China Considering Policies to Boost Auto Sales; Focus on Alternative Energies

Dean Kamen Unveils Stirling-engine Extended Range Electric Car

UnionLeader. Inventor Dean Kamen has unveiled a prototype extended range electric vehicle, the Revolt, which uses one of DEKA Research’s Stirling engines as a range extender. (Earlier post.)

A DEKA Stirling engine. Click to enlarge.

Based on an Think City electric car, the 2008 DEKA Revolt two-seater offers about a 60-mile range on a single charge of its lithium-ion battery pack. The Stirling engine range extender is fitted into the truck compartment.

Kamen said he is in discussions with Think Global over the use of the Stirling technology, but hopes that they will be the first of many automakers to adopt the technology. Kamen hopes a production version could hit the market in two years.

Stirling engines are external combustion engines, using a heat source external to the cylinders for the conversion of heat energy to mechanical energy. The DEKA Stirling can use a range of fuels.


Healthy Breeze

...and we care why?

They don't list price of the vehicle. They don't list fuel economy when running on gasoline or diesel.

Flex fuel capability is less interesting than exquisite fuel efficiency on some common fuel.


Stirling engines are about as efficient as gasoline engines or possibly better. Due to the external and continous combustion they have quite low emissions (exhaust gas and noise).
Downsides: costs, weight, complex regulation.

Henry Gibson

Philips NV and a US carmaker were proposing a car that used a stirling engine and molten salts to power it. The sealed free piston units of INFINIA that are proposed for solar power might make a better choice for reliability. The advantages of stirling engines are very flexible fuel use, high efficiency and very low NOX. The disadvantages are weight and size.

No electric cars should be allowed on the roads but only plug in hybrids. The limited range of battery cars that is always mentioned has unnecessarily prevented their wider use. ..HG..


Wouldn't be surprised to see the Stirling applied to 3rd generation PHEV range extenders. The fuel flexibility is a primary reason, as we see greater interest in CNG and H2 as alternatives to straight liquid fuels.


As it stands now, the average internal combustion automobile engine only converts roughly 20% of its energy into useful motivational power. Most of the rest is expended through heat loss in various locations.

The Stirling engine does about 30%.

Dean ain't no dummy.


I am not convinced that the Stirling engine is the best choice for a PHEV, ok it is multifuels and very low emission even without after exhaust treatment but its 30% efficiency is not as good as diesel or Atkison, plus it has a fairly low power density compared to a ICE. Last but not least it requires helium (in short supply) or Hydrogen that needs to be replenished regularly since nobody knows how to make a moving piston that can prevent leak of H2 or He under 50 bars.


While dean might be a great engineer, he is lousy at marketing - the Segway being a case in point - he applied 21st century technology to solve a problem that was solved by 1890.
He seems to relish taking the hardest, flashiest approach possible.

Surely, for a PHEV, you want a very small, light engine as you won't have that much space left in the car after the battery.

Also, if the range extender is not to be used very often, it might not have to be optimally efficient, you could give a little on efficiency for cost, size and weight.

John Taylor

If recharging were made available at all major shopping plazas, then Battery Electric Cars would be viable and not need range extenders.

This takes political will, not new technology.

P Schager

The stirling and the PHEV make a first-rate combination. The stirling should be extremely quiet (just like the electric), low-maintenance, fuel flexible, and easy to make clean-burning. The efficiency will be in the ballpark of a diesel and the power density should be acceptable when you consider the aftertreatment hardware it doesn't need and the PHEV application. The stirling will probably be immune enough to getting dirtier with age that it will need few or no smog checks, probably qualifying as an ILEV (inherently low emissions vehicle). Or with a catalytic combustor emissions might be low enough to call into question the absoluteness of the ZEV category.

Some gasoline PHEV designs have reached for reasonable efficiency by running the engine flat-out, turning on automatically to recharge. Customer acceptance is likely to be complicated. (I can just see the silent car suddenly revving its engine up full blast as someone is passing in the crosswalk in front of it.) With stirlings there's no problem.

The fuel flexibility can open the door much wider to biofuel through a variety of biofuel options, removing for all of them the chicken-and-egg problem of establishing distribution and a sufficient density of both customers and stations. And help keep the prices stable and prevent anti-competitive games by having enough fuel-independent customers. Stirling doesn't care about octane or cetane or viscosity or a lot of other characteristics, and it isn't hard to make the fuel system handle corrosiveness or low-temperature phase issues; you could run on everything from vegetable shortening to viscose to sugar. The well-to-wheels economics could outweigh any engine efficiency advantages from diesels. In an emergency you could run on pine needles from the side of the road--take that anti-green SUV partisans, whose bragging rights have centered on purported go-anywhere capability! (As for Dean's Hummer, we're gonna have to let that slide for now...) It's a caveat that you could also run on a dirty fuel such as powdered coal, but I'm sure this can be controlled.

Possibly the efficiency could be improved with a turbogenerator combustor. Certainly it can be if the heat source is a solid oxide fuel cell's "waste" heat. Then you'd be looking at something like 60% overall efficiency. At larger scale you use turbines for that, but at car scale I think stirling is best. Here if the SOFC fails the stirling could fall back on a simple burner, so reliability requirements would be eased enough to help with the cost.

Stirlings for cars have been tried in the past and given up on, but the key objection has been the warm-up time. In a plug-in hybrid that has plenty of electric power, this ceases to matter. Thermocycle lifetime won't be challenging either. That's why this is so timely. Also the engine is polite enough that charging while parked could be an option that reduces the engine size need, satisfactorily for some.

Another site put the engine at 300 pounds and 15 kW; that will do for PHEV.

As an aside, I question whether DEKA should be contending for that Revolt name with another Norwegian company, ReVolt Technology, which is kinda in the same business (gunning for it).

But I hope DEKA can come out with this at reasonable cost in reasonable time; if so I'm sure it will be a winner. If they need any public support for this, it is a lot more deserving than 95% of what Detroit wants money for. Most all PHEV makers should be candidates to use it.

Alex Kovnat

The Stirling engine - hybrid electric concept was experimented with as early as ~1969, when GM came up with the "Stir-lec" experimental vehicle. Given the use of large low-speed Diesel engines on ships, it would be interesting to see if a giant Stirling engine would be more efficient in that application.

The big problem with the Stirling engine, is need for helium. Those among Green Car Congress participants who have access to Photonics Spectra will note, in their October 2008 edition, an article on helium which emphasizes the need to use alternatives whenever possible, and to recover and reuse helium as much as possible.

Les and Jane Oke

It's good to see so much new competition in the area of building electric cars.
Have you considered doing your own electric car conversions.
More info at

All the best,
Les and Jane

Roger Pham

Free piston engine genset with thermal efficiency above 50% should be much more efficient and compact than Stirling genset. Re-look at the Free Piston Floating 4-stroke engine in the following link for details:

This a completely mass-balanced (low-vibration) and 4-stroke engine (low-emission), in comparison to previous Free-piston engine proposals. Since it's has highly-variable compression ratio, it should be able to use about any kind of fuels fed into it, utilizing HCCI method of combustion for very low-emission without costly post-combustion clean up of exhaust.


Trehugger: There's no need to replenish the Helium gas this undoubtedly uses, typical Stirling engines run inside a pressure vessel, keeping the high-specific heat capacity gas inside. You might notice the large number of bolts around the lower chamber in the picture. I bet that's the pressure vessel. Although Hydrogen gas has nearly 3 times the specific heat capacity of Helium, it's been out of favor since the Hindenburg.

Heat is transferred into the engine by a heat-pipe from the burner unit. The heat pipe can use molten salts as the medium. The free-piston Stirling engine that Sunpower tried to develop used molten sodium. I forget the name of the book about Sunpower, it's a great read on why innovation is so difficult.


I would rather use a cyclone external combustion engine it has most of the advantages of a Stirling engine but much better power density.

Henry Gibson

There are ways to control both helium and hydrogen loss. Hydrogen makes the best working fluid and there is not enough of it to be any more dangerous statistically than just driving a few miles.

Hydrogen is the magic fuel of the past ten years. It is what helped kill the electric car in California. Cheap, compact and production ready fuel cell cars with compact hydrogen storage were just a few months away from production when ZEV rules were eliminated.

What is not well known is that the Hindenburg was the first commercial passenger fatality for more than thirty years of operating german airships filled with hydrogen. Far more people have been burnt up with jet fuel to worry about hydrogen anymore. ..HG..

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