At the Project Camelot "State of the Planet" conference (August 1st, Amsterdam) there was a demonstration of a Geet/Pantone hybrid engine generator.
Instead of just fuel it uses a steam/fuel mixture that reduces fuel consumption and emission, while maintaining power output. The exhaust is re-used to heat the water/fuel mixture.
The setup as shown in this video has a fuel consumption reduction from 1,7l/h to 0,3l/h.
The emission reduction:
CO1: from 2.1 to 0.13
CO2: from 2.4 to 0.34
O2: from 6.2 to 13.3
You can build this with a couple of radiator pipes, some tubes and other cheap parts.
Read full description, manual, instructions at: