Driving Modes
Gas only: Intended for highway driving
Air only: For speeds below 43 mph, where the engine stops and hydraulics power the wheels 80 percent of the time
Combined: Powered by the gas engine and hydraulic motor
Feasibility
Hydraulic systems are an established technology. The motors and pumps are simple, relatively low-cost, and easy to package—as are epicyclic gears—so the Hybrid Air could be lighter and cheaper than a gas–electric hybrid. And unlike chemical batteries, hydraulics have fewer recycling issues.
Roadblocks
With no big battery to run the air conditioning at standstill, the engine would have to run, using gas; a crash-safe accumulator large enough to hold the requisite amount of pressure (up to 4351 psi) might be heavy enough to negate any benefit. With unproven reliability, hydraulic systems could find it difficult to gain consumer acceptance.