water.... It's not absolutely free but 22oz of water from my tap in the house is like .0026 cents and that can generate enough HHO to mow my lawn 4 times witch is about 8 hours of run time.
It's actually very interesting and possibly the most simple thing in the world to accomplish. Pass electric current through water H2O, blow the molecules apart and presto changeo Hydrogen and Oxygen.
We know that - but where are you getting the free electricity to generate the HHO?.
It's NOT an efficient process, you would be better off using the electricity to charge a battery, and use a battery powered mower.