An internal combustion engine essentially requires a fuel which must have sufficient calorific value to produce enough power, and oxygen for the combustion of fuel. In normal vehicles fuel will be supplied from a fuel tank equipped with it. And oxygen will be taken from the atmospheric itself. Under normal conditions the percentage of oxygen present in atmospheric air will be around 21% of the total volume. Studies shows that by increasing the oxygen percentage in the inlet air increases engine performance and reduces emission produced by the engine.
International Letters of Chemistry, Physics and Astronomy (Volume 48)
S. P. Shivakumar "Engine Enhancement Using Enriched Oxygen Inlet", International Letters of Chemistry, Physics and Astronomy, Vol. 48, pp. 37-49, 2015