Free piston engines could dramatically improve efficiency of ER-EVs
Filed under: Emerging Technologies, EV/Plug-in, MPG
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Over the last century and a half engineers and designers have created a wide array of different combustion engine designs. In spite of certain inherent advantages of some of these designs, the traditional four stroke reciprocating engine has continued to dominate the transportation space. Many of these alternatives have worked in principle but have never been adopted for several reasons. The most important reason is typically the problem of converting the chemical energy of combustion into rotating drive torque to turn the wheels. This has been the case with so-called free piston engines. In its most basic form, the piston in a free piston engine simply oscillates back and forth driven by the alternating combustion energy at either end of a cylinder. Such a system has very low friction compared to a traditional engine.
Conventional engines used today are typically about 25 percent efficient. That is, one quarter of the energy in the fuel they consume is available to drive the vehicle. The actual combustion process is actually extremely efficient converting almost all of the available chemical energy in the fuel into heat energy. The problem is that about one third of that energy goes out the exhaust pipe in hot exhaust gases while another third goes into the engine coolant and oil too keep the temperature of the internal components of the engine below their melting point. Read on after the jump to learn how free piston engines can overcome some of this.
[Source: TreeHugger]
A significant chunk of energy is lost to friction in all the moving and rotating parts such as the camshafts and crankshaft. The absence of these parts and lateral forces inherent in a conventional engine means dramatically less friction as well lower weight. The problem of converting the oscillating mechanical force of a free piston engine remains with a conventional car. However the power of a free piston engine could be harnessed as part of an hybrid electric drive system.
Magnets can be mounted on the piston rod inside of the cylinder with electric coils around the outside of the piston. As the pistons and rod move back and forth the magnets induce a current in the coils. This current can then be used to charge a battery in a series hybrid or ER-EV drive system. Such a system could be up to fifty percent efficient in transforming the energy in the fuel into electrical energy. A free piston engine like this could even be used with an HCCI combustion process that could improve the overall efficiency. This compact design could potentially be the best solution to a problem of a range extender for electric vehicles.
Reader Comments (Page 1 of 2)
11-05-2008 @ 8:29PM
GSP said...
If this is a good idea for EREVs, it also is a good idea for gen sets. I wonder why all gen sets sold, from Honda portable units to 5 MW industrial monsters, all use a cranshaft to run a conventional generator? Since the free piston engine is not a new idea, there probably is a good reason.
Also, I don't see why the free piston engine is more ameanable to HCCI than a conventional one, is there some info on this?
GSP
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11-05-2008 @ 11:39PM
tankd0g said...
I can tell you why these arne't used in portable generators. If the engine worked like this colorful diagram it would be great. However, it's missing a few essentials, like an oil pump, oil, a way to manage that oil, a cooling system, a way to time it etc. etc. somehow I don't think this would be as cheap to build as a 5000W generator with a conventional motor and stator setup.
11-07-2008 @ 10:19PM
Chris M said...
While a free piston engine is mechanically simpler, it requires much more sophisticated controls. The linear generator must convert just the right amount of momentum into electrical energy - too much and the engine stalls, too little and the compression gets too high, causing knocking and reducing efficiency and possibly damaging the engine. The linear generator must also act as the starter motor.
The advantage for an HCCI engine is that type of ignition requires precise control of compression ratios that must change when environmental conditions change. Since a free piston engine can alter the compression ratio "on the fly", it may be the best hope for achieving a practical HCCI engine.
11-05-2008 @ 8:43PM
EVan said...
@GSP
Good point about gen sets.
I think the answer has to do with the fact that nearly all internal combustion generators are made by companies that already build piston engines. Honda, Briggs and Stratton, Caterpillar (commercial/industrial) and Cummins (also commercial/industrial).
I don't think the market for small gen sets has been large enough in the past for companies to put that much R&D into dedicated engine development. Hopefully with the advent of ER-EV's that will change.
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11-05-2008 @ 11:01PM
tankd0g said...
This is a sterling engine is it not? One step down from the perpetual motion machine. Won't be holding my breath on this one.
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11-05-2008 @ 11:22PM
Mike!!ekiM said...
Why is it perpetual motion?
Each piston head gets a burst of gas, at the right time, to reverse piston travel. The advantage seems to be it doesn't need to turn a crankshaft and a flywheel.
11-05-2008 @ 11:26PM
John Rowell said...
No, it's not a sterling engine, it is just a very simple internal combustion engine that generates back and forth motion instead of rotational motion. As such, it cannot be used directly for power without first converting the back and forth motion into alternating (AC) electrical current. And GSP makes a good point: given its simplicity and efficiency, it's hard to see why genset makers haven't been taking advantage of this design.
BTW, a sterling engine isn't a perpetual motion machine, they're currently used in concentrating solar power applications and some industrial processes.
11-05-2008 @ 11:34PM
tankd0g said...
I didn't say it was perpetual motion. Christ...
11-06-2008 @ 12:16AM
Zot said...
I wonder if you could have one cumbustion chamber with two ( or more ) pistons getting blasted away, then using springs to push them back to the middle.
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11-07-2008 @ 9:54PM
Chris M said...
Some free piston engine designs use springs to keep the piston from "bottoming out" if the compression fails, or to prevent the compression from getting too high at either end.
11-06-2008 @ 12:34AM
Flahooler said...
Please don't go painting the "pie-in-the-sky" picture of free-piston engines without at least making some reference to the downside. Yes, there are some very good reasons why these are not in widespread use today.
For example, in a traditional engine, the piston's movement is constrained by the rods and the crankshaft. This creates additional friction and energy loss, but it also helps to even out small variations from cycle to cycle (fuel injection systems aren't perfect). In contrast, a free-piston engine has no such control mechanism. Cycle-to-cycle variations will result in incorrect ignition timing and reduced performance.
Solving this problem requires an active control system to constrain the piston's movement. This could be done through a sophisticated monitoring and control system (utilizing the coils and magnets), but with increasing complexity you also get increasing cost and reduced reliability.
I'm certainly not saying these engines don't have a future....engineers can certainly solve the problems with time and effort. However, this is a classic example of how the marketing hype doesn't quite live up to the engineering reality.
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11-06-2008 @ 3:30AM
Savitaipale said...
Another minor problem that failes to be mentioned, is that with only two cylinders, and no possibility for flywheel, the motor has to be a two stroke. Yet more problems thinking of fuel consumption and emissions.
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11-06-2008 @ 8:07AM
Benton said...
@ Savitaipale,
I think a 4 stroke could be possible if this engine had 4 cylinders and an H shaped connecting rod that joins them all together. As one cylinder fires, its adjacent cylinder would be on intake, while its opposing cylinders are on compression and exhaust. Obviously the valves would have to be solenoid operated.
What do you think?
11-06-2008 @ 10:07AM
GoodCheer said...
Benton: That's just about exactly what I was thinking, except I was thinking of 8 combustion chambers. With 4 chambers (two "pistons") the connector between the pistons would be shearing with each stroke, and the piston assembly would be subject to torque. The reaction to that torque would be bearing pressure on the side of the cylinder wall, increasing friction and causing wear.
With 4 pistons all connected in the middle that torque could be eliminated by firing in two non-adjacent chambers on the same 'side' of the engine on each of the 4 power strokes.
Of course another possibility would simply be to have the pistons coupled electromagnetically. The current being driven by the motion of the firing piston could be used to drive the adjacent piston during its compression stroke. That way you can get back to 4 combustion chambers (2 pistons), and you can also reduce the vibration generated to near 0 since the pistons don't have to be going the same direction. Again, by going to 4 pistons (or I suppose 6, if the mass of the central piston was twice that of the two flanking pistons) you could have a PERFECTLY balanced, vibration-free engine.
There are a lot of cool things you could try.
11-06-2008 @ 4:51AM
kert said...
Its doable
http://www.lceproject.org/en/principle/functional-model.php
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11-06-2008 @ 7:15AM
Herm said...
It is doable, with modern electronics to control the position of the pistons, and modern fuel injectors to inject at the right time.. the other issue is the magnets, they usually dont like shock and hear, but I believe that problem has been solved recently with new magnet materials.
As shown it will be a vibrating machine..
You could also do a version with one of the pistons acting like a supercharger for the other piston.. the coils/magnets would do double duty as the starter.
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11-06-2008 @ 8:25AM
Kardax said...
Something everyone's overlooking so far is that, by necessity, a free-piston engine uses the "two-stroke" cycle. This design would inherit many of its disadvantages, like the fact that the intake and exhaust ports are open at the same time, causing some unburnt fuel to go out the tail pipe...
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11-06-2008 @ 8:46AM
Joe Bloe said...
Uhh, no. With direct injection and external scavenging, 2 strokes have no unburned fuel going out the exhaust and no oil in the fuel/air mix.
11-06-2008 @ 9:26AM
Freedom said...
Good idea, but way too many issues to work out. Why not use a more efficient and proven design from Rudolf Wankel, the rotary engine is perfectly suited for a generator.
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11-06-2008 @ 9:29AM
Freedom said...
A diesel rotary engine would be even better, these guys are really innovating http://www.regtech.com/Radmax_Technology/Rotary_Principle/
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