Injection of nitrous oxide into the combustion chambers of an internal combustion engine as a way to increase power output was discovered by the German aircraft industry early in the Second World War. Thousands of German fighter and reconnaissance aircraft were equipped with the so-called GM-1 system that added nitrous oxide to the intake charge to compensate for reduced air density and less oxygen high altitude. The British Royal Air Force also used aircraft engines with performance enhanced by nitrous oxide. There was no use of nitrous oxide injection by the American military air forces other than very limited experimental use.