**15. Conclusion**

Internal combustion engines are still the most common means of imparting vehicular power even though lately electric and hybrid systems have emerged. Thus legislation is progressively becoming more stringent on various aspects regulating polluting exhaust and environmental impact which incentivize manufacturers to produce ever more clean and fuel efficient vehicles. As discussed direct injection gasoline technology has already been successfully employed to reduce fuel consumption and carbon dioxide emissions for the same power output. Further research on both fuels and combustion technology is currently underway to continue improving the environmental impact. Future work in LNG powered vehicles and homogeneous charge compression ignition (HCCI) are two areas where vehicles' power units can continue reducing environmental impact.
