A question I have had about hybrids is why they don't just use the electric motor as primary propulsion and the gas (Diesel) engine purely to recharge. This would do away with a transmission (potentially, you could use a two speed variant), the alternator, and provide the "instant" torque of the electric motor. The gas engine could be designed to run at some narrow RPM range for efficiency just to drive a generator.
You could have electric motors on each wheel if one wanted AWD and traction control.
I just googled it... Those are known as "series hybrids" BTW.
It seems that the main culprit is conversion losses. At highway speed, using the gasoline engine to power the wheels directly is apparently more efficient than using the gasoline engine to create electricity which is then used to power an electric motor which powers the wheels.
For long sustained drives, the gas engine would be running constantly. If it's not running constantly, it's overpowered for the application and therefore too costly. And if it's underpowered, you'll run out of range or limit your speed because you can't generate enough electricity to both drive the wheels and keep the battery charged long-term. But if the gas engine is running constantly, then there's no advantage of being a hybrid at all, and there's disadvantage from the conversion losses.
And then this also means that the electric motor has to be big and powerful enough to handle all driving duties under all conditions. Which is more powerful than it would be in a parallel hybrid. Which means more cost.
In a parallel hybrid, you have an gas engine that's appropriately sized to operate the car at highway speeds. You have electric motors that make added power for around-town driving, but the gas engine doesn't even necessarily need to spend it's time and power "charging" the batteries all that much b/c you can rely on things like regen braking to provide some of the charge--which makes sense around-town and is irrelevant on the highway.
Just my $0.02.
