There are "obvious" losses in any drivetrain. But, EV's are by far better than ICE's in most driving situations. Only steady-state highway speeds are better in an ICE, and not coincidentally, a series/serial hybrid mimics an ICE on the highway; but it does so ALL THE TIME. So, the actual drive motor is electric which is much better in most situations, and when the ICE is needed, it runs in it's ideal conditions.
Series/serial hybrids have several big advantages over pure parallel-only hybrids:
The ICE *should* be run at a single RPM driving a fixed load. (Which is is not how the Volt uses it -- the Volt is not an ideal series/serial implimentation.) This makes several things possible:
This lets the engine be optimized for that one RPM, and we all know that this is ideal situation -- for parallel hybrids, too. (They run best at highway speeds when they don't need to use their transmission.) So, in the case of an Atkinson cycle ICE, this means it can always be at about 38% efficiency.
This reduces the required displacement and reduces the maximum torque, because the battery can be used as buffer, and only has to meet the average power level.
This reduces the weight of the engine, it's cooling system, the size of the fuel tank.
There is no multi-ratio transmission -- so less weight again. The need for a transmission is an engineering compromise -- the engine cannot meet the torque requirements, and it must be run at all sorts of off-peak conditions.
There is no idling the ICE; by definition, if it running it will be running at it peak efficiency actually doing work at all times when it is running. If you add a stop/start, then this is more complex.
You do not *need* to run the ICE when running at highway speeds, as you would in a true "Parallel-only" hybrid. So for short drives that under the range of the battery, the ICE never comes on.
If you are on a long drive with a lot of it at slow speeds, you cannot extend the range of electric-only mode. So, the ICE cannot contribute if the battery is depleted, and the driving needed is slower than where the engine can be used.
The ICE engine can be located in the car independent of the drivetrain; since there is no mechanical connections to the drive wheels.
Both series/serial hybrids have regenerative braking, and that's a good thing. Both types also run the ICE in it ideal mode, though on a parallel hybrid, the load is only constant on flat ground and no gusting winds. And the speed of the vehicle has to be linked to the ICE's RPM, in order for it to be peak efficiency, so this limits it's flexibility.
Parallel hybrids are more difficult to build: either they drive the rear wheels, while the electric motor drives the front (for better regen), or there needs to be a additional drive wheel. In a parallel hybrid, the ICE must be located to make the best connection the drive wheels, and this means it cannot necessarily be located for best cooling aerodynamics or weight distribution. The GM Precept is the only design I can think of that is a true parallel-only design, and they have a transmission on the ICE, and the ICE is in the back, making cooling more difficult and limiting the storage.
The "fifth" wheel type of parallel hybrid is even harder in many ways. It is aerodynamically and mechanically challenging. Lowering/Lifting the wheel needs to be figured out...
Both of these types of parallel-only setups require a second mechanical drivetrain, adding weight.
If the both the electric and the ICE motors drive the front wheels, then this adds a transmission and a clutch, and a more complex control system.
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In a nutshell, a series/serial hybrid has all the advantages of an EV, and uses the ICE at it's best RPM, and the ICE can be smaller, with all the related weight savings.
A true parallel-only hybrid is harder to implement, and is less flexible in the type of driving, and in it's physical set up. So, while it may have theoretical efficiency advantage (at a particular speed on flat ground and no gusting winds), I think if you cannot have the pure EV range (which is by far the best in most situations), then a range extending series/serial genset extends the advantage of the EV better.
The new NBM "Hummingbird" batteries might make the need for hybrids moot.
Edit: The X-Prize Knockout Round is important to consider.
The average of the 6 hybrids that used their ICE's during the contest (which did not including the FVT) was
61.26MPGe Please note, these are all parallel hybrids.
The average of the 5 internal combustion drive cars was
82.92MPGe.
The average of the 12 vehicles using electric drive MPGe (I'm including the FVT in this) was
134.7MPGe.
So the lowest MPGe of an electric drive; the AMP'd Sky was 86.7MPGe (Tango was 86.8), while the best of a car with an internal combustion is the Edison2 #97 at 101.4. (Actually, the FVT has a ICE powered generator onboard, but did not need it *at all* in the X-Prize. It would be great to see how the eVaro does for MPGe in charging mode!) The hybrids all were all below the 67MPGe -- except the WWU at 92.5 (and the FVT).
The FVT got 152.5MPGe while carrying the weight of the genset, and the genset would have to be pretty horrible to lower the average MPGe to below the WWU, which was by far the best hybrid -- and it (the WWU) was a EV drivetrain combined with an Insight drivetrain; so it was a parallel hybrid/parallel hybrid hybrid, so-to-speak.
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