The Era of the Hydraulic Hybrid Truck

by | Jun 11, 2013

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In heavy stop-and-go delivery and refuse collection routes, drivers are likely to brake hundreds of times each day. The act of braking is now the catalyst for making vehicles go and being environmentally green. The kinetic energy that moves a vehicle forward until something forces it to slow down or stop can be captured and reused. Usually, this energy dissipates as heat from friction in the brake linings. In hybrid vehicles that use regenerative braking, that same excess kinetic power is converted to potential energy that can either be used in the next launch or stored in a hydraulic accumulator or electric battery, until it’s needed to propel the vehicle.

In medium and heavy duty trucking, electric hybrids are not new. But hydraulic hybrids, especially in what is known as the advanced series configuration, are more recent additions and growing in importance.  A hybrid powertrain uses at least two power sources.  It can recover, store and reuse power either electrically or hydraulically. Typically one source will be the engine or prime mover. The second source can be from a battery or ultra capacitor in electric hybrids, from a flywheel in mechanical hybrids, or from an accumulator in the case of hydraulic hybrids.

The “sweet spot” of the hydraulic hybrid is the stop/start duty cycle. The enormous amount of power required to repetitively accelerate and decelerate is exactly what makes this technology so effective. A hydraulic hybrid can store large amounts of energy quicker than an electric hybrid can during a braking event. And so stop-and-go delivery and refuse collection trucks are particularly well suited to utilize hydraulic hybrid drivetrains.

The fuel efficiency that hybrid vehicles can achieve is impressive, and has been proven by the National Vehicle and Fuel Emissions Laboratory of the US Environmental Protection Agency (EPA) with its industry partners for many years. Local municipalities and private refuse haulers, as well as parcel delivery companies, have begun to incorporate these vehicles into their fleets. For refuse vehicles, results in the field document an average of 43 percent savings in fuel consumption annually and improved engine efficiency over all other options for refuse vehicles in the market today

The Growing Market Need

The increase in the cost of fuel has put pressure on fleet owners to control the costs of their operations. Delivery companies are attempting to meet ambitious commercial vehicle fuel efficiency (fleet miles per gallon) targets.  They are looking to cutting edge technologies either through alternative power sources or alternative drivetrain configurations to help them meet those targets.

Delivery companies, such as FedEx and UPS, have converted portions of their fleets to alternatives and are looking to increase those numbers.  But shifting into alternative technologies is never done lightly and researching and testing the efficacy of available options can be daunting.

Municipalities also have much to gain by considering technologies that achieve meaningful cuts in their operating budgets. But to make a commitment to change, fleet managers and other decision makers first need assurance regarding performance, reliability and life cycle. In many cases they want to pilot the technologies in their fleets for real-world validation of fuel efficiency and other performance benefits.

A growing number of local governments are reviewing data, piloting trials and purchasing their own vehicles. The results from series hydraulic hybrids are undeniable, in the form of fuel savings, extended brake life, improved working conditions and increased productivity.

When a diesel engine burns fuel it also releases emissions into the atmosphere. Assuming annual fuel consumption of 8,600 gallons for a typical Class 8 refuse vehicle (depending on route density and operating conditions), 48 tons of CO2 emissions could be avoided in 12 months, at a 50 percent level of fuel reduction. That is the equivalent of the carbon removed by 1,100 10-year-old trees, according to the EPA’s greenhouse gases calculator.

In addition, because the hydraulic hybrid’s regenerative braking system limits traditional friction braking, brake replacement cycles are reduced from 3-4 change intervals per year, to twice in the life of the truck  (average refuse truck life time of 10 years). The corresponding cost reductions for reduced brake maintenance equates to tens of thousands of dollars.  Overall vehicle performance and noise levels are being documented as well, and drivers report a smoother ride with improved productivity.

Other than paying their drivers, nothing costs fleet owners more than the cost of fuel. The EPA calls the series hydraulic hybrid “the lowest cost, most fuel-efficient power train that exists for heavy-duty vehicles.” Miami-Dade County and the cities of Miami and Hialeah in Florida bought the first advanced series hydraulic hybrids, and have led the way in establishing the parameters for viability of the hydraulic hybrid alternative and documenting the results for the benefit of the second generation of users. Their successes are helping to bring the technology to the attention of private haulers such as Advanced Disposal, Waste Industries and Marin Sanitary Services, which have added the first hydraulic hybrids to their fleet.

The business case for alternative vehicles is compelling, and data continues to be collected across all of the sectors. There is no denying that what is good for business is also good for the environment and reduces our oil dependency significantly. It is only just the beginning of what business and government working in partnership can accomplish as they continue to chart new territory in managing the use of fossil fuel.

Tom DeCoster is HDS business development manager for Parker Hybrid Drive Systems.

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