Hybrids On the Hill

Last week I went to a Hybrid Truck exhibit and briefing near Capitol Hill organized by HTUF (Hybrid Truck Users Forum). On display were about a dozen trucks for various vocations (refuse trucks, school buses, delivery vans, and a long-haul truck
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.

Gasoline Buffers

The main obstacle of electric vehicles is energy storage media. A gallon of gasoline or diesel fuel is about 6.5 lbs. The equivalent weight in batteries is in the neighborhood of 205 lbs. This is akin to carrying a memory stick with gigabytes of capacity compared to reels of film or floppy diskettes.

But there are many advantages to electric power such as the ability to decouple vehicle speed from engine speed, energy recapture during braking, silent operation, and no emissions when operating in electric mode. These factors add up to considerable potential energy savings.

This is why there are numerous projects aimed at developing series hybrids (the Chevy Volt being the first that comes to mind). In other words gasoline and diesel are excellent energy storage media while electric powertrains are much more efficient. So the challenge is to somehow combine the better aspects of the two in order to achieve a superior solution, and putting in place the infrastructure to support pure electric power when the energy density of batteries (or capacitors, etc) have matured sufficiently.

Right now that appears to be a revolutionary chassis combined with a small IC engine and electric drivetrain. By that I mean cars that are dramatically different and lighter than existing vehicles. Conventional designs fitted with electric power wouldn’t be nearly as much of an improvement. We should strive for major gains by thinking further outside the box.

The Future Is Electric

Tomorrow’s cars and trucks will be powered by electricity. Why electricity? Because electricity is a common denominator, a form of energy that all others can be converted to and stored in a battery, whether it’s petroleum, coal, natural gas, hydroelectric, solar, wind, or nuclear. That standardizes the infrastructure and puts all sources on even footing, allowing them to be compared to one another on the same basis in terms of cost, emissions, and efficiency. Plus electric vehicles are quiet, don’t idle, and can easily capture otherwise wasted energy with regenerative braking.

There will be many interim solutions between now and then. Parallel hybrids were the first step. These were cars like the Toyota Prius which used a convention internal combustion engine (ICE) augmented by battery electric power. Next we have the PHEV (Plug-In Hybrid Electric Vehicle) which lets owners plug into an electrical outlet charge the batteries, as well as drive them in pure electric mode for short trips.

Next we’ll see series hybrids such as the forthcoming Chevrolet Volt. Not only is it a PHEV but it’s also a series hybrid in the sense that that ICE is only used as a generator to charge the batteries.

Electric isn’t the only means of energy storage used in hybrids either. Hydraulic pressure is one alternative being used on larger vehicles such as refuse trucks and delivery vans. Hydrogen and fuel cell development continues.

But ultimately it seems the most direct (and final) solution is pure electric vehicles with batteries that are capable of providing performance, range, and recharging comparable to today’s vehicles equipped with ICE engines.