Regenerative Shock Update

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Some of you know that we’ve been working on an energy recovery idea for awhile now, in an attempt to recoup energy normally lost as heat to the damper on a vehicle. Well, here’s a little bit more on that story, where/when it started for us and how it’s coming along.

In the early 1990s I was working with a college team to build a solar powered vehicle, at which time I had, what I thought was a terrific, original, Earth-shattering idea to use a linear type generator on the vehicle’s suspension to recover energy from the suspension travel as the car bounced down the road instead of just turning that motion into heat as standard vehicle dampers do. A couple days later one of the guys on the team came into the lab with an old issue of Popular Science, or some such magazine, from the 1960s, which had an article about a revolutionary new idea being tested by Chrysler to increase vehicle fuel efficiency through the use of regenerative shocks!

Ok, so maybe it wasn’t an original idea, but it was still a good one.

I never had the chance to try it out on the solar car and the magazine article never gave the actual results of the trials by Chrysler. Time passed and so did college. Much like my ability to do calculus, the opportunity to try the idea out just faded away. Then a couple decades later the Automotive X-Prize came along and so did Seven, giving us a new platform on which to experiment.

The old idea was rekindled when we found a publication from Argonne National Lab which was supported by a study conducted by the State of Minnesota on the use of regen shocks in a 3000 lb car traveling at an average speed of 45 mph, with an average road surface roughness of 1-3 mm. The results showed the potential for energy recovery if regen shocks were used at all four wheels was 3000 watts/hr. Based on this information, and the earlier idea I had in college, and finally finding a potential linear motor we could use as a generator we were finally able to make some estimates on the potential energy recovery, and therefore economic viability of using regen shocks.

We estimated that if we could recover 1000 watts/hr, one third of what Argonne’s study said should be recoverable, we should be able to recover 17.5% of our total energy use (based on Seven’s results on the efficiency testing conducted on the test track at Chrysler which gave us a steady state power draw of 6000 watts at 50mph). We thought WOW, that’d be awesome…and also sounds a bit high; we were estimating only recovering ⅓ of what Argonne’s study said should be recoverable. So we moved ahead, got a Linmot linear motor/generator, installed it on one of Seven’s rear lower control arms, and ran some tests.

Unfortunately, the results of those experiments so far have been disappointing. Instead of the low end estimate of 250 watts(per linear generator totaling 1000 for all four) and the potential high end of 750 watts, we achieved output readings averaging 3 watts — nothing to blog home about.

The testing was conducted with the vehicle stationary, manually oscillated through a range of suspension arm travel until steady state readings were achievable on a FLUKE digital oscilloscope. To acquire a steady reading the suspension arm travel was approximately 25mm, an equivalent of 10x the road surface roughness Argonne used in their study, but at a lower frequency.

The data collected did enable us to do some rough estimates of what may be possible with our setup under ideal conditions, however. If we could build a generator with a minimum of 1000 windings per inch, add a 1 to 4 multiplier arm to the input of our generator (basically amplifying the road surface roughness) and stack four of the windings in a series parallel configuration, the theoretical potential could be as high as 30 watts per winding for a total of 120 watts per wheel, giving us a maximum energy recovery potential of 480 watts/hr. These are very rough estimates and represent what we believe to be best case results. Therefore, if we could achieve 50% of these estimates we would be doing well and consider it a successful test. But that would only give us 240 watts/hr instead of our previous estimates of 750; it would be easier and far less expensive to just put a single solar panel on the roof of the vehicle instead of going through the trouble of building and installing four custom generators.

We have not given up this option yet. We’re going to do some more digging into our equipment, specifically the Linmot motor, by doing a CT scan of the motor to find out more about its design and how close to ‘ideal’ it is for our application– after all we are taking an ‘off the shelf motor’ and using it in an application for which it wasn’t designed.

We’ll let you know how that turns out, but for now, as the Magic 8 Ball says: “outlook not good.”

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