It uses a wood gas generator to burn coffee grounds, thus creating combustible gas that is filtered before going to the engine.
So, where does one get enough coffee grounds to provide reliable fuel stock? Starbucks does give coffee grounds to anyone who asks for them. But since coffee grounds make excellent fertilizer and compost material, much of it is already spoken for.
More importantly, how many MPPCG (Miles Per Pound of Coffee Grounds) does it get?