Ok so here's the idea:
make an engine that changes between operating modes based on how much power is required.
Atkinson mode for high economy/low power cruising
Miller cycle mode for accelerating
use a vtec style multi lobe cam and custom ECU to switch between 2 modes
both are under-square engines with delayed intake valve closing
the miller cycle uses intake charge compression; usually a supercharger
if we want our compression to turn on/off our options are clutch or electric
we can't use a turbocharger because both engines/modes have low exhaust gas pressure
Ref: Mazda Millennia 2.3L v6 miller cycle 20cmpg 210hp
we'll scale this back to a reasonable 1L i4 @ 100hp
Ref: Toyota Prius 1.5L i4 atkinson cycle 45hmpg 76hp
our 1L i4 i'll estimate @ 55mpg
the transmission should have a high final ratio; a CVT would allow an ERS, a 6 spd manual would be harder to ERS
Lets plan on having an ERS (Energy Return System) on the transmission to store kinetic energy from braking. We'll use a giant rubber band instead of Lithium Ion batteries cuz its way cheaper.
Possible engines:
1.0L 3-cylinder ECA1 2001-2008 Insight - a rather rare engine.
2002 1.2 L L12A Honda Jazz
1999 - SZ - DOHC (by Daihatsu) 1999 - 1 L (997 cc) 1SZ Toyota Platz?
Toyota 2004 - KR - DOHC - 1.0 L (998 cc)
1KR
* Toyota Passo/Daihatsu Boon
* Toyota Aygo/Citroën C1/Peugeot 107
* Toyota Vitz/Yaris (2005–)
* Daihatsu Cuore L276 (09.2007-)
* Subaru Justy (2008-)
its undersquare, has VVT-i, aluminum block, 07-08 engine of the year.... its a winner