It is highly unlikely that gasoline SIE (homogenous charge) thermal efficiencies will ever meet the PNGV goal 3 target of 40% (cycle averaged). Current and past efforts aimed at direct injection (stratified charge) SIE have demonstrated significant increases in thermal efficiency over homogenous charge counterparts, however, will likely fall short of this target. Severe emissions and durability challenges have hampered implementation of this approach.

The open-chamber Diesel remains as the most likely candidate to meet the aforementioned target (current status has peek efficiencies in the 40% range with cycle averaged efficiencies somewhat lower). Well-known emissions, cost and power density challenges persist.