Free-piston engine: GM ran one on whale oil in 1956, and Toyota wants it back
Free-piston engine: Pescara patented it, GM ran the 1956 XP-500 on it, even on whale oil, and Toyota built a 42% efficient one for EVs.
Hyprex
Free-piston engine: Pescara patented it, GM ran the 1956 XP-500 on it, even on whale oil, and Toyota built a 42% efficient one for EVs.