highly dependent on your driving habits. If you brake hard a lot, you are likely to use the friction brakes more, and see less benefit. On the other hand, if you can manage to drive a long distance without using the brakes at all, you might well get almost nothing back from the regenerative braking, but still get the best gas mileage of any scenario.

    What I’d really like to know is the theoretical efficiency of the system. What percentage of the kinetic energy of the vehicle can the brakes recover? What percentage of this energy can the battery system usefully store? And what percentage of that energy can then, in turn, be converted back into kinetic energy? It’s interesting to see some real-world data about how the system works in practice, but I’m also very interested to find some reliable data on how it works in theory.

  4. Matthew Pelham says:

    I found this article very interesting.

    However, I have to agree with you, there a frustratingly few sources of information on the subject. I am currently beginning the early planning and design phase of an electro-cycle and was hoping to be able to build the brakes onto a rolling chassis. The most promising system would be similar to TRW’s, which uses electromagnetic braking systems that cut down on normal braking wear-and-tear as well as restore lost energy. Hopefully I can build a system that requires as little maintenance as possible!

    I am not a gear-head in any capacity, but have been doing basic electricity, propulsion and welding lessons in my free time and feel relatively well-read on the subject. When I get the plans done, I would be happy to send them to you for your own perusal.

  5. Matthew Pelham says:

    Sorry, correcting myself. The TRW system does NOT use electromagnetic braking but rather uses hydraulics. This is still promising, as it is an electric free system. Their site is acting up on my computer, however, and I have not been able to locate any further information on their corporate site. However, I am almost positive that the crucial component is the “brake volume displacement simulator” which I have included a link for at the bottom.

  6. Mark says:


    Thanks for the link! The type of hydraulic energy storage systems described in that article are one of many different systems for retaining energy gained from regenerative braking that I’ve seen talked about in a variety of places on the web. As with the electrical systems used on most gas-electric hybrids that include regenerative braking, the claims about the efficiency of such systems run the full range, from hopelessly inefficient to nearly perfect. As with the electrical systems, I suspect that the truth is somewhere in between, though I have no hard data to back this up.

    I’m more of a software guy than a wrench-head, but I’d be interested to see what you’ve got, when you’re ready to share. The “Contact” link at the top of the page has the info you’ll need.

  7. Shock&Awe says:

    I rented a Prius hybrid for business travel in the Las Vegas area this year and was curious about the efficiency of regenerative braking when compared to the energy content of a gallon of gasoline. There is nothing on the internet that anwers my curiousity, so here’s my input. I am an electrical engineer, with specialization in alternative energy, so this was an ideal geek study. Using the Prius energy display, I calculated that regenerative braking only replaced about 2.3 tablespoons of gasoline during a 1 hour city driving test. Using additional information from the Prius energy display, I calculated that the Prius engine consumed about 1.5 gallons of gasoline during that same test. If you convert the energy units to Watt-hours for these two quantities, the calculated regenerative efficency is 0.9%. This is a surprisingly low number, but I suspect that it is the daily reality of how most of us use regenerative braking on a hybrid car. I would assess that regenerative braking for hybrids is more about “green” marketing and not about braking performance and efficiency. The technical reality is that electromagnetic regenerative braking cannot be used solely to stop a 2000 lb car from stop light to stop light. City traffic will always require friction braking, with a very small contribution from electromagnetic regenerative braking. Take a look at my approach and let me know what you think about it. I suspect that the hybrid car purists will be pissed and that American muscle car guys (like me) will be laughing their asses off. Either way, the truth is front of us.

  8. Jeff in the US says:

    I can’t answer your question about efficiency of regenerative braking – I was googling for the same info which is how I ended up here. I did find a statement at that “Indeed, without regenerative braking and coasting, the Prius would probably not be much more efficient than a standard gasoline car.” But I find this highly unlikely – there are clearly substantial MPG benefits from shutting down the idle engine and from using electric engine at low revs when a gas engine would be very inefficient.

    I can, however, say without fear of contradiction that the previous contributor “Shock&Awe” is just lying through his teeth. He is obviously wrong that “electromagnetic regenerative braking cannot be used solely to stop a 2000 lb car” since it has been used for decades to stop much heavier trains and subways. He is also clearly NOT an “electrical engineer”, let alone one “with specialization in alternative energy”, and his claim that he “calculated” that regenerative braking saved “2.3 tablespoons” out of 1.5 gallons of gas is laughable. If he were a real engineer, he would be more interested in telling how he reached this amazingly precise conclusion, rather than bragging about how macho he is with his “Shock&Awe” “American muscle car”. In short, he is a blow-hard and a fraud. I hope you find an answer, but it won’t be from him.

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