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... a free-piston 100kW multifuel generator that weighs 100kg, will fit under the hood of a subcompact car¹, has no rotating parts, is 50% efficient overall², and will run on anything from biodiesel to hydrogen. At 25kW per 125cc module, this should eat conventional small generator manufacturers for lunch.
[1] The four-module 100kW unit is 11 inches square by 26 inches long. You could put a single 25kW module between the rafters in your garage, or under the kitchen sink, and in terms of space usage you'd scarcely notice it was there.
[2] Pempek claims electrical efficiencies as high as 95%.
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How does that compare to the electricity generated by any of the current hybrids out there? (esp. the prius)
How would it fair as an engine replacement for some/various current sub-compacts (recognizing you also have to put more batteries and an electric motor into it)?
How does it compare to the various electric cars out there (the Zenn, for example)?
How many of them could I put into the engine compartment of my Saturn L series? ;-)
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I've heard discussion of sprung vs unsprung weight, but I've never understood what exactly the deal is. What makes it bad to have that much extra weight in the wheel instead of in the car body?
(and, why not put those motors into the engine compartment, and have them drive the trans-axle or transmission, instead of putting them at the wheel?
other than the whole question about "how much room is in your engine compartment"? :-} )
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The difference between sprung and unsprung weight is quite simple: it's which side of the suspension it's on. Sprung weight is just ... onboard weight. Its motions are damped, and only indirectly affected by road inputs. Unsprung weight has to move with every imperfection in the road surface. The more weight there is, the stiffer the suspension has to be to control it, the less compliant the suspension can be, and the more energy is transferred through the suspension to the body. The lighter the wheel, the more perfectly it can follow the road surface, and the less it transmits impact forces to the car's structure when you go over a bump. A heavier wheel acquires more momentum from a bump, and it takes a more powerful spring to stop the wheel's upward motion, and a stiffer shock to absorb the energy, so the suspension does a poorer job of following the road surface, resulting on poorer road-holding and a harsher ride. The ideal wheel and tire would be almost weightless, which would allow it to be mounted on an extremely compliant suspension that could follow the road surface perfectly while transferring very little energy to the vehicle's structure.
The reason for using direct wheel motors is basically efficiency. There are two reasons why we need a driveline and transmission on vehicles powered by reciprocating engines — first, a reciprocating engine's power output curve is peaky, and it only makes useful power within a relatively narrow range which does not start at 0. So, we need multiple sets of gears to keep the engine operating within a speed range within which it produces useful power and torque, and we need a clutch or torque converter to allow the engine to keep running while the vehicle is stationary. We can't just leave the engine connected to the output shaft all the time and start it when we want to move, because it needs to be spinning at a certain minimum speed to start at all. If the engine is directly connected to the wheels, the starter motor needs to be able to move the entire vehicle to get the engine started.
Secondly, it's not practical to put an entire internal combustion engine with a clutch and gearbox at each wheel. The problem with this is the total power losses through the transmission and driveline may be as high as 30%.
With an electric motor, we don't need a transmission because the engine produces useful torque and power all the way from zero. (In fact, electric motors can produce huge amounts of torque even from a standing start.) Since we don't need a transmission and the only connection is electrical, it's practical to eliminate the entire driveline and all of its transmission losses, which is like getting free power, plus it's mechanically simpler to boot.
The only problem is unsprung weight, and that's one of the reasons room-temperature superconductors are the holy grail for electric vehicles — a superconducting electric motor would not only be extremely efficient, but it could be extremely small and light for its power output.
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The other big win of direct wheel motors is you don't need an axle at all. With the motor built directly into the hub, there can be almost no intrusion of suspension/driveline parts into the interior volume of the vehicle. But to make it really practical to do, you really need superconducting motors.
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since the motors are so small, they'd fit easily under the floor boards!
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Otherwise, looks sound in principle. A lot of energy is wasted as simple, throw away friction when a car's brakes are applied.
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See the Napier Deltic opposed-piston diesel engine (http://en.wikipedia.org/wiki/Napier_Deltic) for an earlier use of opposed pistons to get very high specific power output in a diesel engine, though Pempek has taken the idea much further.
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Vaporware pimpage is running amok wrt uber effiecient people haulers.
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so what stage are they in? have they tested a prototype at least?
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