An ideal hybrid solution uses an efficient system architecture that could meet new mandates and make a huge impact on the environment all while using existing technologies, components and designs in an innovative way.
By Barun Acharya and Germano Franzoni
The idea of a fully electric or hybrid truck is not new. Attempts were made as early as the 1980s to introduce hybrid trucks and buses, but had limited success. So why is the concept so much more realistic today? One of the main drivers is stricter environmental mandates. New polling shows that Americans’ concerns about climate change have surged to record levels. Today, tightening emissions targets, combined with growing directives on the part of many countries to ban fossil fuel use by 2030, are pressuring the global industry to move away from diesel engines.
The momentum to decarbonize is pushing mobile OEMs to think beyond cleaner engines and, ultimately, eliminate diesel engines over time. New de-carbonization policies, supported by carbon pledges and legislation, are driving significant R&D investments in many electrified vehicles, including refuse trucks.
In addition to regulatory drivers, however, mobile electrification requires technical breakthroughs, such as significant improvements in batteries and overall efficiency in electrified systems. The more efficient the motor, motor controller and hydraulic components, for example, the smaller, lighter and less expensive the battery system that is required.
A major obstacle in the adoption of fully electric trucks is the high cost of batteries and the added weight and size of the batteries necessary to power not only the driving, but also the transportation of loads and hydraulic functions that are common on refuse vehicles. According to the 2018 UPS /GreenBiz research study, batteries for electric vehicles are continuously improving in both technology and cost. The 2018 Bloomberg New Energy Finance Electric Vehicle Outlook reports that the average lithium-ion battery price dropped by 79 percent from 2010 to 2017, while battery average energy density improved 5 to 7 percent per year during that time. Research is ongoing into different battery cell designs and chemistries that will help minimize weight and costs.
Why Hybrid Versus Fully Electric
There are some in the industry who believe it is worth the wait for fully electric refuse vehicles to reach full commercial production. Indeed, there are instances where 100 percent electric refuse vehicles are already in use. The New York City Department of Sanitation, for example, acquired a Mack electric garbage truck in 2020 as a test vehicle. It was happy enough with the overall performance to place an order for seven more electric rear loader trucks.
This is not surprising since New York is a heavily populated, congested city with narrow streets. The same could be said for nearly any other large city, such as San Francisco or Chicago.
However, for much of the U.S., and especially in rural parts of the country, conventional refuse trucks still have an advantage because of the larger distance between neighborhoods and the collection site. It is these areas where fully electric refuse trucks are still not commercially viable. Why? Because larger distances require a larger battery to meet the power and range needs. A problem with larger batteries is that they weigh more, which leads to a decrease in payload. Beside weight limits, this increase can be problematic on older roads or bridges with weight restrictions.
In contrast, an electric hybrid system offers a more cost-efficient alternative. It combines some advantages of the electric solutions (e.g., smart energy usage and lower emissions) while keeping the performance and range benefits of a traditional diesel engine. Such added performance is critical for a refuse vehicle when you consider that the average garbage truck typically collects between 1,000 and 1,500 trash can loads per day.
Eventually, the industry will be able to overcome the current problems that limit the feasibility of fully electric refuse vehicles. Fuel cells, for example, may be one solution. For the time being, however, given current power density storage technologies, it is more realistic that an electric hybrid system could serve as a bridge solution.
There are multiple benefits of a hybrid system to consider beyond cost. First, it can be implemented with readily available components without dramatically changing the chassis architecture. It also uses a small battery pack that does not affect the payload capability of the vehicle.
Opportunities for Electric Hybridization: Work Versus Propel
There are two good opportunities for electric hybrid on refuse vehicles: the truck’s propel and its hydraulic work functions. Advantages of electric-assist propel solutions include storing the braking energy of the refuse vehicle into batteries. During vehicle braking, the electric machine in series with the propel system turns into a generator and stores the potential energy from the refuse vehicle into the batteries. When the driver hits the accelerator pedal, the electric machine becomes a motor and assists the vehicle in its acceleration. This repeated exchange of energy between the electric machines, the engine and the wheels keeps the battery size minimal with respect to a fully electric driveline.
Work functions include a variety of high-load services responsible for lifting and dumping loads and compacting trash, whether it is with an automated side loader or front-end loader. Requiring precise force control and high reliability, these services have traditionally been driven by centralized hydraulics.
An Electric Hybrid System for Controlling Work Functions
An electric hybrid system for work functions (implement) control uses hydraulic pumps with a through-drive configuration that are piggy-backed to a permanent magnet electric motor. Using a traditional clutch-shift PTO, this subassembly is mounted on the truck transmission. The electric motor, controlled via a motor controller (or inverter) is also powered by a compact battery pack.
When the truck stops to collect the garbage cans, the PTO de-clutches and the pump is driven by the electric motor. When the truck drives between stops, the PTO is engaged, the pump runs the packer function and the electric motor works as a generator to
recharge the batteries. Since the engine is not loaded at idle, fuel consumption, emissions and noise are all reduced. In addition, the pump size can be reduced as much as 40 percent because its speed is no longer tied to the engine idle condition. This results in added cost, weight and installation benefits. Various pump and valve technologies can be incorporated depending on the truck method of collection (side/front loader), its geometry and the type of control required.
Enhanced Possibilities by Combining Two Hybrid Systems
The performance and value of the overall system becomes even more attractive when the two hybrid architectures (for propel and work functions) are combined, sharing the same small battery system. The same vehicle controller unit (VCU) can be used to control both systems and is responsible for monitoring and optimizing the entire truck in terms of fuel efficiency and productivity. For example, the energy recovered during braking events can also be used to power the work functions, leading to further improvements in efficiency and reduced emissions.
The further addition of a mobile IoT (Internet of Things) solution can add improvements to efficiency and performance by continuously measuring the hydraulics, the driveline, the electric motors and the engine. The vehicle can be tracked during the workday, and the path or route can be continuously optimized based on weight loaded or fleet maintenance needs.
Challenges Still Exist
As this article has demonstrated, fully electric trucks still do not represent a viable option for many parts of the country given the limitations of the current battery technology. Once the battery technology catches up such that smaller, lighter weight batteries can provide sufficient power for longer routes, fully electric refuse vehicles could represent a highly attractive option.
In the meantime, the technology exists for hybrid systems to be fully implemented today. The ideal hybrid solution uses a small battery pack connected to a smaller engine based on the concept of an electrical motor/generator on the drive train that assists during vehicle braking and acceleration. The propel systems and work functions are tied together using the same battery to power them both. This efficient system architecture could allow us to meet the new mandates and make a huge impact on the environment all while using technologies, components and designs that already exist. | WA
Barun Acharya is a Global Industry Market Manager with Parker Global Mobile Systems. He has an M.S. in Mechanical Engineering and an MBA in Finance and has 14 patents. He can be reached at [email protected].
Germano Franzoni is a Senior Systems Engineer with Parker responsible for system design and business development within Parker’s Global Mobile Systems team. He earned a Ph.D. in Mechanical Engineering, with a specialization in hydraulics, holds two patents and is the co-author of the textbook, Hydraulic Fluid Power: Fundamentals, Applications and Circuit Design. He can be reached at [email protected].