How it works: Hydraulic hybrid vehicles
UPS created this video to explain the inner workings of its hydraulic hybrid trucks.
UPS released this video detailing how its hydraulic hybrid trucks work.
Essentially, the system uses its pressurized hydraulic fluid to motivate a pump that turns the wheels, propelling the vehicle forward. Under braking, the same pump is used to repressurize the fluid, recapturing some of the lost energy.
Once it drops below a preset level, the gasoline engine kicks in to drive a second pump, which drives the wheels, while repressurizing the system.
What's interesting about this system is that the gasoline engine doesn't appear to have a direct connection to the drive wheels, relying wholly on hydraulics to transmit the power.
Of course, it all makes sense when you see it in action.