Hybrid Vehicle Drivetrain explained

Series hybrid - EV that use a serial drivetrain receive mechanical power from the electric motor(s) 'only' and uses a smaller battery pack compare to BEV. The pack could be charged (in-motion) by any source of energy: Small ICE called REx, Diesel, Fuel Cells, Portable Thorium Nuclear Reactor.
Pros over the Parallel Hybrid: The series hybrid is almost 2 times less Energy Consuming than parallel one. But it is less efficient than a BEV. Great example: i3 REx has combined 118 MPGe vs Toyota Prius - 52 MPGe Theoretically, a series hybrid can be driven inside a city without the REx while the unreliable ICE is OFF or 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 series hybrid drivetrain has been used on ocean ships, nuclear submarines, locomotives, and other really powerful vehicles to save some fuel. In May 1954 Baldwin built a 4,500 horsepower (3,400 kW) steam turbine-electric locomotive. GE had hoped to deliver a prototype steam turbine locomotive in 1937, the series Hybrid locomotive. Series hybrids are also called Range Extended Electric Vehicles (REEV).

Parallel hybrid - ICE and electric motor work together in parallel power combinations. The Synergy Unit on the front axle contains an Electric motor and ICE as 'never' separated engines. This is the most inefficient drivetrain because inefficient ICE is a traction motor too and the ICE kills not only an environment it also kills the efficiency in a hybrid system. Moreover, Why to pay double for complicated Prius with 52 MPG while you can pay about $12,000 for Toyota Yaris with 42 MPG?? Both are polluters and Yaris is more reliable than Prius. An example of Parallel hybrids would be a Toyota Prius or a Chevy Volt. Almost all old models (2000) parallel hybrids permanently pollute and only a few new models are PHEV, those are may drive up to a hundred miles on small traction battery pack by an electric motor only and temporarily do not pollute. The parallel hybrid pollutes less than an ICE because any hybrid regenerates and stops ICE on low energy consumption (low speed) moreover short stops eg. a traffic light. Parallel hybrid drivetrain had been used "only in small vehicles" due to complexity and inefficiency from late 1980 (as a production HEV) and it is just an unsuccessful experiment in the time of weak batteries, or maybe it is an ICE nostalgia; Versus, series hybrids have been used for more power vehicles, for a long time. Toyota seawater FC navy generator (invented in 2018) would be for a while.

series+Parallel hybrid - The front axle is driven by ICE and the rear axle driven by an electric motor. It is an efficient and less polluting than ICE drivetrain while it works as a series hybrid. Good example would be the Volvo V60 produced for European Market only or BMW i8. Usually, Volvo V60 drives only 20-30 miles on a highway as a series hybrid while the ICE (in this case REx) 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 series hybrid. The another parallel mode: ICE drives the car while rear electric motor 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 and few BMW models are only series+Parallel hybrids with sync complicity between 2 diferent engines. The challenge is -- to sync the electric motor with the ICE in parallel mode only.. Many organizations and companies on the Planet felt on the challenge. I rented the Volvo V60 2017 in Stockholm in 2018. I like this car but not the efficiency of the car in Parallel Mode only that is less than my Prius 2004 efficiency. 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 the rear electric motor works independently and the hybrid system has been defined as the series Hybrid.

In the parallel hybrid case, there is not good sync between engines because energy consumption measured in km/l (MPG) encreased more abruptly than it would be on my Prius at a similar speed, considering the fact: Prius has better drag coefficient than V60. This is my preliminary speculation. I did not collect enough data on energy consumption to judge the sync properly. The V60 is a great hybrid with complex multiple mode changes and Volvo leadership in safety, a close competitor to "i" series hybrids from BMW, but not the Tesla rival. Another exelent car of series+Parallel hybrid would be an exotic supercar - Koenigsegg Gemera 2020 with ICE on front axle and twin electric motors on rear one.

➛ Hybrid Drivetrain

1. series with Fuel Cell FC - Zero Emission
1. series with REx - May pollute during a short-range

2. Parallel - Mostly pollutes
3. series+Parallel - Non-polluter on a short-range

MHEV - Mild Hybrid Engine Vehicle is Simplified Parallel Hybrid that polute more than the Parallel hybrid, but cost much less due use of: mhs

The MHEV does absolutely the same functions as the Parallel Hybrid:

The unified around the World 48VDC MHEV system used to have a 42VDC old-conventional voltage. The initiative came from the VW group in 2018

HICEV - Hydrogen Internal Combustion Engine Vehicle

One of the odd misconceptions is: People confuse Hydrogen Fuel Cell Electric Vehicle with Hydrogen Internal Combustion Engine Vehicle, which is an ICE vehicle, but ICE burns Hydrogen instead of gasoline. That is more dangerous and requires ICE reinforcement because Hydrogen is more combustive than Diesel or gasoline.
History showed that HICEV did not fly because of the more expensive fuel, the reinforced ICE cost, and a distinctive ICE service cost. But in 2020 an Australian Co. found an adequate proportion for a 'partial Hydrogen injection' mixed with Diesel fuel. It is still in severe testing in the harsh Australian desert using heavy trucks. Preliminary results of the independent testing are:

  1. Reduced Diesel consumption in the range of 5–13%
  2. Lowered Diesel Particulate Matter Pollution by 25–80%
  3. Removed 7–25% of Carbon Monoxide!