[Jackson]

A more or less conventional engine is being used as the range-extender for the first E-REVs, which only makes sense: this is what we already have.

However, this engine was developed in a world where the powerplant of a car must produce useful energy at all speeds: requiring engineering trade-offs which hurt efficiency at any one speed.

Ideally, when thinking of a range-extender, one might expect a completely different kind of engine to eventually prevail: one which might have a relatively narrow power band, but an extremely high efficiency, lower weight, smaller size and be able to handle a broader spectrum of fuels.

A tiny turbine comes to mind immediately, but would require exotic materials and close tolerances likely uneconomical to implement on a large scale.

It turns out that a new engine design, a Nutating Engine, is much lighter in weight, smaller in size, using conventional materials. It has been developed for Military uses so far; and it can even run on heavy oil. It has many of the properties of a simple turbine, with few of the constraints. It even recently won an award:

http://www.technologynewsdaily.com/node/9830

A more technical description may be found here:

http://scitation.aip.org/getabs/ser...00126000002000294000001&idtype=cvips&gifs=yes

A couple of diagrams, pictures:

I'd love to get a take on this as a potential range-extender for a future E-REV.

 

[Jason M. Hendler]

Interesting device. You are correct, the ICE range extender selected for the Volt was intended to perform well in acceleration, which isn't important for a generator application.

I would offer up the Mollor compound rotary engine as well. Any device that spins, rather than reciprocate, would be a huge advance.

Link

Also, there are rotary air engines that would be great zero emissions range extenders as well.

Link2

I suspect the piston engine will eventually go away, as the Atkinson engine in the Toyota Prius demonstrates.

 

[kubel]

Building one engine for one specific car isn't economical for automakers. I would imagine the ICE for the Volt will be used in many ultra-compacts produced by GM brands, so a bit of universal compatibility is needed.

Once the concept takes off, and we see it implemented in more vehicles, I'm sure more efficient genset motors will be built.

 

[Jackson]

Fair enough, but I did say "future."

Also, if we keep in mind what these engines are actually for, and use a little imagination, we could find that a single new, compact/lightweight engine might be used for multiple E-REV vehicles in surprising ways.

For example, since the purpose of the engine is to generate electricity, there are no mechanical linkages to transmissions, drive shafts, etc. to worry about. What would prevent a micro-light E-REV (2-seater), a 4-seater (Volt) or future crossover SUV from using the same engine ... in different numbers? The 4-seater could carry two identical engines rather than a separately engineered intermediate engine, the crossover could have three instead of a seperately engineered large engine, and the micro-light only one.

I admit there would be some plumbing complications for intake and exhaust, but the rest would be all wiring.

If they are compact enough, you could look at the generators as though they were batteries, which is what they are in a purely electrical sense: Need more power? Stack 'em up.

This would have the added benefit of an engine failure leaving all but the micro-light E-REV drivable, to some degree.

 

[Jason M. Hendler]

What would prevent a micro-light E-REV (2-seater), a 4-seater (Volt) or future crossover SUV from using the same engine ... in different numbers?

No need to have different numbers of motor-generator sets, you only need to run the same motor-generator set at different speeds. For a small car, you can run it at 2,000 rpm and for the larger vehicles, you can run it at 3,500 rpm, just as an example. It may not be a perfectly efficient, but you wouldn't have to tool up for multiple designs.

 

[Jackson]

No doubt some adjustment could be made to the power output of one engine design, but these would also require that the generator portion attached to the engine be sized differently. This would also be a stock-handling/design for-multiple-cars issue.

In the case of a 2+2 (with one engine running faster than the micro-light in my previous example), or a 5-seater automobile (with both engines running faster than the 4-seater's), this might be worth the re-tooling for an already established engine design. Since the Nutating Engine uses an external combustion chamber, it may be that a different combustor design could be used while retaining the same rotor disc. There would come a point, though, where modification of a single engine's operating characteristics could achieve no further output.

However, it shouldn't be forgotten that a faster running generator might not last as long as one running more slowly, and it will have different cooling needs.

It may also be possible to make a Nutating Engine with multiple rotor discs, or one disc with two combustors (or some combination) which could still achieve some commonality of parts.

However, there is the idea of redundant operation of multiple engines which is appealing, and there is also this possibility to consider:

In your future E-REV Suv, on a hot day, you could turn on your A/C, and only light off one of the vehicle's 3 engines to power it, boosting the range only a tiny bit. Stop and go traffic might only require 2 engines, with the third only lighting after you've merged onto the Interstate, or after driving at illegal speeds for over an hour. Again, it doesn't matter which engine is which. The engine which ran to power your A/C one day might be only the second to come on the next day (to balance wear). That's an issue for the control system programming.

I'm not sure how compelling engineering just the most efficient speed might be, but multiple engines would give a vehicle a wider power range without sacrificing that most efficient mode.

 

[Jason M. Hendler]

I'm not sure how compelling engineering just the most efficient speed might be, but multiple engines would give a vehicle a wider power range without sacrificing that most efficient mode.

It's not just a matter of slight differences in efficiency, it is a matter of costs - 1X, 2X, 3X the cost of the motor-generators, additional costs of accelerating and decelerating additional motor-generators that aren't running, etc.

True, the motor may be the same, but the gearing into the generator and the generator itself may be different, if the power output is expected to be greater.

 

[Koz]

I think it's very likely a rotating engine of some sort will be the generator engine of choice for future EREVs. It could be nutating, Moller (Wankel), micro-turbine, Radmax (axial-vaned), or some yet to be developed rotating engine. The nutating engine looks very interesting and well suited for the application. I think it is important to weight the important parameters for the EREV range extending application. In my mind they are:
-Cost: most important (10)
-Specific power by volume(KW/volume): most important (10)
-Reliability: most important (10)
-Maintainability: Important (6)
-Efficiency: very imoortant (8)
-Emissions: Important (6)
-Specific power by weight(KW/kg): less important (4)
-Scalability: Important (6)

I've ranked cost and specific power by volume with the most important parameters because this is what will allow for more battery capacity.


So, the simplest, smallest, and cheapest solution with at least good efficiency would be the best. From the information known about the crop of rotating engines mentioned above, I think the Moller and nutating engines look the most promising.

When developing EREV specific engine solutions, they should be optimized around 2 power levels of ouput with a general operation level taking precedence for non-towing vehicles. For towing capable vehicles both levels should be of equal importance and perhaps a 3rd optimized ouput of lesser importance may be required. Since the battery and traction motor are the heart of the drivetrain, this should leveraged to simplify the ICE offerings for EREVs. This another benefit of the elegant EREV solution. A 53KW genset with a small, inexpensive engine will suffice for a broad range of vehicle types: micro, subcompact, compact, mid-size sedan, full-size sedan, minivan (?), wagon, etc. Basically all vehicles with a reasonable Cd*A. Larger vehicles could utilize a genset in the 75-100KW range. Look at the more recent battery discharge graphs that GM released. The plan to discharge the battery to around 30% for initial AER, then follow charge/discharge cycles of 10-15% during Range Extended mode. This means they will be trying to operate the ICE at it's most ideal load for as much of it's operation as possible. It looks like most of the power required beyond this ideal load will be supplied by the battery, unless prolonged higher load conditions exist (e.g. highway speeds along with a grade).

 

[Jackson]

Okay, so maybe multiple engines were "over the top." Thank you for the information, Koz. I wonder if we really appreciate just how flexible "flexible" really is.

There might only be one on board generator, but perhaps more than one source of electricity. One often mentioned option is rooftop solar cells (probably not worth doing until multi-junction cells are available at anything close to a reasonable cost).

Could someone design a co-generation device employing thermoelectric cells to further convert waste heat from the primary generator? If someone comes up with something really odd like a disposable, sugar-powered fuel cell utilizing engineered microbes, and space were available under the hood, couldn't you add it? Things not worth doing on their own as a primary source of energy might still be worth doing on a Volt-like vehicle only because it can help.

If the history of technology teaches us anything, it's that new breakthroughs are often limited in application by the mind-set of the previous technology. The first automobiles were "horse-less carriages," which eventually evolved into the personal transportation machines we know today.

===

Getting back to the Nutating Engine, an external combustion chamber and continuous combustion opens up many possible fuels for automotive use. There is a short list of things sold as fuel today which are liquid, intended to be burned in some way, and which require no special storage considerations (such as dewars or pressure tanks):

Bunker Fuel (ocean shipping, "heavy" oil)
Home Heating Oil
Kerosene/Aviation (jet) fuel
Diesel (including biodiesel)
Gasoline (all grades)
Ethanol (including E85)

Other fuels are in development, but these are available now.

Of the six, only Flex Fuel can burn more than one: gasoline and/or E85.

Continuous combustion could allow a properly designed Nutating Engine (or Turbine Engine) to burn all six.

There's an outside chance that a continuous combustion engine could be made to run on anything from warm bacon grease to Jack Daniels.