Another Google search pointed me to these web sites:
Lightning Hybrids uses a bio-diesel engine to power its car. That's a good choice for a hybrid because a diesel running at constant speed is an efficient power producer. You don't see more diesels in conventional cars because gasoline engines have better performance at low speeds. That better performance isn't necessary in a hybrid car because you use the high-torque electric, or in this case the hydraulic, motor at the low speeds.
Even though Lightning Hybrids' 100 hp engine is capable of deactivating unneeded cylinders, it still seems a bit large and heavy at 100 pounds. My Prius' gasoline engine has a 70 horsepower rating, and I feel it has plenty of power. The 2010 Prius model sports a 98 hp engine. In my Prius, the gasoline engine and the electric motor work together to produce a net horsepower of 110, the 2010 model combination produces 134 hp.
I've read that only about 40 hp is really necessary for most cars at 70 mph, so the excess must be a macho thing.
Electric or Hydraulic? Which is more efficient in delivering the fuel energy to the wheels? Which provides better regenerative power storage: batteries or accumulators?
The accumulators are sure to win the green contest. They're just a high-pressure cylinder with an inner spool separating the oil from a compressible noncombustible gas, maybe something as cheap and available as nitrogen.
I want to learn more about hydraulic hybrids.