We're using the components of household negative air ionizers, to ionize the air going into engines.  We have learned that we need to keep the supply voltages steady with regulation, and when we get close to the maximum desireable at idle, to keep the rig shut off via a temperature switch, because too much will foul the plugs.  Gasoline right now, but we'd love to see diesel tried, not the least bit because...no plugs!

Lots more detail here:  https://motormercy.ponderworthy.com