E-bike Generator Challenge

Challenge your thinking some by envisioning a generator at the bottom bracket of bikes. No chain, no sprocket, no derailer, no shifters, no cassette. Only an electric motor, battery, controller (with high-efficiency regenerative braking) wires, throttle, and a generator. The E bike would evolve in an interesting way.

Is anyone doing this already? Thanks in advance for any discussion.

You got the footloose. Not for me.

http://www.fullycharged.com/Mando-Footloose-IM

It's not technically complicated at all, but it's a terribly inefficient way to transfer power from pedals to wheel. We have already invented far better systems for that.

dustNbone said:

...it's a terribly inefficient way to transfer power from pedals to wheel.

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I would have to disagree. A generator can be 97% efficient, transfer the power directly to the electric motor or store it in a battery if not immediately needed (storage has lower roundtrip efficiency). 100 watts of power optimized for the humans peddle rpm and ergonomics makes lots of sense. This makes as much sense as a mechanical gear, cassette and chain scenario. The cost reduction should be significant, weight reduction marginal. Optimization of rpm for exact cadence will increase overall system efficiency. For people who can't pedal, but use a hand crank, this is a no-brainer better scenario than the current expensive mechanical solutions available.

energyi

As one that agrees regarding the inefficiencies of a system like that, but smart enough to know better than say it can't be done, I'll be waiting for your proof of concept model. Hopefully on display prior to the day I end up pushing up daisies.

energyi said:

dustNbone said:

...it's a terribly inefficient way to transfer power from pedals to wheel.

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I would have to disagree. A generator can be 97% efficient, transfer the power directly to the electric motor or store it in a battery if not immediately needed (storage has lower roundtrip efficiency). 100 watts of power optimized for the humans peddle rpm and ergonomics makes lots of sense. This makes as much sense as a mechanical gear, cassette and chain scenario. The cost reduction should be significant, weight reduction marginal. Optimization of rpm for exact cadence will increase overall system efficiency. For people who can't pedal, but use a hand crank, this is a no-brainer better scenario than the current expensive mechanical solutions available.

energyi

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97% seems unlikely. Heck, 98% is pretty much the best case scenario for a crank and chain. So the mechanical aspects of your generator scenario would probably cost ya 3% before you ever move an electron. A quick look online shows generator efficiencies peaking at around 91%. Converting the generated electricity back to torque would have its own inefficiencies. Typical motor efficiencies are around 85%. I seems really unlikely that you'd be able to match a simple chain and sprocket setup for efficiency. Certain you wouldn't be able to match by cost - otherwise most bikes would be built this way.

There is an ES member who could probably give a lot of practical information on this. I forget his name, but his project is the Electrom. That bike uses a generator only pedal drive after the bike reaches 6 mph. Up to 6 mph, it uses a mechanical coupling as well as the generator. I'm betting he'd be a good resource on net efficiencies.

http://www.electrom.ca/

There are a number of bikes and trikes that have been made this way here on ES, and others elsewhere. Sometimes it's called a Serial Hybrid Cycle.

The first one I recall, Lowracer, unfortunately no longer posts here, and all of his videos are gone but many of the pics are still there.

Justin_LE's solar challenge trike does this at the rowing end, IIRC.

There's a few that show up in this search, but there's more that don't:
https://endless-sphere.com/forums/search.php?st=0&sk=t&sd=d&sr=topics&keywords=trike+generat*+pedal*&start=1

energyi said:

I would have to disagree. A generator can be 97% efficient

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It can be, but bike generators won't be. You get those 97% numbers for utility scale generators at one operating point. These monsters operate at one speed all the time, and are only 97% efficient at one power level (usually maximum.) They can get there because weight and size (and sometimes even cost) are no object.

For a bike generator, you have to gear it up to run at a reasonable speed. That's another loss. Or you can run directly off the pedals, but then you are designing a VERY low RPM generator with huge magnets, and your eddy current losses go way up. You would be lucky to hit 80-85% efficiency with one.

Next you have to convert AC to DC. You can do it with diodes, easy. Diodes guarantee a loss of between .5 and 1 volt. On a 48 volt system that's another loss. Call that 98% efficient.

Next you have to charge batteries. Lithium ion batteries are very efficient coulombically - for every amp hour you put in you can get an amp hour out. But they are not efficient energy wise, because you are charging at 4.2 volts per cell and discharging at 3.8 or so. So that's another 90% efficient step.

Next you hit the motor controller. You can do pretty well with these - 95% would not be unusual for a good, well-matched controller.

Finally, at the motor, you can see 85% peak efficiency. But for a bike ridden in the real world of hills and changing loads, your average efficiency is going to be closer to 75%.

So your overall efficiency is: .80*.98*.90*.95*.75 = 50% efficiency. Now, you could improve that with custom designed (and expensive, and heavy) components. But let's say you're rich, and do an awesome design job and get that to 70%. Do you really want to throw away a third of your pedaling power? I don't know about you, but I need all the legpower I've got when I am on a conventional bike.

For people who can't pedal, but use a hand crank, this is a no-brainer better scenario than the current expensive mechanical solutions available.

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Justin did a pretty good job with one for not a lot of $$. The commercial ones out there run $1500 and up - but your solution would likely be a lot more expensive.

amberwolf said:

Justin_LE's solar challenge trike does this at the rowing end, IIRC.

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If you mean the Sun Trip trike, I don't think so. The rowing mechanism connects directly to the main crank drive. I recall Justin talking about trying to match the drum diameter of the pull cable so that the rowing motion would tend to match up well with the standard crank cadence.

[youtube]5Ne21GhaTMs[/youtube]

Ah, well, originally he'd planned for a rowing-end generator; its' even still in the title. I didn't realize (or at least, didn't remember) that it didn't work out that way.

I recall one ES member had done it on a faired recumbent, 2 wheel velomobile. I agree cranking a generator is terribly inefficient as compared to a bicycle drive train, not only in energy conversion losses. The extra weight has to be part of the equation too.

Yet, I have seen worse ideas. Like this guy who wanted to regen the front wheel to power the rear.

It does have a few limited advantages, but efficiency is not one of them.

You could charge the battery some while stationary, like at many stop lights, or keeping the DJ powered up at a party, or charging a house battery off grid, at night.

There are several versions of e bike, that can also charge a battery (just not at each stop light), it just requires lifting the wheel off the ground, not a bottom bracket made into generator.

Great discussion, thanks for all the insights. The Mando Footloose and the Electrom are great examples of what I was requesting. Thanks. ( http://www.fullycharged.com/Mando-Footloose-IM and http://www.electrom.ca/ )

These designs will evolve and bottom bracket generators will become more efficient. I like that the Electrom is beginning to utilize the advantages of the evolution to a generator, lower profile and fairings. Eventually I would think that the ergonomics for the most efficient positioning for a constant wattage generation, for the distance, will be a big driver for overall system efficiency. Again, this would be for a bike or trike that already has an EV motor; already has a motor controller, battery, throttle, wires. And I would envision the generator to be DC, not AC and that the DC would go directly to the motor when it is needed eliminating storage losses.

Thanks again for the discussion.

energyi

energyi said:

And I would envision the generator to be DC, not AC and that the DC would go directly to the motor when it is needed eliminating storage losses.

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All generators are AC. You can convert them to DC via a mechanical commutator, electronic commutator or diodes.

Note that bicycle motors are AC as well. The common term BLDC stands for brushless DC - but requires an external controller to convert the DC battery voltage to the variable frequency AC that the motor windings actually need.

Permanent Magnet DC Generators. My hub motor regens DC.

energyi said:

Permanent Magnet DC Generators. My hub motor regens DC.

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In this case, the Carbon brushes (mechanical commutator) rectify the AC Voltage.

energyi said:

Permanent Magnet DC Generators. My hub motor regens DC.

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That's because there are either commutators or rectifiers converting the generator's AC to DC.

A motor that is 97% efficient is very expensive. Ever seen what those kinds of motors cost?
Then you're gonna lose a few % in the controller both ways unless you use an expensive controller.

So this is a very expensive way to have less efficiency than a string of metal hooks and pins..
If you eliminate the bike chain entirely then you need another motor on the rear.. and since you're not using gears, then you need an oversized motor to take hill climbs and efficiency will be nothing like 97% on the drive side. More like 80.. 90%

Now you have a ~$3000 bike chain that's far less efficient than a real one.

A better option for a 'hybrid' bike is a tiny DD hub with 0.2mm laminations and ultra ultra tight windings so that it only gives you ~1nm of losses from the iron lamination stack.. pair it with the most expensive low friction bearings.. you can build this mythical tiny DD for about $500.

..or you build a geared motor with an electrically actuated clutch so that when you hit the brake, you get regen. When you hit the power, you get power.. otherwise you are freewheeling.

The expense of the custom motor could be offset by using a smaller battery since this is a hybrid bike, right?
Allow the user to add a bigger one on..

neptronix said:

A motor that is 97% efficient is very expensive. Ever seen what those kinds of motors cost?

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Yep! I worked with a Sunraycer team for a bit. $5000 for a motor isn't unreasonable when it means the difference between second and third place - but it's not something that most people are going to be interested in.

Sounds like a very fun project.
It's amazing what's required to squeak those last few percentage points out of an electric motor.

Lucky for us, there's emrax motors today and they don't cost a bundle and a half.
http://emrax.com/wp-content/uploads/2016/12/emrax_188_technical_data.pdf
But making a small super efficient bike sized motor is apparently a huge and expensive challenge.

The limited efficiency is still worth it, in specific applications. Example, pedal your e bike to charge a small house battery, that gives you one light at night, and charges your cell phone or other small battery devices. This works well in the third world, where house power might not exist at all, or cannot be affordable yet. Presumably in the am, you might be able to charge the e bike at work, or some other place away from home. Pedal gen might suck balls, but not as much as missing that important call, like you get to work today.

dogman dan said:

The limited efficiency is still worth it, in specific applications. Example, pedal your e bike to charge a small house battery, that gives you one light at night, and charges your cell phone or other small battery devices. This works well in the third world, where house power might not exist at all, or cannot be affordable yet. Presumably in the am, you might be able to charge the e bike at work, or some other place away from home. Pedal gen might suck balls, but not as much as missing that important call, like you get to work today.

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This is known in the industry as the 'first watt'. The first watt is the most valuable to the consumer, and also the seller. Like an addictive drug, 'the first one's free' then they're hooked, they want/need more. We are all power addicts. We're always trying to squeeze out the final few watts... in the name of efficiency.

To pedal for charging is only making sense when you need the energy for some other purpose than moving. Why would one want to pedal more energy and carry more weight than watts required for the ride? Masochism or physical exercise addiction?

I think it is great idea for those who are on high powered ebikes who still want to have good excersice.

Personally i am looking for a system that i could install in my bike using standard bottom bracket in my velomobile. The chain is so long and after fitting hubmotor in the rear wheel it is hard to make a smooth chainline so it doesn't rub anywhere making a lot of noise...

Any suggestions would be welcome. I don't give a damn about efficiency, just want to have more or less natural resistance when pedalling the generator.