i think i've seen that before and they've used different engine design only for compressed air.
ang plano ko sana dati ay i-convert ung mga regular engine (ICE) to run by compressed air pero it needs high pressure air at around 500-600 psi.
ang puede lang maka-produce nung ganung pressure is ung compressor ng pang SCUBA or regular air compressor na may booster.
kaya nag decide akong i-drop na lang ung project ko dahil una, it's too expensive to buy the SCUBA air compressor at pangalawa, gagamit pa rin ng ICE na kailangan pa rin ng oil change (although matagal bago magpalit may possibility na magkaroon ng leak), etc.