Joining Forces

As a consumer, you can make changes to your daily lifestyle in order to counteract the effects of the rising fuel prices. Those options, however, don't suit the obligations of commercial vehicle users. Refuse has to be picked up, parcels need to be delivered, and the whole scope of other services still need to be provided, regardless of fuel prices.

The only practical response to rising fuel prices for nearly all of the vehicles used in these applications is to improve their efficiency, and the best technology available to do that today is hybrid technology.

A hybrid powertrain system reduces fuel consumption by blending the power generated by the internal combustion engine with power from other stored sources of energy on board. And of course, when you reduce fuel consumption, you also reduce the polluting emissions that are an inevitable by-product of burning hydrocarbon fuel.

Electric and hydraulic

There are two different kinds of hybrids, electric and hydraulic. Electric hybrids more easily support other on-board electronics and have a high energy density — best suited to applications that have a lot of cruising time because they can support longer engine-off operation.

Electric power is generated by the diesel engine and through regenerative braking, which recovers power which would normally be wasted as heat and brake wear. That power is then stored, typically in a battery pack, and delivered back to the truck through an electric motor that delivers energy back to the drivetrain, or to auxiliary power systems on the vehicle to power equipment or tools.

While electric hybrids have a high energy density, hydraulic hybrids have a higher power density which provides a better power boost for acceleration "off the line." The hydraulic hybrid systems work best with intensive stop-and-go applications such as residential refuse pick-up, because of the use of stored energy through regenerative braking.

According to NextEnergy, Detroit, MI, a non-profit corporation, a hydraulic hybrid system can recover and use up to 70% of the kinetic energy that is captured during braking. Hydraulic hybrid vehicles (HHVs) reuse stored energy that would normally be lost during braking in conventional vehicles, creating a cost effective system, while also eliminating idling and reducing engine operation for fuel efficiency.

Regenerative braking is a process by which the hydraulic kinetic energy from braking — which pumps low pressure hydraulic fluid into a high pressure accumulator — is stored for future use. As the fluid enters the high pressure accumulator, it compresses nitrogen within the accumulator, which then pressurizes the hydraulic system. The stored energy is used for accelerating the vehicle in conjunction with, or in place of, the engine.

Glenn R. Wendel and Travis W. Jackson, engineers in the Vehicle Systems Research Dept. at Southwest Research Institute (SwRI) write, "an important characteristic of nitrogen gas is that as oil is pumped into the high-pressure accumulator during braking, the gas pressure increases exponentially. Energy is stored as more and more fluid is pumped into the accumulator at higher and higher pressures." The stored compressed gas in the accumulator forces hydraulic fluid through a hydraulic motor to power the wheels.

Other benefits of hydraulic hybrid systems include the use of less-costly components, no battery requirement, and the use of composite material for reduced weight and more efficiency.

Work is being done to increase the maximum operating pressure, currently around 5,000 psi to around 7,000 psi. As accumulators increase in pressure capabilities, a hydraulic hybrid system will be able to sustain engine-off driving for longer periods of time.

"It is definitely not a question of one technology being better than the other. Hydraulic and electric hybrids simply have different ‘sweet spot applications," says Brad Bohlman, business development manager, Advanced Technology, Eaton's Fluid Power Group. "The choice of which to use in a particular application is highly dependent on the nature of the application itself, specifically the duty cycle of the vehicle in question.