Back in the 1970s, I thought the "idea" of a Series Hybrid was pretty neat.
The definition of a Series Hybrid is an internal combustion engine driving a generator, which then drives an electric motor. A battery might be inserted into the idea to allow some power buffering. The ICE usually runs at a constant RPM, and is thought to be more efficient that way.
The trouble is in the reality... Too many energy conversions, losses at each conversion - Gas to mechanical energy, mechanical to electrical energy, electrical energy back to mechanical.
Now let's look at that "optimized for constant RPM engine". Constant RPM engines are all over the place. No magic. You may have more than one at your house. Gas engine lawn mower, lawn tractor, most (not all) AC generators, etc. The lawn equipment is governed to 3600 RPM unless you back off the throttle. The AC generators run at 3600 or 1800 RPM depending on design. None of these have a constant load. The governor opens/closes the carb throttle plate to keep RPM at the governed speed as it drives the changing load.
The engine in a Series Hybrid has to do the same. The load on its generator is certainly not constant. There is no big magic increase in chemical to mechanical energy efficiency there.
The Chevy Volt - The Volt is an EV with an on-board gas-powered battery recharger. It's claim to fame is running in EV mode, and then a way to travel additional miles after you ran the battery down. If the battery is discharged, it becomes a Series Hybrid.
Speaking of the Volt, some may remember the big brouhaha that came out of EV purists when they discovered that, gasp, the Volt, in certain conditions, actually has the gas engine driving the wheels directly! Oh shame! (yes, I'm being sarcastic about the big to-do over that).
The mode was running down a highway at speed, level or near-level road. I read that directly coupling in mechanical power from the gas engine in that situation, improved overall efficiency and overall range. While running in that mode, it acts like a
Parallel Hybrid.
One more thought about Series Hybrids - The concept DOES have a use - They are in operation all over, have been for many years. Pulling very heavy loads. "Diesel", actually Diesel Electric locomotives. A diesel engine drives a generator, which drives the electric motors. No batteries. The engine throttle is adjusted to match the load and the desired speed. A 16-notch manual throttle control was the most common. On winter nights, when most of the vegetation is gone, I can hear freight trains notching up throttle in the distance, to head up a slight grade, after leaving lower-speed blocks.
The whole purpose of Diesel Electric is that no mechanical transmission is needed. Mechanical transmissions in high power applications are a real problem!
A lot of mining equipment is also Diesel Electric for the same reason. For some, the piece of equipment is all electric, and has a big "extension cord" plugged into a wheeled parked big Diesel-powered generator unit (don't run over or cut that cord!
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