Mack Trucks was one of several manufacturers announcing breakthroughs for commercial hybrid operations. Source: Mack Trucks. ENLARGE image

Sometimes procrastination pays off. A year and a half ago, hybrids had barely proven themselves in light-duty operations, were being used experimentally in medium-duty commercial operations, and were virtually nonexistent in Class 7 and 8 applications. Although some cities operated hybrid buses, hybrids were primarily a consumer-market phenomenon.

So I put off writing about them.

Then Arvin Meritor, Eaton, PACCAR, Freightliner/Sterling, International, and Mack/Volvo began announcing breakthroughs in their commercial hybrid operations, leading me to predict that heavy-duty hybrids will be cost-effective within five years—sooner, if the EPA provides sufficient incentives.

Many of us will be driving hybrid pickup trucks and SUVs well before that. The Big Three automakers already market Class 1 pickups and SUVs, albeit at a healthy premium.

With improvements in hybrid technology and higher-than-average gas prices, the greatest potential for fuel savings may be in Class 3 through 5 vehicles. As is any vehicle using a power take-off (PTO), they're well suited for hybrid technology. Hybridization enables the PTO to operate electrically when the engine's off. When the battery runs low, the engine automatically recharges it; then shuts off when it's fully charged, usually in less than five minutes.

Most Class 7 and 8 vehicles are used in over-the-road (OTR) operations, where steady state driving cannot, it was believed, take advantage of the primary benefit offered by hybrids: the ability to capture and store energy when slowing or stopping. Heavy-vehicle hybrid power plants were mostly used by refuse packers and other vehicles making multiple starts and stops. Then Wal-Mart started evaluating hybrids in OTR use and found that even when coasting down a slight downgrade, energy could be recaptured. But I digress.

Hybrid History

Just so we're all on the same page, let's define what “hybrid” means and how hybrids work.

Technically, any vehicle that uses two or more types of power is a hybrid. Introduced in 1934, diesel-electric locomotives had the first hybrid engines, which operated at the most fuel-efficient speeds regardless of load. A generator converted mechanical energy to electricity that powered motors at each drive wheel, eliminating the need for transmissions and differentials that cause energy-consuming friction.

Not all hybrids combine internal combustion engines with electric motors, although that's where most manufacturers focus their development efforts. Mines have used diesel-hydraulic hybrids for decades. The engines run pumps that pressurize hydraulic fluid, which drives hydraulic motors at each wheel.

Some vehicles have hydraulic accumulators that use the resistance of oil under pressure to recover energy during braking and release it as needed. Hydraulically stored energy can be “slammed in” for quick bursts of acceleration, and rapidly absorbed for emergency braking. Refuse vehicles use this system because it works well in applications that require hundreds of starts and stops daily.