In which hydrostatic transmissions get more efficient.
So, you’ve certainly heard of hydrostatic transmissions, and might even own a lawnmower with one. Engine’s connected to a hydraulic pump, you control the swashplate on that pump to control the direction and amount of fluid moved, and the fluid drives a hydraulic motor. Upshot is, you get a horrifically inefficient, but fairly cheap CVT (actually an IVT, because powered neutral is possible).
Similarly, you’ve certainly heard of diesel-electric locomotives and serial hybrids, where the engine is attached to a generator, and the generator then supplies electricity to one or more electric motors. Really, this is the same concept as the hydrostatic drive, just converting all of the engine power to electricity, rather than using hydraulic fluid as the medium.
And, you’ve probably heard of power split devices in the context of hybrid electric vehicles, especially those from Toyota, Ford, and some of GM’s hybrids. If you haven’t, you might want to read this article that I posted a few months ago.
So, in power split transmissions, you have an engine (well, except for when you don’t have an engine, but that’s beside the point), you have one motor taking power from the engine, you have a second motor receiving power from the first motor, and you have a planetary gearset that connects all of this together, and allows the engine speed to be set independently of output speed, forming yet another form of IVT. However, because some of the engine power follows a mechanical path to the wheels (sometimes it is 0%, but sometimes it’s as high as 100% of engine power), it’s more efficient than a serial transmission.
However... who says that those motors have to be electric? I give you the Fendt Vario, in which those motors are hydraulic instead of electric.