Serial hybrid - EV is driven by an electric motor(s) 'only' and uses a smaller battery pack compare to BEV.
The pack could be charged by any source of energy: Small ICE called REx or Diesel, Fuel Cells, Portable
Thorium Nuclear Reactor. The serial hybrid is almost 2 times more efficient drivetrain than parallel one.
But it is less efficient than an EV. A great example is the BMW i3 with REx.
Pros Over the Parallel Hybrid: a Serial hybrid can be driven (theoretically) without the REx while the unreliable ICE in a repair shop.
The most important fact to comprehend: The REx or small ICE is mechanically apart from the driveline and supply electricity to traction battery only, like an electrical generator on a submarine. The serial hybrid drivetrain has been used on ocean ships, submarines, trains, and other powerful vehicles.
Parallel hybrid - ICE and electric motor work together in parallel power combination. The Synergy Unit on the front axle contains an Electric motor and ICE as 'never' separated engines. This is the most un-efficient drivetrain because un-efficient ICE is a traction motor too and the ICE kills not only an environment it also kills the efficiency in a hybrid system. An example would be a Toyota Prius or a Chevy Volt. Almost all parallel hybrids permanently pollute scince year 2000 and only few new models are PHEV, those are may drive up to a hundred miles on small traction battery pack by electric motor only and do not polute. The parallel hybrid polutes almost 2 times less than an ICE.
Serial+Parallel - The front axle is driven by ICE and the rear axle driven by an electric motor. It is an efficient
drivetrain while it works as a serial hybrid. Only one example would be the
Volvo V60 produced for European Market only.
Usually, Volvo V60 drives only 20-30 miles on a highway as a serial hybrid while the ICE tries to keep a small
battery pack to be charged. It is a very complicated system (with many modes of driving) as the parallel hybrid or/and serial hybrid.
The typical serial mode: ICE drives the car while rear electric motors may work as a generator for charging the
traction battery pack. Also, the system could be considered as a PHEV because you can plug the Volvo into any outlet to charge overnight.
The V60 is only one production Serial+Parallel hybrid on our planet.
That is logical confirmation of the system's complicity! The challenge is --
to sync the electric motor with the ICE in parallel mode.
Many organizations and companies on the Planet felt on the challenge. I rented the Volvo V60
in Stockholm in 2018. I like this car but not the efficiency of the car in Parallel Mode. According to my data collection
from the V60 instrumentation panel and few experimentations -- Volvo engineers made the 'smart sync': if the electric motor rotates slower than ICE, than
electric energy from the electric motor regenerates to the traction battery, else: the system switches the ICE to a neutral drive.
The most important fact to comprehend: The Volvo V60 drivetrain acts both ways:
On high speed it combines power from both engines and the hybrid system has been defined as the Parallel Hybrid.
Sometimes both engines work independently and the hybrid system has been defined as the Serial Hybrid.
In the parallel hybrid case, there is not good sync between engines because the energy consumption measured in km/l (MPG) encreased more abruptly than it would be on my Prius on the similar speed, including the fact: Prius has better drag coefficient than V60. This is my preliminary speculation. I did not collect proper statistics on the Energy consumtion to judge the sync properly. The V60 is a great car with smood multiple mode changes. The V60 is the best hybrid than I was ever driven, except serial hybrids from BMW.