Being a relative neophyte when it comes to hybrid technologies, I got excited when Eaton Corp. invited me to its proving grounds in Marshall, Mich. to take a spin in one. I’m familiar with some of the technology and the anticipated applications of hybrid power systems, but when the Eaton people put me behind the wheel of a Peterbilt Model 320 refuse collection truck, I was, to put it mildly, amazed at what an hydraulic launch assist (HLA) could do.

Simply put, this HLA system uses a pressurized hydraulic accumulator tank to help brake the vehicle and then launch it again from a stop, using energy recovered during braking. It works like this:   

Regeneration:

During braking, the vehicle’s kinetic energy drives the pump/motor as a pump, transferring hydraulic fluid from the low-­pressure reservoir to a high-pressure accumulator. The fluid compresses nitrogen gas in the accumulator and pressurizes the ­system to a maximum of 5,000 psi, Eaton says. The regenerative braking captures about 70 percent of the kinetic energy produced during braking.

Launch Assist:

During acceleration, fluid in the high-pressure accumulator is metered out to drive the pump/motor as a motor, propelling the vehicle by transmitting up to 2,550 lb ft of torque to the driveshaft.

In hybrid-speak, this type of system is known as a “parallel“ hybrid hydraulic system. The vehicle‘s powertrain is supplemented by the addition of the hydraulic system. Parallel systems are best suited to vehicles operating in heavy stop-and-go duty cycles, such as refuse trucks and buses, where energy previously turned into heat during the braking cycle can be recovered and reused to launch the vehicle from a stand still.

The other option is a “series“ hybrid hydraulic system, where the conventional drivetrain is replaced by the hybrid system, as in energy is transferred from the engine to the drive wheels through fluid power.

Efficiency is improved by operating the engine at a “sweet spot” of best fuel consumption, and using a hydraulic ­modulator to control wheel speed, and through energy recovered using regenerative braking.

Clearly, certain applications lend themselves better to one hybrid system or the other. Neither of them — at this point in time — are particularly well suited to over-the-road applications, but that could change as the technology evolves.

Eaton provided three vehicles for the test drive, the Peterbilt 320 refuse truck, a Ford E450 passenger van typical of ones you see shuttling weary air travelers around the airport, and an International P&D chassis. The first two had hydraulic parallel systems installed; the delivery van had an electric version of the same system, using batteries to store the energy recovered from braking rather than an hydraulic accumulator.

The systems are the same in principal — once-wasted energy is recovered by either a battery or an accumulator and put to productive use, reducing fuel consumption — but performance characteristics of each system lend themselves better to different applications.

Hybrid electric systems (HEV) have much higher energy storage capacity, but lower power output capabilities. HEV systems are often found on utility chassis, and use the recovered energy to power on-board equipment without running the engine. Hybrid hydraulic systems (HHV) typically regenerate more energy than the electric systems and they have higher power output capabilities, but for shorter duration, i.e., until the accumulator is discharged.

Stop ’N Go:

Driving the passenger van first, I got a taste of the what the system can do. Traveling at about 50 km/h, I applied the brake, and barely had to touch the pedal. The hydraulic system did most of the work.

Like an engine brake, the pump and accumulator pulled the truck right down to idle.

Launching was cool. I just touched the throttle pedal — not really applying any throttle — and away the van went, with far less noise than a traditional start up. It got up to about 40 km/h on its own before any throttle was needed. That was in economy mode. I fear passengers might complain about a power-assisted launch with an aggressive driver at the wheel. Performance mode is quite dramatic.

The refuse chassis performed equally well, equipped with a higher-capacity system. The retarding force during braking is terrific. I think this could put an end to the squealing brakes common to heavy stop-and-go applications. This additional retarding force can more than double brake life, Eaton claims.

I got the truck up to 40 km/h without the throttle, but admittedly, I wasn’t racing from driveway to driveway. In the real world, I expect drivers to use power mode to get the job done faster. It might be possible to hit half a dozen or more driveways without ever touching the throttle.

Once the initial stop has been made, the accumulated energy is reused to move the vehicle to the next driveway, where the subsequent braking event would recharge the system, and on you’d go ‘til the next street. The system is said to be 70 percent efficient, so some power would be required at some point, but it seems to me, it would be minimal.

The shuttle bus performed similarly. Given the way those drivers operate the buses, they’d be accumulating hydraulic power as they make their way around the airport, and when the accumulator fully charged, they’d be able to launch without the aide of the engine, theoretically. In my experience those drivers would stay in power mode, and keep all the passengers at the back of the bus by virtue of the G-forces experienced at launch. 

Eaton is claiming fuel economy and emissions reductions benefits in the 20-30 percent range and a 2-3 year payback is possible with this technology.

As we’ve lamented before, the cost of systems such as this make broad market penetration a challenge in Canada. The U.S. offers substantial incentives to offset the cost of these systems, but here in Canada, the best we can come up with are carbon taxes. www.todaystrucking.com/news.cfm?intDocID=20066

So don’t hold your breath waiting for the garbage truck on your street to be a hydraulic parallel hybrid. Did I mention, the extra boost of power from the accumulator eliminates a bunch of engine noise too? I’d pay for a little quiet at 7:00 a.m. when my garbage truck comes by.

Drivers can select the level of retarding capability, and
a status bar show the level of accumulated charge.