Hydrogen-injected internal combustion engine
From Halopedia, the Halo wiki
The hydrogen-injected internal combustion engine is a motor unit used by the United Nations Space Command. It is featured in the M274 Ultra-Light All-Terrain Vehicle and M12 Force Application Vehicle.
The AMG 12.0 liter version used in the Warthog emits a whiny hum. It produces at least enough power to propel a three-and one-quarter ton Warthog to 125 km/h.
The 1000cc four-stroke version used in the Mongoose ULATV emits a rough growl, especially when starting up.