A hydrogen and natural gas fuel mixture for internal combustion engines is provided for vehicle engines such as those used in standard production engines for automobiles, trains and lawn mowers. The gaseous fuel for operating a vehicle combustion engines includes approximately 21 to 50% Hydrogen...http://www.google.com.au/patents/US5787864?utm_source=gb-gplus-sharePatent US5787864 - Hydrogen enriched natural gas as a motor fuel with variable air fuel ratio and fuel mixture ratio control 
