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RevoPower’s Gasoline-Powered Bicycle Wheel

The Wheel’s basic design.

RevoPower, a Denver-based company, is developing a 2-stroke engine-powered replacement front wheel for bicycles. The 25cc hub engine in the Wheel delivers 1hp (0.8 kW) peak power at 7,500 rpm and supports a top speed of 20 mph. Fuel economy is an estimated 200 mpg.

The Wheel meets CARB/EPA standards for small off-road engines, and features an idle-stop function that cuts off the engine at speeds below 5 mph. (Conversely, the rider needs to hit 5 mph before the engine will engage.) By stoping the engine at low speed or rest, RevoPower avoids the issue of idling emissions, as well as the problem of cooling the engine when the wheel is not rotating.


The company has developed seven prototype engines, two hand-built and five from Komatsu. RevoPower is working with another manufacturer on a stratified charge two-stroke engine with port geometry that will meet the emissions requirements.

The Wheel itself weighs less than 15 pounds (5 kg), and uses a standard one-quart fuel tank (although other designs are possible). Noise emitted by the engine is less than 65 dB at a distance of 7.5 m.

Swapping out a conventional bike wheel with the powered Wheel takes about 30 minutes. The Wheel will be available in 2007, and will cost about $400.

RevoPower has put about $1M into the development of the Wheel so far, and is raising an additional $3-7 million.

(A hat-tip to Richard Earl!)




I am not a fan of small 2 cycle engines. They make noise and pollution, the power output is almost an afterthought. The EPA standards are not very strict and the amount of pollution put out by a one HP engine may exceed a full size 100 plus Hp Prius.
Bikes this size are best served by an electric motor in terms of noise and pollution.
Perhaps someone could post the g/HP or g/mile emissions on these small engines.

Harvey D.

A real misfit where clean electric non-polluting bikes are availble.


Actually, bikes of this size are best served by the use of two human legs. If you're gonna use some sort of external power, agree, however, that electric would be better.


actually the idea is pretty old,

already in 1987 they started to produce these bikes, i own one just for fun of 1988

Eric S. Johansson

is one source of emissions analysis. two-stroke engines persist because they are a huge win for small motor applications. for the same mass, you get twice the horsepower. and yes, because two strokes are terribly dirty, we are going to have to suck it up and convert to weaker motors made even weaker by low-energy density fuels like ethanol is an article on fuel cells and scooters.

As for electric bikes, they are toys. They can't carry any sizable amount of load. If you travel any distance i.e. 20 miles or so, it's effectively a one-way trip unless you have four to six hours to spend in between arrival and departure. for small vehicles like scooters and motorcycles, liquid fueled engines will be a win for the for the foreseeable future.

FWIW, when thinking about scooters, don't think about the 49 CC systems. Think about 150 to 500 CC. I'm buying a 250 CC (four stroke, liquid cooled) because it lets me get up to highway speeds without it overpowering my skill level.


Future scooter owner here.

I've looked at the possibility of an electric bike, and the price/performance ratio wasn't good enough for me.

At least someone is looking into the biggest single complaint about bicycling: The sweat. Now if they could work on the funny pants part, we'd be set.

Mark A

This is a great design idea. The packing of it is very well thought out. And until 2 stroke, weed eater type, engines are totally outlawed, which they arent at the moment, I dont see a big problem here. Of course if everyone, and thats everyone, turned in their 6 mpg excursions and suburbans for 6 of these gas powered bikes, then the resulting pollution problems would need to be taken care of. Of course we would save alot of fuel in doing so.

For now, a nice toy to market, but will not make much of an impact. Who would pay an extra $400 on top of the price of the bike, for this system? Only ones who regularly now use a bicycle. This will not get us out of our 4 wheel vehicles.

Thomas Pedersen

Hmmnnn - navigating dense city traffic with 15 lbs extra weight in the front wheel..? No thanks!

It sounds to me like a dirty, loud lawn mower engine... 65 dB from 7.5 m is quite a lot. Standard cars sound off less than that from 1 m.

Too bad they wasted $1M on that project... :-(

Rafael Seidl

Eric -

two-strokes do not get twice the power out of the same displacement, for several reasons:
- displacement is bore * stroke of the piston
- the bottom quarter of each stroke is reserved for gas exchange
- combustion products are retained in the chamber (10-30% EGR depending on scavenging method)
- fresh charge must be compressed to effect scavenging, in small engine typically in crankcase

Bottom line: typical power gain relative to four-stroke is more like 50%. The price is HC emissions as high as 30x those of a four-stroke engine of similar power. Note that it is possible to build four-strokes small and powerful enough for chainsaws, with substantially reduced HC emissions.;site=a4e/lng=en/do=show/alloc=3/id=2712

As for recharge times, with modern batteries it should be possible to bring those down to 15 min off a regular 110V 15A household circuit. Assuming a 50% efficient charger and 14V battery voltage, you'd need a 14.5Ah battery. Btw, some of the fancier electric bikes are rather expensive toys:


Sid Hoffman

I'm also concerned about the noise, like others have mentioned in the standard scientific method is to measure sound levels at 1 meter. In fact, the rider's head is probably not much more than 1 meter from the wheels anyway.

Noise levels generally drop in half for every doubling of distance, although this is where it gets confusing since the real sound level drops in half ever 3 db, even though human perceived levels are every 10db or so. Even if we take the more conservative 3db, 65db at 7.5 meters would be 74 db at 0.93 meters, which is very loud. At 10db doubling, it would be over 90db at 1 meter.

Agreed that a human/electric hybrid bicycle makes more sense both from noise and clean energy perspective.

Eric S. Johansson

Rafael Seidl: thanks for the correction on power output. I'm looking forward to a pollution tax + buyback program on engines to drive 2strokes out of general use.

And yes, e-scooters are expensive toys. when they can match the range and speed of a 250-300 cc scooter, I'll think about them. ;-)

thanks again for the correction.


"As for electric bikes, they are toys. They can't carry any sizable amount of load. If you travel any distance i.e. 20 miles or so, it's effectively a one-way trip unless you have four to six hours to spend in between arrival and departure. for small vehicles like scooters and motorcycles, liquid fueled engines will be a win for the for the foreseeable future."

Most electric bikes out there fall into the category you describe. However over the last couple of years there's been some huge improvements in wieght loss speed and range. I've personally built lightweight bikes that go 30 miles at 20mph no pedalling.
There are lots of excellent kits out there for cargo see

There's a little less available in the way of complete bikes but there are some great models out there.
Have a look at the ezee torque

One of the great things about electric bikes is that you can expirement with some fantastic battery tech that will takes years to be adopted in cars. Some people are even starting to use a123 systems cells

Anyone looking for info on electric bikes should take a look at or the yahoo group power-assist Newbies are very much welcome!

John W.

Nick F thanks for the links! They are helpful.

Two-strokes, historically anyways, have indeed produced roughly twice the ponies of equal sized four-strokes, because of double the powerstrokes, and simpler engine design with less power-sucking mechanisms like cams, rocker arms, related bearings and chains, etc, etc. Anyone who grew up with dirtbikes knew the two-strokes in their powerband were scary fast: a typical 250cc two-stroke would put out around 50-55 horse compared to roughly half that for a 250cc four-stroke equivalent, at least a decade or more ago. I'm not in the scene anymore so I'm not up on the current generations. Mind you, four-stroke dirtbikes have had a major renaissance lately, and have come a long way, so I can believe two-stokes are not as advantageous anymore, power-wise. However, I do doubt very much the power advantage has been cut in half.

That said, I'm not promoting two-strokes, because they do polute much more, though they have been cleaned up substantially in the last several years. Golden Eagle Bicle engines has a very good engine-assist set-up too: and though they don't have the slick in-wheel packaging of this setup here (it sits outside of the rear wheel usually), it does have this advantage: you can use any engine you desire, including clean four-strokes.

Eric S. Johansson

NickF: I'd like to see stokemonkey work with: ( I have a folding model) :-)

fuel cell scooters

electric scooters

dirty search for electric scooters


It would be interesting if something like this could be fueled by an ethanol/svo blend or some other biofuel/blend that works well in a 2 stroke. That way they could market a 'green' fuel for their otherwise dirty motor for those a bit more concerned about emissions and get more free press.

This would make an interesting addition to an electric bike, giving it a third source of power. Hills around here mean lots of opportunity to recover energy, so gas alone doesn't make enough sense.

Rafael Seidl

Eric -

you're welcome. A recent long-term study conducted by the Austrian automobile club OeAMTC in collaboration with the Univ. Techn. Vienna shows that total HC emissions in 2005 (total 175,100 metric tons) were split as follows:

solvents - 44%
home furnaces, gas stoves etc. - 23.6%
industry* - 14.7%
agriculture - 4.1%
cars - 4.1%
two-wheelers - 3.7% <--- NOTE
off-road machinery - 2.7%
commercial vehicles & buses - 2.0%
power plants** - 0.6%
aircraft - 0.5%

* includes evaporation at during vehicle fill-ups

* Austria has extensive hydroelectric dams and just one coal-fired power station.

The important thing to note is that on-road two-wheelers produce almost as much HC as all of the cars. This is because of those small engines running on two-stroke gasoline (premixed with lubricant) that do not have three-way catalysts (too much back pressure).

John W -

I meant bmep rathen than nominal power. Due to the simple construction without valves, it is possible to rev a two-stroke higher to achieve the 2x nominal power you indicate. Doing that sharply reduces your engine' life expetancy, though, unless its one of these

Final point: the biggest reciprocating engines in the world are two-stroke turbodiesels with crossheads and exhaust valves in the calinder head for straight scavenging. These 60,000-100,000 hp monsters are 15m tall and built by Hyundai under license from MAN. Cylinder bore is about 2m. Running at just 60 RPM, they allow the huge propellor of a contain ship to be fitted directly to the crankshaft.

Ergo: not all two-stroke designs are evil, just the ones that blow lots of unburnt HC out the exhaust.

John Ard

I'm a bit of a redneck so bear with me. I have a hard time seeing how a bike would handle with front wheel drive. There has to be a nearly vertical learning curve on figuring how to not fall off this thing and burn your face on the engine.


Erick, the Wheel can operate on E85, and on biodiesel (with appropriate changes to the engine).

Thomas Pedersen


Correction, the bore is about one meter, not two. Until recently, the largest MAN engine employed 98 cm bore. I believe the largest is now 108 cm.

The stroke, however, is more than two meters.

The strongest engines are mostly made for container ships that travel faster than oil tankers. For that reason, and physical limitations of propeller size in harbours, these engines run faster; around 100 rpm.

A note one CARB/EPA regs for small off-road engines: Read, lawn mowers, i.e. very lax regulation! I distinctly remember a study 10-15 years ago comparing emissions from a lawn mower with a SAAB 2.3 litre engine (particularly clean). Mowing an average sized lawn caused as much emission (mostly unburnt HC) as a 4500 km trip with the SAAB engine. That corresponds quite well with my subjective feeling (headache) after using a lawn mower.


Bike like this should go for 500mpg, 200mpg is not enough.

Why go thru the trouble mounting a 2 stroke on a bicycle? Might as well get a motocycle! Which confirm have more then double the payload of a bike, high MPG, and more power.

Electrical bike, quiet, charge at home, clean, zero emmission and whats bother you so much just to excercise your legs a little?


sebastian wrote:
"actually the idea is pretty old,

already in 1987 they started to produce these bikes, i own one just for fun of 1988"

Actually, the Honda's first product was the same concept in 1947.


Eric S. Johansson:

Wow thats a nice bike (I'm very jealous ! ) ). I'm not sure whether the stoke monkey would fit
Recumbents generally make an excellent platform for electric assist as they are already very efficient. You might want to look at the expensive but good quality mid drive kit

there is also the cyclone kit

and a range of diy options some using similar silent induction motors like on the stokemonkey. If you look at the photo section on the power-assist i think there is somebody who's done a conversion on the same bike


According to IMO Annex VI, of MARPOL 73/78, regulation 13, a two stroke heavy fuel slow speed diesel with the one meter bore and over two meter stroke can put out 17g NOx/kW-h.
Perhaps sopmeone could put that in context with a normal truck engine, but they are rated g/mile.

They burn fuel from .5 to 4.5 % sulfur with 2.5% being the average, so the SOx is very high.
They have no PM rules other than smoking in port limits.

They put out more pollution than any other engine type except off road two strokes and they at least do not have much SOx.
By changing the fuel to normal diesel low sulfur (500 PPM) like in a truck, and SCR with urea, the pollution goes down about 90%.
The fuel eff. goes down only slightly.

Other than


People, Sorry but you are so far off on modern two stroke engines.

First of all the 50cc two strokes are euro 3 capable (not official yet because they have not finalised the intro date) and have catalytic converters. They make more power that the older not cat versions.

Ironicaly the 4 strokes have great dificulty in meeting legislated CO levels - go figure.

Then of course there are the direct injection technologies that provide very competitive engine out emissions.

Also 200mpg is pretty good for a small engine runing at that speed. The stratified charge engine should drop HC's by about 70%, CO by about 80% and reduce fuel consuption by about 25%.

As for noise. The europen limits for MOPEDs is 72dBA at, surprise surprise, 7.5 meters so 65 dBA is going to be real quiet. Sitting on top of a moped that gets the 72 dBA number whilst riding around on a normal day, you are are aware of the engine but it is subjectively lower that the tire noise of a passing SUV. I would say that at 20mph on this little guy you are not even going to hear the enginge above the general taffic noise.


PS. how do I know this you ask. This is my job!


Say, isnt that engine going around with the wheel? Doesnt that cause fluid buildup in the head? If it is a typical piston port 2stroke, there's no way out for oil buildup, that cant be good.

Here on the left coast, we've seen tons of little 50cc 'gopeds' causing traffic hazards and noise pollution. This is not an elegant solution, but a marketing niche. Lazy americans who dont want to peddle. Where are the micro-hybrid moped/scooters? Is direct injection diesel the only future for motorcycles?

Innovation is what we need, not resurected design.

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