Editor's note: The debate over whether series or parallel architecture is best for plug-in hybrids will be waged in the marketplace. GM and Fisker sit on the series side; Ford and Toyota sit on the other. And EDI, a company founded by UC Davis professor Andy 'Father of the Plug-in' Frank, serves both markets. The question is further complicated by the fact that not many of these vehicles are on the roads. Here's an analysis from attorney and energy enthusiast Jason Jungreis on a matter that we hope to visit a number of times in 2011.

Let us suppose that, through a combination of forces including mileage or emissions requirements, fuel prices, and technology maturation, it becomes de rigueur for vehicle manufacturers to focus their energies on electric vehicles with extended range capacity (EV-ER): after all, such vehicles sooth range anxiety by carrying some energy-dense fuel instead of a huge number of heavy and expensive batteries. Manufacturers probably understand the EV side of the equation -- sure, battery chemistry has ever-changing flavors, yet the configuration of the balance of the system (motor, controller, charger, management system, electric peripherals, etc.) are pretty well decided.

However, we don't have a known ideal extended range device to provide electricity and perhaps also motive power. What are manufacturers' options, and which is best?

Clearly, there are a variety of options, including: conventional gasoline ICE; diesel; HCCI; rotary; turbine; Stirling; two cycle; split intake and power stroke designs; ethanol fuel cell; and hydrogen fuel cell. Similarly, there are just as many factors to consider in determining the pros and cons of each model. Some considerations include: does it define whether the vehicle will have parallel or series hybrid architecture (i.e., is the range extender capable of moving the vehicle directly or can it only be used to produce electricity for the batteries/motor; manufacturing cost; fuel economy; fuel flexibility; size; weight; noise, vibration, and harshness (NVH); longevity/maintenance; usability in the full spectrum of real world conditions; and implications for battery pack size. Because there are so many elements to consider, let's compare each of these considerations.

The first critical decision is parallel versus series architecture. If a vehicle is to be used for driving long highway miles, then direct mechanical drive would seem an advantage: otherwise, you have to accept the Rube Goldberg machine-like process of taking mechanical motion, converting it into electricity, shunting that electricity through a controller, storing it in the battery, pulling it back out of the battery, pushing it back through the controller, and running it through the motor to once again get mechanical motion -- and accept the loss of energy at each step.

Engineers might point out that this system allows the engine to run at optimally designed speed and load and therefore to be efficient enough to make up for these energy losses. Besides, using this approach, there is no need for a mechanical transmission. It also vitiates the need to engineer for all those annoying on-throttle / off-throttle / part-throttle situations, and as such, complex fuel management, NVH driveline lash, and a host of related issues are sidestepped. However, series design also requires a commitment to a large battery pack, with its attendant cost and weight issues.

But wait -- there's more to consider: interestingly, through-the-road parallel does away with the need for two motors. This is clearly a point in its favor, along with providing 4-wheel-drive capability, to boot. Plus, a smaller drive motor can be used in parallel because the engine can help. This would be a clear point for parallel, if it wasn't for the fact that if the engine is an ICE and it only helps now and then, it can't heat up its catalytic converters and must now run inefficiently (d’oh!). Additional considerations include anticipation of improved batteries, which would slot nicely into series design; the lower top speed or need for a more expensive motor in series designs due to the motor's single-speed transmission; and the modularity of a series design that would enable more batteries to be used instead of an engine and broaden the variety of possible ways to extend range. This last point may be particularly convincing for manufacturers: one platform will enable both pure battery-electric vehicles and a choice of range extenders based upon fuel availability choices or technology advances.

GASOLINE
Parallel v. Series: either
Manufacturing Cost: medium (well known)
Fuel Efficiency: approx. 33%
Fuel Flexibility: gas or ethanol
Size: medium
Weight: medium
NVH: good
Longevity/Maintenance: complex but known
Real-World Usability: very good
Battery Size Needed: offers flexibility

DIESEL
Parallel v. Series: either
Manufacturing Cost: medium (well known)
Fuel Efficiency: approx. 40%
Fuel Flexibility: biodiesel
Size: medium
Weight: heavy
NVH: fair
Longevity/Maintenance: complex but known
Real-World Usability: very good
Battery Size Needed: offers flexibility

HCCI
Parallel v. Series: better in series
Manufacturing Cost: medium
Fuel Efficiency: approx. 40%
Fuel Flexibility: gas or ethanol
Size: medium
Weight: medium
NVH: good
Longevity/Maintenance: complex, less well-known
Real-World Usability: good
Battery Size Needed: may offer flexibility

ROTARY
Parallel v. Series: better in series
Manufacturing Cost: medium
Fuel Efficiency: approx. 30%
Fuel Flexibility: gas or ethanol
Size: small
Weight: light
NVH: very good
Longevity/Maintenance: complex, less well-known
Real-World Usability: very good
Battery Size Needed: may offer flexibility

TURBINE
Parallel v. Series: series only
Manufacturing Cost: medium high
Fuel Efficiency: approx. 30%
Fuel Flexibility: excellent
Size: medium
Weight: light
NVH: low vibration, high noise
Longevity/Maintenance: very good
Real-World Usability: good
Battery Size Needed: large pack

STIRLING
Parallel v. Series: series only
Manufacturing Cost: medium
Fuel Efficiency: approx. 50%
Fuel Flexibility: excellent
Size: large
Weight: medium
NVH: very good
Longevity/Maintenance: good
Real-World Usability: fair (slow to start, unclear how to incorporate)
Battery Size Needed: large pack

TWO CYCLE
Parallel v. Series: better in series
Manufacturing Cost: medium
Fuel Efficiency: approx. 40%
Fuel Flexibility: gas or ethanol
Size: small
Weight: light
NVH: good
Longevity/Maintenance: complex, less well known
Real-World Usability: good
Battery Size Needed: may offer flexibility

SPLIT STROKE
Parallel v. Series: better in series
Manufacturing Cost: medium
Fuel Efficiency: approx. 40%
Fuel Flexibility: gas or ethanol
Size: medium
Weight: medium
NVH: good
Longevity/Maintenance: complex, less well-known
Real-World Usability: good
Battery Size Needed: may offer flexibility

ETHANOL FUEL CELL
Parallel v. Series: series only
Manufacturing Cost: high
Fuel Efficiency: approx. 50%
Fuel Flexibility: ethanol only
Size: medium
Weight: light
NVH: excellent
Longevity/Maintenance: unknown, but should be excellent
Real-World Usability: unknown, but should be very good
Battery Size Needed: large pack

HYDROGEN FUEL CELL
Parallel v. Series: series only
Manufacturing Cost: very high
Fuel Efficiency: approx. 50%
Fuel Flexibility: hydrogen only
Size: medium
Weight: light
NVH: fair
Longevity/Maintenance: unknown, but should be excellent
Real-World Usability: unknown, but should be very good
Battery Size Needed: large pack

Once a series design has been chosen, and assuming there is reasonable exploration of range extenders, what do we think will win?

The data comparison demonstrates factors to consider: for instance, if the cost would come down, then direct ethanol fuel cells could be great -- but they obviously require pure ethanol, which is simply not available and won't be available without a serious commitment. Similarly, Stirling engines could be great, but they're difficult to manage due to their slow light-up. Further, if they're used in series with large battery packs that will be primarily charged from the grid, does the high fuel efficiency matter that much? Turbines have similarly slow light-up, their noise is disconcerting, and they have lower efficiency. However, all of that may not matter if users are primarily grid-charging the pack, and they have the advantage of being able to burn any fuel. Trade-offs abound.

Purpose-designed series engines, such as Lotus' Omnivore two-cycle (http://www.lotuscars.com/news/en/omnivore-engine, http://www.youtube.com/watch?v=fIG9pWldO8U), seem a good way to go: relatively conventional manufacturing processes, high efficiency, not too weird for existing technicians (we stopped calling them "garage mechanics" when they started charging more than any other blue-collar job you can name), and potentially small and light. But the engine is not really an omnivore, as it does not profess to be able to burn diesel (perhaps it could be designed to live up to its name, and then it could suffice for all possible users). There are a number of manufacturers, would-be manufacturers, and would-be developer-licensors that are each searching for the ideal conventional-but-better-but-not-too-weird engine design to fit this bill.

In the final analysis, manufacturers are going to have make decisions that will effect the planet's travelers, and while everyone wants to look smarter than the other guy and drive in the fast lane, no one wants to drive down a dead-end road. Therefore, given the comfort level that existing engineers have with internal combustion, I think it is likely that the future will be series hybrid architecture to satisfy mass manufacturing, marketing, and modularity concerns, with one or another version of a small, light, efficient, fuel-flexible, and semi-conventional range extender. Vive la difference -- as long as it's not too different.

Tags: chevy volt, city driving, diesel, ev, extended range, fisker karma, ford motor company, freeway driving, gas, gas mileage, honda, lithium ion battery, plug-in car, plug-in hybrid, prius