“Lean-mixture-ratio combustion in internal-combustion engines has the potential of producing low emissions and higher thermal efficiency for several reasons.

First, excess oxygen in the charge further oxidizes unburned hydrocarbons and carbon monoxide.

Second, excess oxygen lowers the peak combustion temperatures, which inhibits the formation of oxides of nitrogen.

Third, the lower combustion temperatures increase the mixture specific heat ratio by decreasing the net dissociation losses.

Fourth, as the specific heat ratio increases, the cycle thermal efficiency also increases, which gives the potential for better fuel economy.”

To summarize:

The more efficient HHO engine has more fuel energy converted into useful mechanical energy and less into wasted thermal energy.

This is apparent to the user in increased fuel economy, lower combustion temperatures and approximately 100C lower exhaust temperature.