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Mack Trucks was one of several manufacturers announcing breakthroughs for commercial hybrid operations. Hybrid ready-mix and dump trucks could be available in five years.

Procrastination sometimes pays off. Last fall when I started researching hybrid vehicles, I found that these energy-saving trucks and vans had been proven successful in light-duty operations. But many experts were still skeptical regarding medium-duty commercial operations. Hybrids delivered a full measure of benefits in city delivery, bus, and parcel express applications, but they weren't yet ready for the heavy-duty world in which producers work.

So I put off writing this article, hoping for a breakthrough.

In mid-January, ArvinMeritor, Eaton, Freightliner, and Mack/Volvo all announced breakthroughs for commercial vehicle hybrid operations. Their investment leads me to predict that we'll have hybrid ready-mix trucks and dump trucks plying our construction sites within five years. This could happen even sooner if there are sufficient incentives from the U.S. Environmental Protection Agency to encourage rapid investment.

One thing is for sure: Many of us will be driving hybrid pickup trucks or SUVs well before that.

Just so we're all on the same page, let's define what “hybrid” means and how they work. In its purest definition, technically, any vehicle that uses two or more alternative types of power is a hybrid.

Diesel-electric locomotives, first introduced to the railroads in 1934, were our first hybrids. The design allowed engines to operate continuously at their most fuel-efficient speed, regardless of train speed. A generator converted diesel power to electricity, used directly in motors at each drive wheel. No transmissions, no differentials where energy could be lost. But unlike modern hybrids, these devices didn't try to conserve unused energy. A train's battery system was used for starting the diesel engine and lighting only. Nor were they part of a reclamation system that recovers energy and stores it as electricity.

In the recent past, hybrid engine technology combined other energy sources. Diesel-hydraulic hybrids have been operating in huge mining vehicles for decades. They use diesels to run pumps that pressurize hydraulic fluid. The fluid drives hydraulic motors at each wheel.

But today, hybrid engine technology is focused on efficiently combining internal combustion engines with electric motors.

Hill climbing

The principle of reclaiming energy makes hybrids fuel-efficient.

Development units have shown up to 50% more miles per gallon in commercial service. Diesel-electric hybrids, most common today, integrate motor-generators into vehicle architecture, usually within the transmission housing. There are also diesel-hydraulic hybrids that convert braking forces to hydraulic pressure. Pressurized hydraulic fluid is stored in accumulator tanks, to be released to assist acceleration and to meet increased power demand for climbing hills.

Whether electrical or hydraulic, during braking the vehicle's inertia drives the unit. The vehicle's kinetic energy is converted to electrical energy or hydraulic pressure that can be stored. Without such a storage mechanism, kinetic energy must be converted to heat by the brakes, then that energy is transferred and lost to the air as the brakes cool.

In extreme duty applications, almost all new systems are diesel-electric. When additional power is needed for start-up, acceleration, or hill climbing, electronic controls reverse the motor-generator's functions. It becomes a motor, adding power to the driveshaft. Using reclaimed power from the battery bank gives hybrids outstanding fuel economy and good performance with smaller engines.

When the world's first gasoline-electric hybrid, the Toyota Prius, went into production a decade ago, it depended on lead-acid batteries and comparatively crude computers to control the energy reclamation process. Because of the batteries' weight, this hybrid technology was practical in small cars, but not big trucks.