Hydrogen-injected internal combustion engine
From Halopedia, the Halo wiki
Hydrogen Injected ICE
The hydrogen injected ICE is an engine used by the UNSC, and is featured in the Mongoose and Warthog vehicles. It is unclear exactly how the HI ICE works, but it presumably includes an internal combustion engine (Or ICE abbreviated). The 12.0 liter version used in the warthog makes a kind of whiny hum, and produces a significant amount of power (enough to propel 3 and one-quarter tons of 'Hog to 78 MPH). The 1000cc 4-stroke used in the Mongoose ULATV has a rough growl to it, especially when starting up.
From halo 2 to halo 3 barrels could be found labeled as "Hydrogen Fuel"
