“While maintaining the highest gear ratio and the lowest rpms is optimal for fuel efficiency, it is not optimal for the long term health of the engine parts - which are being starved of power by the low fuel consumption and therefore working harder and under more load and stress.”
That’s irrelevant in a Prius that don’t have gears that the operator can control even if they wanted too. The Toyota system is two electric motors connected by a single speed epicyclic gesr.The input is on the planets with the engine driving the planets, the output is the sun where the second motor is also hard attached too. The first motor which is normally a generator is on the ring all three rotate relative to each other there is never any shifting more can they be disconnected in the basics hybrid form. All three rotate in a mathematical dance controlled by very fast and sophisticated computers and sensors. So how does the Prius sit still with its engine on if it has to spin all three? Clever math...at stop when the sun is stationary and the engine driving the planets forward the sum maths ratio is negative for the ring and it spins...backwards the motor 1 / generator doesn’t care its direction of rotation so the car sits stationary with the ICE on. To pull away M2 starts to out torque in the forward direction which also loads the ICE and causes M1 to slow down eventually stop and reverse direction as speed increases that cross over point is 35 mph at which the ICE has to run to keep M1 from spinning too fast. Normally a Prius launches with the ICE off and stationary with M1 and M2 both running forwards somewhere before 35 mph M1 slows down and that loads up the ICE planets in the forward direction spinning it up and starting it then all three dance at their commanded speeds. So no the user can never hold a higher gear in a Prius it doesn’t have gears in that way.
The plug in Prius has a one way dog clutch on M/G 1 so it can disconnect and allow the ring to spin freely which keeps the ICE stopped and let’s M2 drive the car up yo 80 mph using electric alone. The ICE has a dedicated starter to bring it up to speed so the ring gear is then moving forward and when M/G 1 spins up yo matching speed the dog clutch engages and M this time working as G1 plus the ICE joins the party at any speed above 35 mph.
Honda has a better plug in system it’s simply two electric drive units one drives the wheels the other is a generator attached to the ICE normally the two never touch the electric motor drives the wheels and the engine drives the generator its all electric no gears at all. This way no complicated epicyclic gear unit nor the high speed controls for it. You get plug in mode natively but trade some fuel economy by going ICE to electric to electric at high speeds. At low speeds it’s always more effective to go electric to electric hence is why hybrids get double the mpg at city and traffic speeds. Honda said ok at motorway speeds we will just link the two electric units together via a simple one way clutch and drive the final drive ratio directly via the ICE. You get all the benefits of a serial hybrid in grid lock and keep the economy at motorway speeds by not using the electrics for anything but regen braking, and peak torque for passing and acceleration. Elegant and simple is the Honda way. It means you have to have a huge motor 2 capable of driving the whole mass of the vehicle at any speeds unlike the Toyota system where M1 and M2 plus the ICE can all three drive the car at any time.
Thanks - I know next to nothing about electric or hybrid design.
All cars have motor rpms and tire revolutions - whether manual, automatic, hydro, ICE, hybrid or electric.. and therefore they all have a ratio between the two motions. At one extreme of that ratio you use less fuel - but put more stress on the the motor.
I get your point though, that with a hybrid like the Prius, the driver’s strategy for obtaining optimal fuel efficiency would be much more complicated than with an ICE, and would require a keen understanding of the design and what conditions trigger the ICE to kick in or lay back.