Super-Efficient Free-Piston Engines Could Replace Traditional Gas and Diesel Engines
by Michael Graham Richard, Gatineau, Canada on 10. 2.08
Free-Piston Engines: The Future?
While we wait for battery and hypercapacitor breakthroughs to allow us to go 100% electric and ditch internal combustion engines, series plug-in hybrids (electric motors turn the wheels and a gas generator kicks in when the batteries are empty) are our best bet to dramatically reduce transportation-related oil consumption and CO2 emissions.
The first few series plug-in hybrids (like the GM Volt and Fisker Karma) will use regular 4-cylinder gas engines as generators, but free-piston engines could potentially change all that somday: 1) They have very low friction, 2) only one moving part, 3) are about 50% efficient (about TWICE as good as gas engine and better than diesel), 4) and they generate electricity directly. Read on for more details.
How a Free-Piston Engine Works?
Invented in the 1920s, free-piston engines are finally starting to look like they could hit the big time. "Having a cheap and efficient way to generate electricity is becoming more important as automakers develop electric vehicles with onboard generators for recharging the battery pack and extending range."
Indeed, in a free-piston engine there is no mechanical connection between the piston and a crankshaft. The pistons aren't connected to anything! "Instead, [in the model looked at by researchers] two opposing pistons just shuttle back and forth inside a chamber. To generate electricity, the pistons could be equipped with rows of magnets that shuttle past metal coils to create an electrical current." This design also makes it easier to achieve homogeneous charge compression ignition (HCCI), which further increases efficient. We also assume that the whole package would be lighter than a traditional engine, helping extend all-electric range on plug-in hybrid cars. Very promising design.
Free-Piston Engines are Efficient and Flexible
Technology Review writes:
these engines could be 50 percent efficient at generating electricity--close to the efficiency of hydrogen fuel cells and much more efficient than conventional generators. Free-piston engines are efficient in part because they have fewer moving parts than conventional engines do. The engine configuration also makes it practical to tune the engine so that the fuel in a combustion chamber burns very quickly. Faster combustion allows the engine to get more work out of a given amount of fuel, improving efficiency. It can also improve emissions.
Indeed. the flexibility mean that a free-piston engine could optimize itself on the fly to burn different fuels: Gasoline, diesel, natural gas, hydrogen. It could be the ultimate flex-fuel engine!
The Future
These won't be on the market overnight, but many car makers are doing R&D and trying to figure out how to overcome the downsides of free-piston engines (how to muffle the sound, how to better control burning cycles, etc). We'll have to wait and see.
Via Technology Review, Next Big Future
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This could be used in generators too, not only in car. Pretty cool!
@Mike Nice article
*****
While I like the concept and hope it bears fruit please understand the are some significant concerns as well
And yes I bold highlighted some of the above text
Imagine a plug-in hybrid made of carbon fiber, with an aerodynamic shape. 80 miles all-electric range, and a small free-piston engine generator as backup. You can run it on diesel, or biodiesel, or natural gas (captured methane from landfills?). Would be pretty cool. Charge the batteries with solar or wind, and you've got a winner.
>This could be used in generators too,
What if the CAR IS THE GENERATOR ? Plug in the car to power the home.
>This could be used in generators too,
What if the CAR IS THE GENERATOR ? Plug in the car to power the home.
LONG way off. A really long way. And their claims of 50% efficiency are hugely suspect, not only are you going to have burn efficiency losses, you'll have generational losses. I seriously doubt the entire system would hit 50%, and you'll still have to drive an electric motor to get the power to the wheels, which is an additional loss.
Also, this looks like a two stroke design, which means it will need forced induction of some kind plus direct injection to be anywhere near clean. Since there's no output shaft you've either got electrical-motor supercharging (and a loss of efficiency) or turbocharging (which would make this engine difficult to operate at low speeds). And you can forget about running diesel and gasoline in the same engine since with a fixed piston on the other side you can't really do variable compression.
by the time they figure this thing out, LiOn batteries will be six cents per KHW.
It's still reciprocating, so the whole mass has to come to a full stop and start in the other direction twice per cycle. Induction works best in a loop or circle if you're looking for conservation of momentum.
The intake and exhaust have valves, so there's more than one moving part.
I'd like to see somebody try to doctor this deal onto the crank of a Wankel. No opposing forces, ideal for high RPM, great HP/Weight ratio.
You'd probably need more than one set of these firing out of sequence or you'd have a horribly shaky engine, 4 would be smooth and you'd get a nice straight 8 sound!
Starting would be an issue too, although you'd probably reverse the procedure and use the electricity to magnetically start it.
On a side note, two of those coils in the diagram are essentially wasted. Notice how no matter where you move the piston two of those coils will have a magnet in them? Well, the coils only produce voltage/current when the amount of magnetic field going through is changing. Those two center coils aren't going to see much, if any, change in the magnetic field that goes through them.
That doesn't even get into the more serious problems with the design pointed out above.
Seems like if you had pneumatically or electrically operated valves, you could make it a four stroke for less pollution (I suppose you could have an electrically operated camshaft as well)
One of the major advantages of a free piston engine is its inherent variable compression ratio design and therefore its inherent ability to run HCCI. Simply put a charge into the cylinder and assume that it will detonate at say X CR. If you are wrong, the piston will keep moving and eventually the CR will be high enough to detonate the charge at which point the piston reverses motion and starts the cycle over on the other side.
The downside to free piston engines is extracting power. The built in alternator approach is flawed in that the reciprocating mass is enormous and that magnets like neither shock nor heat.
The most obvious from of energy extraction is hydraulic but then you have to deal with hydraulic losses.
Has some kinks and flaws like people have pointed out, but the idea is good. I have a problem with the quality of produced electricity. A rotating generator produces a nice even oscillation and is easily conditioned, but the conditioning can become difficult with this design. But maybe at constant speed and with proper magnet layout it can produce good characteristics as well.
The design looks 2-stroke. I think they can make it run cleaner, if they design it so that no fuel mixture from intake is channeled directly to exhaust but lubrivation is another issue. Is it going to be a fuel additive?
GreenPlease, I dont think the mass of the magnets will be an issu with high grade Neodynium magnets, but they are temperature sensitive. Then again, this design allows the generation to take place further away from the combustion chambers than sketch would illustrate and it can be kept cool enough.
Looks to me like a 2-stroke engine. The only thing I remember about 2-stroke engines is that their fuel efficiency is crap. So how did they solve this????
to the 2 Stroke engine comment-> your right this is a 2 Stroke but they are using a fuel injector to solve the fuel efficiency problem, BUT in a 2 stroke you need to get oil to the piston rings and 2 stroke injected engines dont last very long (maybe you will get 75000 out of it) because the oil is not delivered in the fuel. I have been working on this problem for a long time. It looks good but they are overlooking this issue and others
1)how do you deliver oil to piston rings (maybe you can use rings and materials that dont need it)
2)This would run Very rough if you dont have some sort of flywheel to even the stroke -its compressing one side when the other is combusting (you need more than one moving part)
3)there still is a total loss of heat - it looks aircooled (the technology in Bruce Crower’s Six-Stroke Engine could be used to capture this energy)
Good luck to them but you dont want a engine that only captures 50% if there could be technology that could get 75% or better. The ideas are out there and just need to be put together!
Thanks for your time
Mike
Is it just me, or should we not be calling this engine "piston free" when it has two pistons in it? Crankshaft free would be more accurate.
Since this converts electricity without losing efficiency translating linear mechanical motion in to rotational motion via the crankshaft, I wonder if a wankel engine (which generates rotational motion directly) could be made just as efficient? Any engineers out there know the answer?
@Rasmus
That would be pretty inefficient. Large scale combined-cycle natural gas turbines are 89% efficient. Electrical losses over transmission lines average 7%, so that's 82% efficiency to your house. Assuming all the kinks are worked out of this, you'd still only be at 50% efficiency with your suggestion.
Also, unless you're off the grid, you don't want to be burning fuel to produce your electricity when there are so many other ways to make electricity.
@ KinofCain
I agree it's good to be skeptical. However, current ICEs are 25% to 30% efficient with all the extra losses to friction, gearing, and motion conversion (linear to rotational) that this design doesn't have, so just removing those is going to get you pretty close. And the engine doesn't need to run at a range of RPM; it can be designed and tuned to run at the most constant efficient speed since it will be recharging batteries, not providing the mechanical energy to move the wheels. Electric motors are about 90% efficient, so the losses are minimal. Batteries have only doubled in efficiency in the last 30 years or so, so don't expect huge gains on that front. And lithium is mostly imported, so we'd be trading one foreign dependence (oil) for another.
"...to burn different fuels: Gasoline, diesel, natural gas, hydrogen."
Don't forget ethanol, millions use it in Brazil as principal fuel
@JSDryer
The correct name of this style if engine is free-piston not piston free, and the use of the word free is, as you surmised, to highlight that the piston(s) are not connected directly to a mechanical crankshaft used by the piston driven engines we normally see in automobiles.
If they wanted to have a longer more descriptive name it would be crankshaft-free-piston-engine.
Thanks, TP. Interesting the way my mind transposed that, even though I read the phrases several times thoughout the article.
So, if it has trouble varying speed like a standard engine, why not hook it up to a series hybrid and have it run at the most efficient setting as needed?
There is a good reason most of these early industrial engine designs never took off. Not all good ideas can jump through the technical hoops that physics presents us with. The devil is in the details.
That being said, it's always fun to ponder their potential and use what is right about their design in different ways. Hell, Henry Ford came up with the modern assembly line after being inspired by the meat packing industry.
According to the following:
"many car makers are doing R&D.....We'll have to wait and see."
It will never happen!!!
...unless, someone else does it other than the car industry!!!
@Brian
Because of the efficiency they used to use these engines for compressors and generators ... there are other industries that need engines like these.
I know how to solve the 2 stroke, control, and vibration issues:
Take the pistons and attach them to a central shaft through linkages. Attach that shaft to a flywheel. Now a 4 stroke is possible because the pistons have momentum to create the compression stroke. Additionally, the geometry of the linkage defines how far the piston is allowed to move, preventing crashes into the valve.
Next, add a second set of pistons to cancel vibration. Run it off of the same central shaft to control timing.
...And now you've got a conventional flat 4 piston engine and a bunch of reasons why nobody makes free-piston engines.
@ Peter,
Doesn't that sort of cancel all of the efficiency gains?
first glance it seems workable
piston one fires
if for some reason there is no air / fuel charge in piston 2 what prevents a hard strike into the head assembly?
instant damage
not a workable concept on a number of levels
@Steve
There have been many working versions of free-piston engines in use in real world situations and not just in labs. As I mentioned above they used to be used for compressors and generators because of their efficiency.
So it's not a concept -- it's a reworking of a proven system.
The drawing above is probably not a CAD image of the entire assembly -- ie it might just be missing some very relevant details to address your concern
The last couple of comments are correct, the concept of the free-piston engine is not new. The technology was developed and utilized heavily in the early oil industry to run sucker rods off of a large fly wheel. These engines burned the natural gas that seeped from the nearby oil wells. To see this early technology check out any number of the collector club web sites devoted to early steam and gas powered engines. You can also contact the Drake Well Museum to get more information.
As a means of loading the free piston engine just hook them to a" cam to crank" crank shaft and use an alternater or generator as a flyweel. this would allow the motor to generate ADDITIONAL power, while connecting the sets of free pistons to a load of sorts for stability.
free piston engins or sometimes refered to as linear motors are two cycle. they would be more efficient but would be harder to meet future & more stringent emvionmental & emission codes.
their compact design & light weight make them ideal for small emergency generation at camp,work & home sites.