A growing demand for machines that cost less to operate, together with regulatory pressures for lower emissions, are leading construction-equipment manufacturers to electric and hybrid drives over traditional hydraulic and mechanical ones.
Hybrid drives typically let OEMs use smaller diesel engines and operate them at speeds where they are most efficient and run cleanest. And the drives can recover and reuse energy that was previously wasted as heat — which means smaller cooling systems.
But the eye-popping advantage is lower fuel consumption over machines with more-conventional powertrains — which saves money and lessens environmental impact. And hybrid machines can offer productivity gains and often require less maintenance. Here’s a look at some recent developments.
Caterpillar, Peoria, Ill., is set to begin delivering the D7E, said to be the industry’s first electric-drive, track-type tractor. It eliminates the torque divider and mechanical transmission found on the company’s D7R, instead using the diesel engine to drive a brushless-ac generator.
Electric current from the generator routes through a solid-state power inverter and to ac-electric motors connected to the differential steering system that propels the tracks. The propulsion module has no moving electrical contacts and relatively few moving mechanical parts. Also, the motors are sealed and liquid cooled, making them suited for the same rugged environments as today’s tractors. According to Cat officials, the system delivers long life with low maintenance.
The inverter also supplies dc power for accessories such as heating, air conditioning, ventilation, and the water pump. Thus, the D7E has no engine belts, which reduces parasitic power drag on the engine and downtime for belt replacement.
Among other reported benefits, D7E’s electric drivetrain generates high torque at low speeds and, because there are no friction clutches, driveshafts, or belts — and fewer moving parts overall — it more efficiently transfers engine power to the ground. And with no gears, there are no peaks and valleys in power-train efficiency — the continuously variable electric drive is efficient over the entire operating range.
Because the engine drives a generator instead of a transmission, it doesn’t rev as high or lug as low. It runs in a narrower, more-optimal range between 1,500 and 1,800 rpm, compared with 1,600 to 2,200 rpm on the D7R. That improves fuel efficiency and lowers emissions. And the system doesn’t generate as much heat, reducing cooling loads and fan power requirements.
Compared to the D7R Series II, the new D7E is 10% more productive and burns up to 20% less fuel. So in a typical dozing application, it will move about 25% more material per gallon of fuel, say Cat officials. It is also expected to reduce lifetime operating costs by about 10%.
In addition to environmental benefits, the D7E meets a growing demand for powerful yet highly maneuverable track-type tractors. More and more dozers in this class are used for site-development work that demands a mix of straight dozing power and tight maneuverability, and it is difficult to balance these two capabilities in a single machine. The electric drive delivers both heavy-dozing and fine-grading performance, according to Cat.
Volvo Construction Equipment, Asheville, N.C., is developing the L220F Hybrid wheel loader that company officials say will offer more power, better performance, and the potential to reduce fuel consumption by up to 10%, compared with current versions. This not only reduces fuel costs but also provides significant environmental benefits — such as lower CO2 emissions.
Volvo’s design is a parallel-hybrid system. Both an electric motor and the diesel engine connect to the mechanical transmission and propel the vehicle. (In a series-hybrid design, on the other hand, the diesel engine does not connect to the driveline but instead powers an electric generator that feeds electricity to drive motors.)
The heart of the L220F Hybrid is an electric motor called an integrated starter alternator motor (I-SAM). Fitted between the engine and transmission, the I-SAM is coupled with a lithium-ion battery with a several hundred watt capacity.
The I-SAM is basically a powerful starter motor with sufficient capacity to propel the loader, on its own, from a standstill up to a speed where the diesel engine takes over. A CPU governs gear shifting and the power split between diesel and electric power. The I-SAM also runs as a generator/ alternator for recharging batteries and regenerative braking.
The I-SAM overcomes the inherent problem in diesel engines of low torque at low engine speeds. Traditionally, the operator revs the engine to get more torque, but the electric motor delivers high torque from standstill, which gives quick response at lower engine rpms. The I-SAM generates up to 700 N-m of torque and 50 kW of peak power.
And because the diesel engine is not needed for the initial (peak) burst of energy, a smaller engine more suited to the average energy demand of the equipment can be used. This lets it operate in a range that reduces harmful emissions, such as NOx.
In many applications, up to 40% of a wheel loader’s time is spent idling. When stationary, the hybrid system shuts down the engine. Thanks to an electrically powered (not belt-driven) a/c compressor and fan, the electronic and climate-control systems can run for up to an hour even with the engine off. The I-SAM restarts the engine as soon as the operator hits the throttle.
All this lets the engine remain off when it would otherwise be idling — and the operator doesn’t need to overrev to get sufficient working torque. Thus, the biggest fuel savings from diesel-electric hybrids in the construction industry are likely to come from machines that undergo frequent starts and stops. Early indications are that hybrids need no more maintenance than diesel-only equipment, and electrical regenerative braking will likely reduce wear on service brakes.
LeTourneau Technologies, Longview, Tex., uses diesel- electric drives in its 50-Series loaders and dozers. Ac generators replace gears, transmissions, and torque converters, while electric-drive motors eliminate axle components such as differentials, universal joints, and driveshafts found in mechanical drives.
A major benefit is better fuel efficiency, says the company. The electric drive lets the engine operate at a near-constant rpm, eliminating the constant acceleration and braking of massive rotating engine components associated with mechanical drives. Lowering these fuel-robbing inertial factors reduces fuel burn by up to 20 gallons/ hr, depending on model and working conditions. Savings can easily exceed $100,000 annually, according to LeTourneau.
The company’s 1,050-hp, L-950 wheel loader with a 54,000-lb payload capacity, for example, uses brushless switched-reluctance (SR) motors, high-power insulated-gate bipolar-transistor (IGBT) switches, and digital controls. The rotor has no magnets or windings; stator poles contain a winding, similar to the field of a dc motor. The durable design limits motor maintenance to replacing two bearings every 20,000 to 30,000 hr. Independent fourwheel drive with wheel-speed control also improves traction and limits slippage in poor operating conditions, which can double tire life versus conventional loaders, say company officials.
In terms of performance, the SR traction motor offers a virtually flat torque curve. This means high torque at low speeds to minimize stalling, plus consistent power at any speed. The result, says the company, is faster response to operator inputs, greater digging power, and better overall performance versus mechanical-drive equipment. In addition, electric drives do away with the need to shift gears, which improves cycle times and reduces operator fatigue.
A hydraulic motor normally turns the upper structure of an excavator, but several manufacturers are switching to hybrid drives. In these, the diesel engine powers an electric swing motor that also runs as a generator when the turntable slows, storing braking energy to help the engine accelerate the upper deck on the next move.
At the recent Intermat 2009 in Paris, Komatsu, Rolling Meadows, Ill., announced availability of its PC200-8 Hybrid crawler excavator to international markets. The excavator was launched a year ago in Japan and about 30 units have been sold to date.
The diesel engine drives a generator while an electric motor turns the upper structure. The proprietary Komatsu Hybrid System also generates electricity when the upper structure slows while turning, stores it in a capacitor, and uses it to assist the engine via the power-generation motor when the engine accelerates.
The machine uses a capacitor because they instantaneously collect, store, and discharge electricity, making them a better fit than batteries for construction equipment, according to Komatsu.
Company officials report the design cuts CO2 emissions and uses about 25% less fuel than a standard PC200-8 hydraulic excavator. And on one unit, at a sludge-disposal job site where the machine’s upper structure turns frequently, fuel consumption dropped 41%.
Case Construction Equipment, Racine, Wis., unveiled a prototype hybrid materials-handling excavator at Intermat. The elevated-cab scrap handling machine will potentially reduce fuel consumption by up to 40%, with a substantial cut in exhaust gas emissions, according to Case.
The unit’s diesel engine powers hydraulics for the boom and track drive, and also drives an electric generator which, in turn, powers an electric slew motor and scrap-handling magnet.
Under low loads, high-efficiency capacitors store surplus electricity. As loads increase, the electrical system complements engine output, allowing for near-constant engine speeds that reduce fuel consumption and noise.
In repetitive operations, such as scrap handling, where machines continually turns from side to side, electric swing motors use less energy than hydraulic motors and regeneration stores excess electricity in the capacitors for later use. Integrating the magnet generator into the hybrid drive, rather than powering it via the hydraulic pumps, also takes advantage of stored electrical energy.
Doosan Infracore, Suwanee, Ga., has also begun developing a hybrid excavator with the goal of cutting CO2 emissions and fuel consumption by 35%. The excavator will convert excess power from a diesel engine during rotation and other operations into electricity, and store and use it to augment engine output during high loads.
Doosan expects it will save about $14,000 in fuel per unit per year thanks to better fuel efficiency, based on a 22 ton-class excavator operating 10 hr/day, 200 days/yr, with diesel priced at $1.04/liter. Project partners include electric-motor maker Komotek, power and electronics parts producer ADT, Seoul National University, and Aachen University of Technology in Germany.