I seem to recall that the maximum efficiency of electrolysis of water is only about 70%. And that the maximum theoretical efficiency of the Carnot cycle in an engine is just under 70%, but the actual efficiency in modern engines is considerably lower.
So already 50% of the power is gone. IE, 2hp (about 1500W) from the alternator nets about 1hp additional power at the engine. Then the alternator is not 100% efficient. The battery is not 100% efficient at storing and releasing power.
Even if everything was 100% efficient, you'd have broken even.
BTW, there is no such thing as "HHO gas". It is H2 and O2 mixed.