Hybrids On the Hill
tractor) featuring technologies we've read about. These included electric and hydraulic hybrids, not only for the propulsion but also for auxhiliary loads.
These make the most sense in stop-and-go applications. Every time a vehicle comes to a stop, kinetic energy is converted into another form. Traditionally vehicle brakes have done so by converting that energy into heat and dissipating it to the
atmosphere, a process that wastes a lot of energy.
What a hybrid system does, whether electrically, hydraulically, or in some other means, is capture that energy for later use.
In some of these trucks that energy can be used for propulsion as well as for PTO (Power Take-Off), such as powering the hydrualic ram in a refuse truck, power tools at a work site or the lift bucket for power crews. This work can be done using stored energy rather than idling the engine to produce the power.
Hybrid electric trucks use the same operating principles as those of most hybrid cars. They capture braking energy and convert it into electricity, which is stored in batteries (or capacitors).
On the other hand, hydraulic hybrid trucks capture braking energy via a hydraulic pump and two connected accumulators (tanks which store hydraulic fluid under pressure). One tank is a low pressure tank, the other a high pressure one. When the vehicle slows, the pump forces more fluid into the high pressure tank, increasing the stored energy for later use (see the Parallel Hydraulic Hybrid diagram).
To my knowledge, the vehicles on display were parallel hybrids and the sense was that series hybrids are on a longer time horizon [correction: the UPS parcel delivery van present was a series hybrid, thanks Eric].
The main issues with implementation, of course, are reliability and Return-On-Investment (ROI). These vehicles have yet to be deployed in large numbers so there are questions about how they'll perform in the real world, if they'll deliver the
expected benefits. Further, because of the low production volumes the costs are still considerably more expensive than conventional vehicles.
It's a classic challenge. That's why it's important for policy makers to help make the hump smaller and encourage industry to find the answers to the two questions above.