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I am hopeful that it is a serial hybrid. And there are several big advantages of a serial hybrid:
*Engine can be a lot smaller because it only has to run at one RPM where it is the most efficient.
*It does not need a transmission to spread out its torque band.
*It does not need to have the ICE mechanically adjacent to the wheels.
A parallel hybrid also needs a generator -- the Prius has one.
The battery in a serial hybrid does not necessarily need to be bigger than in a parallel hybrid. It only needs to be large enough capacity to not run low on a charge while the ICE is running.
The advantages of a serial hybrid are that the electric motor(s) are much more efficient than any ICE, and they can be hub motors, eliminating any drivetrain losses. Or they can have just a single reduction gear, which has lower losses than a multi-gear or CVT transmission.
The generator and ICE are directly linked, and can be located independently from the wheels. If the generator is larger than it would be in a parallel hybrid, this is more than offset by the lack of a transmission.
As mentioned above, the ICE can be run at it's designed torque peak, at a single RPM where the engine can be optimized to minimize the internal losses. It can be located in an ideal location for low drag cooling, and since it is a smaller displacement than a parallel would require, the cooling system is proportionally smaller; and aerodynamically minimized, too.
A 500cc ICE also weighs less, and has less internal friction, and it has to pump less air. The (flex) fuel tank is said to be just 4 gallons. It is a flex fuel ICE, so tuning for different fuels is easier when you only have to worry about one fixed RPM.
In comparison, the VW 1L has an 800cc diesel and it is a parallel hybrid -- and it has a 7-speed dual clutch transmission. NO transmission is mentioned on the 1/X. The VW 1L seats just 2 people and it weighs 380kg/838 pounds which is 88 pounds less than the 1/X -- so why does it have a 60% larger displacement diesel with a 7-speed transmission and an electric motor. It needs to have start-stop tech in order to save fuel when stopped. (Of course a serial hybrid has an auto starter, but the timing of it is based on the state of charge, rather than the "request" of the driver.)
The VW Up! Lite is a closer match to the Toyota 1/X (both carry four people) and the Up! Lite weighs 1530 pounds which is 604 pounds more than the 1/X, and it get ~1/2 the fuel mileage. Diesel has ~20% more energy by volume than gasoline.
So, a parallel hybrid that weighs 88 pounds less and carries half the people, and has an ICE that is 60% larger displacement gets roughly the same mileage (170mpg vs ~150+mpg); and the same parallel hybrid drivetrain in a 4 person car that weighs 604 pounds more gets less than half the mileage (70mpg vs ~150+mpg).
Diesel should be 20% better, anyway. And why does it take 60% more displacement and a 7-speed dual clutch transmission? The 1/X carries the same people as the Up! Lite, but delivers the FE of the 1L, with a smaller engine.
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