Right but then why not use a more efficient and reliable engine? The motors will last hundreds of thousands of miles. The power circuitry should have a similar run if kept cool. Why have an ICE that's more likely to fail with lower thermal efficiency?
The engine used in Toyota's most popular cars runs in Atkinson most of the time with a 40% thermal efficiency.
Because the engine isn’t the only thing being optimized. Suppose your options are 50 miles of effective plug in range and a 40% efficient engine or a 35% efficient engine and a 100 miles of plug in range. The second option might use a lot less gas.
It’s not just the cost of batteries that’s a concern resulting in short plug in hybrid ranges. Weight is a real limiting factor when you also need a gas engine and large fuel tank.
Atkinson engines have poor power to weight ratio at maximum efficiency, if you’re going to maintain that 40% then the engine needs to be oversized and under utilized. This is why many hybrids have seemingly much more power than they need. Thus the 194HP Prius vs 75 HP on a CX-90 plug in SUV.
Also it’s not just that you can have extra Li+ it’s also pushing around less dead weight when the engine is off. So you can get fairly close to a 50 mile range difference depending on exact setup.
Alternatively you can have a smaller Atkinson engine and a larger reserve on the battery, but that also costs range while still being heavier.
It could make more sense to use a conventional engine, but get extra expansion (the kind an Atkinson/Miller cycle engine gets from the longer expansion stroke) by turbo-compounding. That is, exhaust gases (still at greater than ambient pressure) go through a turbine that is coupled to the output shaft, or to another generator, rather than being used to drive a compressor as in a conventional turbo. Or, attach a motor/generator to a conventional turbo and be able to have it operate in both modes and use the battery to help overcome turbo lag.
It might make sense to look into a turbine engine on a hybrid. Though that’s probably been investigated I doubt anyone has invested the kind of time and money needed to make a real shot of it.
> The engine uses port and side direct fuel injection systems (PFI/GDI, referred to by Toyota as D-4S); a cooled, external exhaust gas recirculation system (cEGR); and a wide range of authority variable valve timing with electric phasing on the intake camshaft and hydraulic phasing on the exhaust camshaft. Atkinson Cycle is implemented using late intake valve closing (LIVC). Effective compression ratio is varied by varying intake camshaft phasing.
The engine used in Toyota's most popular cars runs in Atkinson most of the time with a 40% thermal efficiency.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7425626/