Can Hybrid-Electric Drivelines Be a Solution for Off-Highway Vehicles?

May 11, 2016

Gunnar Stein

Packaging of Oerlikon Fairfield’s hybrid-electric drivetrain system illustrated in a front loader.

Today's construction, agricultural and mining machinery is driven almost entirely by high-performance diesel engines. Although these units are relatively economical, off-highway vehicles still account for a significant proportion of the world's fuel consumption. According to a study conducted by the Swiss Federal Office for the Environment (FOEN), this vehicle category accounts for a whopping 19% of annual diesel consumption. For companies in the construction, agricultural and raw materials industries, high consumption is reflected in higher operating costs. As a result, demand for low-consumption off-highway vehicles is trending upward in both industrial nations and emerging markets.

With Oerlikon's hybrid drivetrain, the diesel engine has no direct drive to the wheels but instead provides a generator with power. Power inverters convert the electricity produced by the generator and feed it into electric motors in the front and rear axles that drive the wheels.

A more efficient option comes in the form of hybrid-electric driveline systems. In themselves, they are nothing new and have been long established in the automobile sector. But even giant dump trucks have been driven by hybrid diesel-and-electric combinations for many years. And recently, hybrid electrically powered wheel loaders and dump trucks have started appearing on the market. In the extended off-highway market, systems like these offer an efficient alternative to conventional drivelines. Operating under the Oerlikon Fairfield brand, Oerlikon’s Drive Systems Segment has now developed a hybrid-electric powertrain for off-highway vehicles.

Driveline configuration

Gunnar Stein, Chief Technology Officer in Oerlikon’s Drive Systems Segment.

In this innovative driveline concept, the diesel engine has no direct drive to the wheels but instead provides a generator with power. Power inverters convert the electricity produced by the generator and feed it into electric motors in the front and rear axles that drive the wheels. The drivetrain features much better efficiency than traditional torque converter drivelines. It also enables energy recovery and allows the engine to run in best fuel-economy operating conditions. This means fuel savings of up to 30% overall. On top of this, a smaller and lighter combustion engine is sufficient to generate the power needed.

But the increased efficiency in the drivetrain is not the only advantage. As a rule, off-highway vehicles usually have to carry out a number of additional functions, such as manipulating a shovel. Operating these functions today calls for complicated hydraulic systems with relatively low efficiency. Now, these auxiliary functions can be electrically driven or even be replaced by electric actuators and motors. Hydraulic systems can be simplified and the system efficiency can be improved due to a higher flexibility.

An important benefit is the system's ability to store energy while operating. The energy regenerated when a heavy construction machine brakes, for instance, can be stored in a buffer. Also, the engine can charge the buffer in low load phases and the stored energy can be used later to even out consumption spikes or drive auxiliary functions. To achieve this, Oerlikon’s Drive Systems Segment makes use of so-called supercapacitors.

These efficient energy storage units can handle significantly more charge cycles and are also able to store and redistribute energy faster than a conventional lithium ion battery. The added efficiency is particularly advantageous when a construction machine completes a large number of stop/go cycles.

Modular hybrid-electric drive

Oerlikon’s Drive Systems hybrid-electric drive is modular in design and comes with all the requisite components. These include the generator, power inverter, electric motors, end drives and capacitors as well as the control unit and software. The system is thus able to accommodate the wide range of demands placed on a driveline solution by various types of vehicles and the conditions under which they operate. While a major producer may require only individual components, small OEMs in particular usually need a complete drive solution. The entire driveline or individual parts of it integrate smoothly with various types of vehicle design.

Below the line, a hybrid drive entails higher upfront costs, but the additional investment is usually amortized within a two-year period of operation. And the benefits for end customers are compelling: lower operating costs, increased productivity and hence higher profitability. As a result, the solution has enormous market potential. Hybrid-electric drivelines will replace a large share of the applications dominated by conventional hydrostatic drives and torque converter drivelines.

Gunnar Stein, Chief Technology Officer in Oerlikon’s Drive Systems Segment, wrote this article for Off-Highway Engineering as part of the annual Executive Viewpoints series appearing in the June 2016 issue.