Great idea! I like to see people trying new things in search of the best fuel economy. Your idea reminds me of the old PTO (Power take off) on a tractor engine. Sometimes used to run added equipement or a Hydraulic pump.
Just a thought here about the cost of added weight and cost of hydraulic valves, hoses, and motors.
Would it be a more cost effective solution to use compressed air rather than hydraulics as a storage medium?
I have seen the compressed air powered cars do quite well in short haul applications. And pnuematic parts are lighter and much less expensive.
Here's a good link to Hydraulic Hybrid "How stuff works"
HowStuffWorks "Parallel and Series Hydraulic Hybrids"
Good luck with your project
