Electromobility and Powertrain Optimization

Electromobility is a key topic throughout the mobile-equipment industry today. What’s behind the push for electrification?  One of the key drivers is the increased environmental consciousness and focus on global warming. The goal is to reduce carbon dioxide emissions in every way possible, and many countries believe that electrification can be a solution. Regardless of your position on this, the reality is that it is being pushed by both governments and customers, and we as an industry must be prepared to adapt.

Another key driver for electrification is the growth in large urban centers. According to U.N. estimates, in the next 30 years, there will be nearly 2.5 billion more people living in cities. This drives a further need for quiet solutions that can be deployed in areas that are already challenged with pollutant gases and smog. Electrification brings some advantages in those areas. No one wants to wake up with the sound of a diesel engine outside their window anymore.

The expected growth in autonomous and connected vehicles will increase the use of sensors, computers and guidance systems, which lends itself well to fully electric vehicles and electrohydraulic hybrids.

And then there are battery advances. Over the last few years batteries have tremendously improved in terms of performance and price. Cost per kilowatt-hour has been reduced by almost half, with projections to further reduce that cost again by half. At the same time, power density is increasing, which allows for easier integration into a vehicle and optimizing the system based on customer needs. Some of this is driven by different battery technologies such as lithium polymer and lithium iron phosphate, which has no rare earth metals. And there will be further improvements in fast recharging.

Some OEMs have moved to fully electric vehicles and claim equivalent performance and equal or lower cost, compared to similar diesel machines. We believe that within the next two to three years, it may be cheaper to build an electric vehicle than a diesel vehicle. A lot of this is being pushed by electric vehicles coming out of China. Electrification will evolve from a few prototype machines to truly an alternative and competitive technology for traditional fluid power. 

Design Considerations
What are some of the impacts for off-road machinery? Companies are now being pushed to develop true zero-emission vehicles. But, of course, the end user wants a cost-effective machine without compromising on performance.

A traditional hydraulic drivetrain includes a fuel tank, diesel engine, pump and hydraulic wheel motor or axle transferring torque to the ground. And there are several alternative architectures. For instance, one option is a full electric drivetrain with electric motors at the axle. Another is a hybrid-electric drivetrain with a diesel engine driving a generator to power electric traction motors.

Let’s instead focus an electrohydraulic drivetrain where batteries, an inverter and electric motor replace the engine, but still driving a pump and hydraulic wheel motors. Machine designers must consider many factors, especially the mission profile: How will the machine be used, and how hard must it work?

Look at duty cycles. Consider all the power requirements of the machine; not just traction for a drivetrain, but the implements, steering and tools as well. Weight distribution and geometry are key factors in all machines, but now we are replacing the counterweight with a large battery pack. Can we install it in a more optimum position or spread it out throughout the machine? How will it affect our center of gravity and our performance?

Design engineers experienced with combustion-engine power plants may be unfamiliar with electrical vehicle constraints, such as how a battery performs at -40°C versus +40°C, how components are protected, and what kind of charging facilities are available.

Sit down with the customer to understand how the machine works and evaluate the mission profile. Then weigh various options, and optimize the best models using detailed and sophisticated tools at a granular level, to understand the impact of each component. Potentially combine everything into one analytical model and evaluate total system performance. Control strategies need to be considered, too. Hopefully, the result is a machine geometry that matches customer expectations for durability, cost, performance, machine autonomy, and range.

One advantage of this architecture is we limit the amount of change. It’s not an entirely new design, but more of an incremental step. Today, pumps and motors are relatively known commodities. Things like electric traction motors are very much an unknown. By preserving a traditional hydraulic drivetrain, we limit that uncertainty to only one component rather than multiplying it by four wheels.
 

Energy Management
Digging a bit deeper into our electric-hydraulic drivetrain example, a hydrostatic drive converts input power to force and speed. It allows a hydraulic motor to operate in all four quadrants – as a pump or motor moving forward or reverse. Due to engine limitations, however, we're often restricted primarily to propel and, sometimes, limited hydrostatic braking.

Electric motors do not share these same limitations. So an electric motor allows full four-quadrant operation. This facilitates things like regenerative braking or energy storage from the swing function on an excavator. That becomes critical because regeneration means less recharging and a smaller battery pack.

Different motors, such as inductive and permanent-magnet motors, offer tradeoffs involving efficiency, cost, and performance. The same holds for components like radial-piston motors, which offer very high volumetric efficiency. With a variable-displacement pump in our circuit we can tailor the right combination to operate at the best overall efficiency. But engineers must consider this within the framework of the entire system.

Consider two different use cases. One machine works for five hours, recharges for two hours (potentially over lunchtime), works for a final hour at the end of the day, and then plugs in overnight. In the second, a machine operates intermittently – work for two hours, charge for an hour, work for two hours, etc. In both cases, there are six hours of work and two hours of charging, but the implications for the machine architecture and dimensioning are very different.

In the first case, with a 3 kW charger, we need a large and expensive 40 kWh battery pack. Compare that to the second architecture where we have intermittent charging throughout the day. Moving to a fast 9 kW charger, we are able to reduce the required battery pack down to only a 25 kWh battery pack, with a tremendous cost savings.

This is an example of how focusing on a few percentage points of efficiency on individual components will not bring the same level of gains as truly understanding the mission profile — and reducing the battery pack and potentially several hundred kilograms off the weight of the machine.

Another option to optimize the drivetrain is looking at multiple pumps. If the main pump is sized to handle driving and auxiliary functions, when only the auxiliaries are working, perhaps the main pump is oversized. In this case, we can add a second, smaller pump that only runs when required. Again, look at overall system efficiency rather than component by component.

In the end, we can optimize energy storage, charging, and how we use the vehicle. We can optimize our control algorithms to ensure that the overall system operates at peak efficiency. We can optimize our energy recovery options: hydrostatic braking, downhill operation and boom lowering. And we can optimize the actual machine geometry itself with multiple pumps.
 

At the time this article was written, Adam Frey was a design engineering manager for North America at Poclain Hydraulics. He has a BSME degree from Grove City College in Pennsylvania and has been in the fluid power industry for more than 13 years. His duties include work within Poclain’s electrical mobility group, which strives to marry the advantages of hydraulics with those of electrical power systems and create highly efficient drivetrains. He is also a collaborator with NFPA’s Fluid Power Challenge, a hands-on education program to further student interest in fluid-power technology and engineering careers.

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