Hybrid systems bridge the road to electrification with small batteries and readily available components.
Electrification aims to eliminate vehicle emissions that degrade air quality and negatively impact public health. From a technical standpoint, electrification makes sense for light- and medium-duty vehicles, such as cars and small construction equipment. However, electrification is more complex for larger vehicles in high-power and high-range applications where maximizing payload is critical.
Many jurisdictions have ambitious electrification goals for heavy-duty vehicles, such as refuse collection vehicles, which contribute significantly to overall emissions. Though the push for electrification is noble, it’s difficult to overhaul an entire fleet of engine-based refuse trucks without the supporting infrastructure. It’s also difficult to part with hydraulics for electric alternatives that add weight and maintenance.
Refuse vehicles like the automated side loader (ASL) are designed around the truck chassis and demand more from electrification than passenger cars. For these vehicles, the primary energy source powers the driving, load transportation, and hydraulic functions. A traditional ASL carries about 15,000 lb of garbage and uses a hydraulic system to pick up 1,000 to 1,500 trash cans daily while compacting the load. Electrifying this vehicle is possible but requires a large battery, which decreases the truck’s payload.
For such applications, an electric hybrid system combines the advantages of electric solutions, such as smart energy usage and lower emissions, while capitalizing on the performance and range of a traditional diesel engine.
Parker Hannifin Corp. proposes a hybrid system that can be implemented with readily available components and a small battery pack without revolutionizing the chassis architecture. The system relies on the diesel engine for driving and an electric motor for hydraulic functions. The idea is to optimize components based on operation conditions to reduce emissions without compromising payload.
Let hydraulics do the work
Parker’s electric hybrid design uses a traditional ASL chassis and doesn’t replace the engine with an oversized battery. It also doesn’t replace hydraulic functions with electromechanical alternatives.
Hydraulics technologies have advantages that electromechanical actuation lacks in ASLs and other refuse collection vehicles. First, hydraulic cylinders are generally lighter in weight than electromechanical assemblies. If an ASL has a linear actuator and motor, the arm must lift the added weight of the assembly in addition to the load. “Even if the system is more efficient because there are no hydraulic losses, if you think about lifting 1,000 cans per day with the added weight, that’s a lot of energy wasted,” said Germano Franzoni, Senior Systems Engineer at Parker. “The power density, especially for suspended weight, is very important.”
Another advantage of hydraulics is the ability to distribute power among different functions. For example, if an ASL has a hydraulic system of 40 hp and five functions that run continuously, a valve can split the power from a single pump and distribute it among the five actuators. Electromechanical assemblies can’t split power like hydraulics and might need up to 120 hp to perform the same work.
Also, hydraulics are accustomed to corrosive and dirty environments, whereas electromechanical actuators are more delicate and less tolerant of such operating conditions. “The number one maintenance item in refuse vehicles is the cylinders because the duty cycles are very intense, and there’s a lot of shock and vibration,” said Franzoni. “If you want to change a cylinder or rework the seals, it’s a relatively quick and inexpensive maintenance item. But if you need to replace an electric actuator, that’s a much more complicated part in terms of cost and maintenance.”
A traditional ASL collects garbage cans with a hydraulic arm, and the packer continuously runs to compact the load inside the vehicle body. During collection, the pumps rotate at low speeds, and the truck engine is idle and loaded to prevent stalling. This operating condition is inefficient and produces noise and higher emissions.
With Parker’s proposed electric hybrid system, the engine isn’t loaded while idle. When the ASL stops at a house to pick up a garbage can, an electric motor runs the hydraulics and relieves the engine, which increases efficiency and reduces fuel consumption, emissions, and noise.
“When the engine works at idle, it’s the worst point for efficiency and emissions,” said Franzoni. “We think hybrid is the right solution, where hydraulics are still on the truck, but the hydraulic pump isn’t run from the engine — it’s run from an electric motor, which also serves as a generator while the truck is driving.”
Hybrid system operation and design
Parker suggests using a hydraulic pump with a thru-drive configuration that is piggy-backed to a permanent magnet electric motor. This subassembly is mounted on the truck’s transmission using a traditional clutch-shift power take-off (PTO). The electric motor is controlled via an inverter that derives power from a compact battery pack.
When the electric hybrid ASL stops to collect garbage cans, the PTO de-clutches from the engine, and the electric motor drives the pump. The truck engine can either idle without being loaded or come to a complete stop, and the pump can run at optimal speed (1,800 to 2,200 rpm) to reach maximum efficiency. As the truck drives between stops, the PTO engages, the pump runs the packer function, and the electric motor works as a generator to recharge the battery. The electronic control unit can modulate the instantaneous amount of charge depending on driving conditions to maintain good traction at the wheels.
The electric motor is the first design consideration for hybrid systems. Usually, electric motors operate most efficiently at high speeds. But when driving and operating an ASL, hydraulic components can limit the motor speed. Therefore, selecting an electric motor that is configurable and adaptable to the hydraulic components’ operating conditions is essential.
As for sizing, the battery can be relatively small, and the pump size can be reduced by half because its speed is no longer tied to the engine’s idle condition. The system can be designed with different pump and valve technologies depending on the arm geometry and the type of control required.
Parker tested its electric hybrid design concept by sending an ASL on a route through a dense suburb in Detroit. The team recorded the engine and electric motor speeds, hydraulic arm pressure and flow, packer pressure and flow, and battery charge during the test. The results showed that the battery remains well charged even when driving phases between pickups are short.
The test started with a fully charged 96 V battery with a stored energy of 10.4 kWh and weighing 238 lb (107 kg). The ASL had a single P1M 65 cm3/rev piston pump with LS controls and electric torque limiting supplying flow to an L90 load sensing valve bank where the arm sections were prioritized over the packer sections. The electric motor was a Parker GVM-210-100 with a peak torque of 110 lb-ft (150 Nm). The motor was controlled using a Parker GVI inverter.
As the ASL drove from house to house, the electric GVM motor served as a generator to charge the battery. The duration of the driving phase depended on the distance between houses, traffic, traffic lights, and other factors. When approaching a garbage can, the driver activated the dead-man trigger on the joystick, the PTO de-clutched, and the GVM motor turned the pump based on flow demand (500 to 2,000 rpm). By the end of the test, the ASL completed more than 300 garbage can pickups without significantly long driving phases, demonstrating that the battery recharged adequately between frequent stops.
The future of electric hybrid systems
Parker’s design and test show that decoupling the engine from the hydraulic functions improves efficiency, integrates easily with engines and standard truck chassis, and reduces component size, noise, and emissions. But the benefits are not limited to ASLs. The system can easily be adapted to similar applications, such as residential front-end loaders (FEL), where the hydraulic arm is installed on a cart attached to the truck’s forks.
Despite the range of applications, electric hybrid systems are still displaced in today’s market, which favors fully electric vehicles. Governments are calling for zero-emissions by target dates and providing funding to produce and purchase fully electric fleets. Regardless of what makes sense from a technical standpoint, there’s no money set aside for hybrid systems.
The original roadmap placed hybrid as a bridge toward electrification. While waiting for infrastructure and battery innovation to meet the demand for fully electric vehicles, hybrid systems provide a right-now solution without much disruption. However, electric hybrid implementation has been postponed until the market shifts.
“We see a space for hybrid in the future,” said Franzoni. “We just don’t know exactly when or how.”
Parker Hannifin Corp.