[DMZFrank]

And burn up the intake, exhaust valves and spark plugs due to the ultra lean fuel mixture. What about potentional detonation and engine knock?

[RSmithOpt]

OK...So why don't vehicles running on LPG or CNG burn up the intake and valves?

We are not talking about leaning back the burn mixture in the cylinder, we're talking about leaning back the amount of gasoline delivered to the cylinder with brown gas supplementing the burn mixture.