Hydraulic hybrids are picking up

Dec. 13, 2007
The hybrid powertrain in a Prius won’t help much if the task is moving tons of refuse.

That’s not to say hybrid technology has no role in heavy equipment. There is a great deal of research aimed at applying hybrid techniques in big trucks and utility vehicles. But the advanced powertrains for such equipment will look a lot different than those of passenger hybrids now on the road.

Hybrid Basics
Most engineers are familiar with electric hybrids, but it pays to review a few basic principles that apply to a broader array of hybrid approaches. The fundamental ideas behind hybrid power are to capture and store energy that would otherwise be wasted during the vehicle’s normal operating mode or to store energy produced during more efficient portions of the operating cycle for use during less efficient operating modes. Both techniques have been in development since the late 90s, and are now, or soon will be, entering large-scale commercial acceptance.

The best example of the first technique is regenerative braking, in which the hybrid system captures and stores the vehicle’s kinetic energy rather than dissipating it as heat. An example of the second technique is a system that captures and stores a portion of engine output during constant-speed operation, then uses it to power equipment or systems when the vehicle is not in motion. This eliminates the need to idle the engine for long periods. The most-effective hybrid systems combine elements of both.

A hybrid system can be configured with either a parallel architecture, which means it supplements the vehicle’s conventional drivetrain, or in a series architecture, which means it replaces all or part of the mechanical drivetrain. Today’s hybrids are either hydraulic or electric, and each has certain advantages.

Hydraulic systems store energy in pressurized accumulators while electric systems store energy in batteries. Both can be configured in series or parallel architectures, and both have been extensively engineered, developed, and tested on a range of commercial vehicles.

It’s All About Duty Cycles
It is definitely not a question of one technology being better than the other. Hydraulic and electric hybrids simply have different “sweet-spot applications.” The choice depends on the nature of the application itself, specifically, the duty cycle of the vehicle.

A key difference between hydraulic and electric hybrids is the duration of the power they can supply. Hydraulic systems are power dense while electric systems are energy dense. For example, the Eaton Hydraulic Launch Assist (HLA) delivers 300 hp with a motor that measures roughly 12 18 in., but only for a matter of seconds. That’s high-power density.

An electric system, on the other hand, will deliver less power at any given instant, but for an extended period of time, measured in minutes. That’s high-energy density.

So, parallel hydraulic-hybrid power makes a lot of sense for a heavy vehicle like a refuse truck that is constantly starting and stopping. There, you can capture the braking energy and recycle it over and over to improve operating efficiency.

An electric hybrid, on the otherhand, tends to be ideal for something like a utility- bucket truck that’s driven to a job site and then sits for extended periods with the engine idl ing to run the bucket. In that type of application, you can charge the batteries on the way to and from the site and also capture braking energy. Once on-site you can shut off the engine and run the bucket on battery power until the batteries need recharging. This could be up to several hours. At that point, the engine automatically restarts, fully recharging the batteries in a few minutes.

A Well-Developed Technology
Eaton is the only hybrid-power supplier to offer both hydraulic and electric systems. In addition to HLA, Eaton’s Truck Group has developed an electric-hybrid system for medium-duty trucks. Next year, it will start delivering them on truck chassis from International Truck and Engine Corp., Kenworth Truck Co., Peterbilt Motors, Freightliner Corp., and others.

The group has produced more than 220 hybrid-powered vehicles — including package-delivery vans, medium-duty delivery trucks, beverage haulers, city buses, and utility-repair trucks — for testing and evaluation. Most of these have been put into service alongside conventional vehicles for head-to-head comparisons.

FedEx Express, UPS, Coca- Cola Enterprises, and The Pepsi Bottling Group have had some of these vehicles on the road more than three years. And 14 public utilities have taken delivery of 24 hybrid-powered repair trucks. Eaton worked with truck-body builders like Altec, Terex, and others. It is also testing in Europe with DAF Trucks, and in Asia with the Beiqi Foton Bus Co., one of China’s largest commercial-vehicle producers.

Lessons of Hybrid Development
Eaton’s involvement in hydraulic- hybrid power began in earnest in 1999 with a prototype development project for a Ford Super- Duty pickup. Along the way it became apparent an even better use of the technology was bigger vehicles that make frequent stops. That led to the current focus on refuse-collection trucks.

Among other things, the initial project specification called for a bent-axis pump, because that geometry has better efficiency at partial loads than an axial piston pump. But the system usually operates at, or near, full capacity and rarely takes advantage of the extra efficiency.

The axial piston pump is a lot easier to incorporate into a parallel architecture because of the through-shaft capability. So, production systems will use axial piston pumps.”

Eaton’s HLA for refuse trucks is an “open” configuration using a reservoir at atmospheric pressure to feed the pump and accumulator. A “closed” system using nitrogen-filled high-pressure and low-pressure accumulators also offers theoretical advantages.

In the closed system, the fluid is never exposed to the atmosphere so there is no oxidation, no ingress of dirt, and virtually no maintenance. It’s a “fill-forlife” system, which ought to be attractive to refuse-truck operators. Or so it seemed.

It turned out, however, that people who maintain refuse trucks are familiar with the conventional open hydraulics used to run the onboard systems. They preferred the open system because it was familiar.

In the same vein, hybrid vehicles must compare favorably to conventional vehicles in terms of performance and reliability. Customers won’t give those up justto gain fuel economy.

Nuts and Bolts
Component selection for a hybrid- power system is not always as straightforward as one might think. For example, most of the HLA systems for refuse trucks will use high-tech carbon-fiber accumulators. These accumulators use the same technology as the latest generation of military and commercial aircraft. They cost more than steel accumulators, but weigh about 300 lb less. The weight reduction is significant because it represents 300 lb of additional refuse per trip.

The HLA system lets trucks accelerate faster. It’s not a big difference, but HLA-equipped trucks can make more pickups. For example, if an automated side-loading refuse truck with HLA makes just 4% more stops, the incremental revenue can be $30,000 or more per year. And the customer still gets the benefits of better fuel economy, albeit a bit less than they would get by not using HLA to increase acceleration.

Hybrids also eliminate idling, a major consideration for line-haul and service trucks. An electrichybrid system can supply the energy for all of the “hotel load” used in sleeper-cabs. Right now, the energy is supplied by idling the diesel. An idling diesel can use a gallon of fuel per hour, and many states already have antiidling regulations.”

Bottom-Line Fuel Savings
Eaton Hybrid Truck reports fuel economy improvements of up to 60% over its conventionally powered test vehicles. And HLA-equipped refuse trucks have shown fuel-economy improvements of 20 to 30%, while reducing emissions by similar amounts.

The next generation of hybrid power systems promises even greater efficiency. One possibility is a series hydraulic system in which a diesel engine drives a variable-displacement hydraulic pump. The pump would be linked to a variable-displacement hydraulic motor either connected to the final drive or directly to the wheels.

The result is an infinitely variable transmission (IVT), which is more efficient than a continuously variable transmission (CVT). Unlike the CVT, the IVT does not need a clutch to get zero driving force. Simply adjusting the pump and/or motor displacement to zero disconnects the engine from the final drive.

The EPA, Eaton, and International Truck and Engine used the technology in a package-delivery truck that showed fuel economy improvements of up to 70% in laboratory testing.

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