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GM and U Mich receive patent on plug-in series hybrid/extended range electric vehicle powertrain using multiple free piston linear alternator engines

The GM/U Mich patent envisages using multiple free-piston engines (240A through 240D in the patent drawing). Click to enlarge.

GM Global Technology Operations LLC and the Regents of the University of Michigan recently were awarded a US patent (Nº 8,261,860) for a plug-in series hybrid or range-extended electric vehicle powertrain using multiple free piston linear alternator (FPLA) engines.

The powertrain system—the patent application for which was filed in 2009—includes an electrical energy storage system—such as a Li-ion battery pack—and an electric traction motor. The covered hybrid powertrain uses the multiple reciprocating free-piston linear alternator engines to generate electric power for the battery pack or the traction motor.

The FPLA units are connected to a power electronics module, which in turn in connected to the electrical energy storage system and the electric motor. A control module determines mechanical and electrical power demands in respond to driver input and a state of charge of the electrical energy storage device. The control module commands operations of the reciprocating free-piston internal combustion engines to generate electric power responsive to the electrical power demands.

Patent Drawing of an FPLA. Click to enlarge.

Each FPLA unit includes includes a reciprocating free-piston internal combustion engine having a single cylinder with a double-ended free piston, which forms two variable volume combustion chambers. Each cylinder has a linear electrical alternator, including multiple electrical coils and integrated iron stators, and the double-ended free piston includes multiple permanent magnets.

When the free piston oscillates in the cylinder during operation, electrical energy is generated by the action of the permanent magnets forming and breaking magnetic fields with the iron stators and electrical coils.

Fuels can include gasoline blends, and alcohol blended hydrocarbon fuel grades. Ethanol blended hydrocarbon fuels can include higher ethanol blends (e.g. E80, E85), and neat ethanol (E99). Alternative liquid and gaseous fuels such as neat methanol (M100), natural gas, hydrogen, biogas, various reformates, syngases, and others may also be used.

Each FPLA operates in an engine-on (ON) state and an engine-off (OFF) state. Each FPLA has at least one resonant operating speed corresponding to mass of the piston and length of the cylinder at which it generates a preferred output power—i.e., a maximum output power or an output power achieved at a maximum efficiency point. The preferred output power is referred to as a rated output power at the resonant operating speed.

The use of the FPLAs offers flexibility in terms of power generation. In one embodiment described in the patent, each FPLA operates at a resonant speed and at a fixed output power, e.g., a rated output power of 30 kW. Alternatively, each of the FPLAs has the same rated output power—e.g.30 kW—and operates at one of multiple output powers, e.g., 10 kW, 20 kW, and 30 kW preferably at the resonant speed, the output power controlled by controlling mass of fuel injected each combustion cycle.

In one embodiment, the FPLAs have different rated output powers, and generate different, preferably progressive amounts of output power in the engine-on state.

The FPLAs can be configured to generate different rated output powers by varying engine design factors including bore and stroke of the cylinder, size and mass of the piston, and other factors. Thus, in one embodiment, the first FPLA unit can have a rated output power of 10 kW, the second FPLA unit can have a rated output power of 20 kW, the third FPLA unit can have a rated output power of 30 kW, and the fourth FPLA unit can have a rated output power of 40 kW for a total rated output power of 100 kW of power for the combined operation of the FPLAs. (Numerical values are illustrative and not restrictive.)




PHEVs with ICE driven generator together with storage battery and control units have been on the market for many years and should not be the subject of further patent rights to limit market applications. An old GM trick?


IMO should consider this drivertrain.


I wonder how they keep the pistons cool, PM dont like a lot of heat or shock.


Try sizes of 7, 14, 28 and 56 kW apiece, yielding 7-105 kW in increments of 7 kW.  That's more than enough for a mid-size passenger car towing a trailer, even up a mountain.

Speaking of varying power, the equation for a simple harmonic oscillator is ω=√(k/m) and the spring constant k of an air spring is proportional to the quantity of air.  Supercharging or turbocharging would both increase the energy per power stroke as well as the engine's natural frequency, so power would go up by roughly the 3/2 power of the intake air pressure.  That's about a factor of 2.8 for a boost pressure of 1 bar, or 40 kW out of the base 14 kW engine.  Power could be varied widely by altering the boost pressure, and excess exhaust energy could be recovered by an alternator on the turbocharger shaft.

Then there's the packaging freedom inherent in the all-electric transmission.  We haven't seen too much of what designers can do when they don't have to deal with driveshafts, but I'd love to find out.


Im interrested to buy if the overalll mpg increase and if the price of the entire vehicle is not higher. Gm seam to be the leader in green technology with the volt, then this new technology. This will be maybe cheaper then what it replace as they said that the pistons are free.


Herm has a point, "PM dont like a lot of heat or shock."

Power is harvested through inducted electricity from linearly free moving PM piston, rather than connecting rod and crankshaft rotation to generator.

Again, a corporation that crushed advanced trolleys, EVs, batteries,.. is controlling new transportation technology advances.

There is no mention of free piston linear alternator engine potential gains or prototype/implementation/marketing dates.

Another taxpayer $40B for new Asian auto plants may be needed.


The possibility on this type of ICE generator-alternator to be mass produced and come to the market place in lower cost PHEVs is very minute. High performance, light weight, lower cost, 120+ kWh batteries may be around years before.

This is another PR move by GM to convince the lower and middle classes, in USA and Canada, that the $40+B bailout was justified. More will come on a regular basis?


Free piston linear alternators may not be a great enabler for PHEVs, considering the great economic barrier of entry of many $100Ms for a brand new kind of combustion engine and electric machine. It is more likly that a conventional crank with conventional rotating generator will do the the PHEV job almost as well, with the benefit of shared tooling with more conventional cars.

But GM is leading in this space and this invention, and their other leading production work, like the Volt, as AD points out, is the evidence of the reality.

GM haters can turn the reality of GM's progress and breakthroughs in green technology into their paranoid world world as evidence of the imagined evil. A large US company succeeding in green transportation doesn't fit their world view as the "US = bad and China = good" or support the "GM is the booegeyman" hypotheses.


It strikes me that this enables you to run your generator at peak efficiency over a much wider range of powers, by completely disabling the units you don't need - which is good.
On the other hand, you now end up with 3-4 ICE engines, where you used to have only 1, adding complexity.

Also, the 4 cylinders will all be different sizes (for different powers), so it is not as if you have 4 engines of the same size, you have 4 engines of different sizes.
Hence complexity and cost of design and manufacture.

Still it is very interesting and I hope they build some prototypes.

I wonder what the engine(s) would sound like with 0-4 engines running at any given time.


Keep it simple, twin opposed free piston generators to cancel out vibration and exhaust noise.. the piston position can be precisely controlled by the magnets. Hopefully they can build these things without using PM.


"GM haters can turn the reality of GM's progress and breakthroughs in green technology into their paranoid world world as evidence of the imagined evil."

frankbank, there is nothing imagined about GM's conviction:

or http://www.cbsnews.com/8301-505123_162-57551668/gm-opens-new-plant-for-chinese-budget-brand-baojun/
http://www.4-traders.com/GENERAL-MOTORS-COMPANY-6873535/news/GM-Confirms-Plans-For-$1-Billion-Russia-Investment-By-2018-14477750/ ..

Many respect what US Ford has done with marketed, high mpg power trains and models, esp. EcoBoost and the C-Max, while GM has it's track record.

Since overpaid DC idiots decided "Corporations are people too"('Citizens United'), GM is an ex-con.

With GM now putting taxpayer $billions into Asia, some question the 'ex'.

Bob Wallace

kelly - it's quite likely that if we had allowed GM and Chrysler to go down then Ford would have followed. Ford has acknowledged that.

A collapse of Ford would mean no "EcoBoost and the C-Max".

All car companies are beginning to do regional manufacturing. Making cars in China to be sold in China is only good business. Good GM business increases the value of the GM stock that we (taxpayers) own. When we sell we may even see a profit on our Detroit bailout.

Already, when one considers what we would have spent in unemployment and welfare costs along with tax revenues received, we're greatly ahead. The GM/Chrysler was a great investment of tax dollars. So far it's cost us about $10k per job saved. We would have spent more than $10k per unemployed person the first year. And all those still-working people must have paid more than $10k into federal, state and local taxes.

Try stepping away from the Fox 'Hate Machine'. It's a brain-rotter.


Try stepping away from the myth that normal bankruptcy, without giving the GM assets to the unions and giving the new GM much favored treatment, would have ended GM.


I am not sure running in the "sweet spot" is worth multiple cylinders, each with a different power output (why not run at or near the single sweet spot and let the batteries absorb and provide, in turn, the difference, else eliminate the batteries).

Nor am I sure that an engine's sweet spot must be so narrow.

Nor am I sure that such a simple combination of well known technology should be patentable.

Certainly not if this is justified by more of "this will make GM more profitable and we own GM, so it is OK".


Fisker, and even GM, eventually figured out that a generator-to-battery-to-motor configuration does not result in peak MPG.

Varying the configuration of the internal combustion engine matters not. Directly driving the wheels like the Prius is the efficient way to go when burning fuels.

The motors can always be driven (without the engine) by batteries whenever necessary, when battery capacity allows.


Bob W, I don't watch Fox, It's a brain-rotter.

But GM has played 'too big to fail' and killed EV tech decades before stealing $40B from taxpayers, plus $10B for Delphi and 30 fold return to the 'Romneys'.

Like the ex-GM/Toyota Numi car plant now turning out Tesla Model S's, the MT Car of the Year, anyone better than the GM corporate gas-guzzler oil culture would have begun.

As America's 'Big One', Ford would have further thrived - perhaps even beyond the "EcoBoost and the C-Max".

GM failed and wouldn't be if taxpayers had honestly been told their new $billions were going to build new Asian auto plant after auto plant.


This is a very good development in the area of energy conversion. Using permanent magnet in high temperature environment is a concern but there may be other ways to overcome this limitation. Using opposing pistons will cancel or reduce vibration related issues.
The main benefit I see with this engine is the ability to control the power flow precisely by electronics. The mechanical energy in the piston needs to be transferred synchronously by tracking the induced emf in the stator coils. Power electronics and control method are very well established for this( linear induction motor controls, magnetic levitation controls, etc) though there may be some critical issues related to tracking continuously varying emf. Overall, it will eliminate many mechanical connections that are inherent in a rotary engine and alternator solution and that by itself should help improve the efficiency of energy transfer. It will be interesting to see when they build some prototypes of this concept.


"Fisker, and even GM, eventually figured out that a generator-to-battery-to-motor configuration does not result in peak MPG. ... Directly driving the wheels like the Prius is the efficient way to go when burning fuels."

This is not true for GM, the Volt can operate in a serial hybrid or parallel hybrid mode.. at lower speeds serial mode is more efficient than parallel.


This conjecture is all absent data. We can't know if free piston gen sets are better or worse well-to-wheels unless and until we know how efficient they are at converting BTUs of fuel into electric power. Yes there are conversion losses, potentially several if the energy is stored in a battery or capacitor en route to the motor. However, if the Free Piston ever achieves greater than 50% thermal efficiency (which was the goal set out by Sandia when they announced their partnership with GM and U Mich. http://www1.eere.energy.gov/vehiclesandfuels/pdfs/merit_review_2011/adv_combustion/ace008_vanblarigan_2011_o.pdf)...then maybe the well-to-wheels will be superior...especially if the get good at producing almost the exact amount of power needed for the motor, with minimal storage conversion losses.

By the way, I don't see how GM gets this patent when there appears to be prior art 14 years earleir at http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA294318.

Roger Pham

Look at the Prius' 48-mpg hwy rating vs. the Volt 40-mpg hwy rating. And then, look at the Prius' 50-mpg combined rating, vs. the Volt's 37-mpg combined gasoline mpg rating. It seems like directly driving the wheel is the most efficient way, because the losses in the generator and motor and power converters are eliminated!


There is no efficiency advantage of free piston vs. crank-slider piston. Losses in heat transfer and in the exhaust would still be quite comparable, and free piston still needs to transfer energy to the wheel drive shaft somehow.

Patrick Free

The perspective is missing behind this interesting article : We're headding to PHEVs optimized arround much larger battery packs, like Tesla Model S & X top bin at 85KWH. Not only these batteries provide much more capacity, but also much more instant power, allowing to move to a drive train exclusively tracted by electric engines, even at maxi power demand points at high speed on mortorways and on mountain roads, the ICE Range Extender being only used as "accasional power Generator to extend the EV car range and eventully also recharge the battery pack once exhausted. These Rage extenders can work at constant speeds, but must me small, efficiant, and reliable when only used a few times per year for longer vacations trips for ex. They will be used more and more rarely and need be reliable used that way and still efficiant. Is that techno good for this and to what extend ?

Patrick Free

GM has buit the Volt as the best possible "real car replacement" PHEV optimized arround an 18KWH battery pack. And they did it perfectly. Question is whom will do the best PHEV optimized arround next Gen Tesla like 85KWH battery pack and beyond. Tesla was 1st to market with that class of "large-like-my-dreams" battery, and the pure electric drive train that can exploit it as a pure EV. But they are missing a Range extender required to make a real car replacement from that drive train. They made a far better pure EV 2nd car, with a much larger range, but still only a second car, you can't take for your summer vacations one time per year, were you need to make >800 Miles per day, in areas where no fast charging will be available before 20 years from now.

Patrick Free

I understand the never-tracting and more and more rarely used range extender ICE Generator engines required for next gen PHEV, could be a different techno than current Gazoline and Diesel Engines established as kings of tracting ICE engines today. So far my bet was this would grant a second life to Micro Gaz Turbines or Rotative engines, but I'm missing objective datas comparing their reliability for few times per year usage, as well as their efficiency to generate the Electric power required for the battery, compared to the current std ICE engines and the alternative one covered here. Basically nirvana for me, if we assume 500Miles can be completed with 70 litters of Diesel, or 130KWH of EV battery, would be an engine consuming less than 70L of Diesel to rechange 130KWH, or <46L for 85KWH... Is that possible in a reliable way for rare usages each year ? (With 85KWH giving >300M range, I could leave with one full charge per week for my wekly local commutes during the year, plus an extra charge for most week ends outside home, and only a few litters of Petrol for very few long vacation trips 2 or 3 times per year maximum in my case...)


Prius is 3000 pounds the Volt is 3800 pounds. It is not comparing the same car with series versus parallel.

Liviu Giurca

A new solution introduced by the site www.hybrid-engine-hope.com describes an alternative range extender configuration. This concept can be used for hybrid vehicles or for electric vehicles with extended range.

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