Understanding design considerations of traction, work functions, braking and more helps users optimize electrified vehicle design.
By Jay Schultz, Business Development Manager, Tom Hickey, Systems Application Engineer, Electrification Growth Team, Parker Hannifin
As mobile vehicle manufacturers begin to develop electrified versions of their product offering, they ultimately face one critical challenge: balancing range with battery costs. This article focuses on the design considerations of the systems that use the battery’s stored energy to run the key vehicle functions and how they can be optimized to reduce total energy consumption and extend vehicle range.
Key vehicle functions and system architectures
Traction – A common trait that all mobile vehicles today share is that they’re mobile and designed to be self-propelled. But besides the fact that they can all move, the actual systems behind this function can differ greatly between markets and vehicle technologies. The most common architecture is an engine-driven transmission, but it’s also worth considering traction systems that use closed-loop hydraulic systems, which consist of one (or two) over-center variable displacement pumps driving a hydraulic motor and drive axle. In either case, the traction function is greatly impacted when re-designing for a battery electric system… specifically because, now that we have a battery, we have an opportunity to benefit from regenerative braking.
To understand regenerative braking, it’s worth mentioning how most vehicles brake today — with the vehicle at speed and the operator commanding a braking event. For example, a disc brake will clamp down on the rotating wheel and apply a friction force that absorbs the vehicle’s remaining kinetic energy in the form of heat. The important takeaway here is that the vehicle’s kinetic energy is now spent. And if the operator wants the vehicle to speed back up, more energy will be required to do so.
Now let’s consider traction in a battery electric system, which mainly consists of a battery (capable of bi-directional power flow when not fully charged), an inverter (which is usually capable of bi-directional power flow), and a motor (which is also capable of bi-directional power flow). With bi-directional power flow, we can now use the electric motor to apply a resistive torque to the moving vehicle, which slows it down. But as this is happening, the vehicle’s kinetic energy is converted into electrical energy and stored back into the battery. If the operator wants to speed back up, the energy just captured while braking can be used, thus significantly reducing the total energy consumption for a given duty cycle.
Work – In this article, we’re mostly referring to vehicles that are designed to do additional functions beyond just moving from A to B. “Work” is the term we use to define these functions, and it comes in many shapes and forms. A garbage truck lifting a can or a wheel loader driving a pile are examples of work. While these vehicles are doing very different things, they are just two among many types of vehicles that leverage hydraulic systems to achieve these additional functions.
The types of hydraulic systems are diverse, but the general concept includes one or more hydraulic pumps delivering flow to a valve, which then determines how much of that flow is going to one or more demanded functions. Downstream from the valve the hydraulic flow reaches the actuator, which ultimately converts this hydraulic power into rotary or linear motion. Combined, these components offer a power-dense solution with the flexibility to deliver the vehicle’s power to wherever the work is being done.
With the available technologies today, it’s hard to find a more robust alternative than hydraulics, and the expectation is that most electrified vehicle designs will still consist of hydraulic system solutions to achieve at least some of the vehicle’s work.
Although hydraulics are a mature technology, it doesn’t necessarily mean these systems won’t change in an electric vehicle design. They certainly don’t have to because there’s no reason you couldn’t just run an identical ICE-driven hydraulic system at a fixed speed off an electric motor. But the traditional ICE-driven hydraulic system typically expects a pump running at a fixed speed, which can be incredibly wasteful when little or no work is being demanded. And again, the key challenge in mobile electrification is balancing range with battery costs, so we are now incredibly picky about eliminating wasted energy.
To address this, we should leverage the ability to dynamically control the pump’s speed. Unlike an engine, an electric motor is designed to be commanded to a dynamic speed or torque value, with peak torque performance available at speeds as low as 0 rpm. This flexible performance at a very high efficiency is unmatched and brings the opportunity for additional optimizations in the vehicle’s hydraulic system design.
It would take another article to dive into the different types of hydraulic systems that could be considered in an electrified vehicle but, before we dive into that step, we must first comb through each work function and ask the following:
- What type of motion does this function demand — linear or rotary?
- What are the function flow and pressure requirements throughout a given duty cycle?
- How does the operator and/or machine command this function?
- Does this function run in parallel with other functions?
Depending on these answers, we can make a better decision on what makes sense from a design standpoint. Ultimately, the system architecture will range between a 1-ePump centralized system to a very decentralized system consisting of many ePumps and electric motors/actuators. As the system architecture moves towards a decentralized approach, power delivery becomes more efficient. But as the design moves in this direction, the number of systems also increases which translates to higher BoM costs.
Will these costs offset the savings captured by downsizing the battery? The answer to that question will determine where exactly a vehicle design should land in the spectrum of centralized-to-decentralized systems.
Auxiliary Functions – These functions support the other two functions mentioned in this article, and the two we’ll consider are steering and brake functions. For steering, many solutions such as steer-by-wire have already been developed for traditional ICE systems, and a lot of these solutions carry over well in electrified systems.
As for brake functions, are those still needed even though we’re now braking the vehicle using regenerative systems? For most cases, yes! Although much of the vehicle braking will be from the electric motor, there are cases where regenerative braking cannot handle the entirety of the demanded braking energy and additional braking power may be required. Or, if the vehicle is parked on a hill, the electric motor shouldn’t be holding a brake torque at 0 speed for too long. Therefore, it is recommended to include a parking brake, which can apply the necessary holding torque to keep the vehicle stationary for an extended period.
Sizing the right components
Traction – For this exercise, we’re assuming the system architecture leverages an electric motor-driven traction rather than an electric motor driving a hydrostat (which still may exist in some electrified system architectures). But in our case, we are driving our vehicle directly with one (or two) electric motors. To size the motor, we first need to consider the following questions:
- What are the peak tractive effort requirements? How do these values translate to torque and speed?
- What speed reductions will be between the motor and the axle? Are these ratios variable?
- What different duty cycles and use cases will the vehicle’s traction system expect to see?
With the answers to these questions, we can select a motor and inverter combination that is designed to deliver the required torques and speeds. The Parker GVM motor series and GVI inverters, for example, bring a modular approach to meeting a diverse range of vehicle traction needs. With a variety of configurations, we can rightly size the system to meet or exceed the performance expectations of traditional mobile vehicles.
Work – The solutions to work aren’t as straight forward in electrification compared to traction and, depending on where we landed on the spectrum of centralized-to-decentralized systems, we’re likely working with at least one ePump. To size the motor(s) for these ePumps, we need to determine the following:
- What is the flow and pressure demand for each ePump over the course of the expected duty cycles?
- After review of the above, what is the corner condition that each ePump will see from a pressure and flow standpoint?
One quirk about electric motors and inverters is that they have two performance curves: a continuous performance curve and a peak performance curve. Selecting the right curve to abide by can be a bit difficult because a duty cycle is rarely just one fixed load. Therefore, the motor/inverter requirements to do this work don’t necessarily need to be at or below the motor/inverter’s continuous performance. This is why it is beneficial to have a good understanding of the vehicle’s duty cycle, as it paints a much clearer picture of what the ePump needs to handle. With the context of demanded pressures and flows, we can select a pump displacement to deliver these flows at a given speed and then calculate the required torque from the motor. This value, along with system voltage, ultimately enables us to select the appropriate motor and inverter.
Beyond rightly sizing the system, some of the largest efficiency gains can be captured by simply changing how the system is controlled. With the ability to vary the pump speed on the fly, the ideal scenario is that, when work isn’t being commanded, absolutely no energy is being spent. For some functions and system architectures, this isn’t possible, as some amount of standby flow or pressure is required. But even slowing the pump to a lower standby speed can reduce energy consumption in this condition.
Additionally, hydraulic components themselves can range from very inefficient to very efficient, so any efficiency gains brought by a component change could translate into lower costs of the battery. Some work functions may benefit from being separated from the hydraulic system entirely and become electromechanically driven. This type of solution removes the inefficiencies of hydraulics entirely and enables energy recovery.
Auxiliary functions – Sizing for braking and steering systems is mostly like sizing work functions. You need to ask about the flow and pressure demands over the course of a duty cycle. The big difference to consider for these types of systems is that they are typically lighter duty than work but also are more likely to require some degree of standby flow. A case can be made to separate these systems from work systems so that the more powerful work-related components can remain off and allow the lower power components to independently switch in/out of low power standby as they’re commanded.
For both work and auxiliary functions, there are a variety of options. One path would be a component-based solution where a manufacturer can buy the hydraulic components, motors, and inverters individually. Alternatively, some companies like Parker offer packaged solutions in the form of configured ePumps and ePTOs. Either option comes in a variety of power levels and interfaces to help meet a given vehicle’s work system requirements.
When choosing a partner to assist in electrifying your vehicles, it’s important to identify a company that has a broad product portfolio and technology experts who can help guide your decision-making and support the design and integration of your next electric vehicle platform.
Parker Hannifin
parker.com
