Hydraulic systems have long been necessary for off-highway mobile equipment because of their power density. However a variety of factors – including regulatory trends and advances in technology and economics – is making electrification the way of the future. Diverse industries are working to rethink their operations and equipment to adapt to one of the biggest game changers to impact consumer and commercial markets in decades.
Electrification is not new. The difference is the convergence of three powerful forces: stricter governmental regulations; ESG initiatives by the top companies in the world; and TCO. As adoption rates for electric passenger cars accelerate, experts predict an increased migration of electric technologies into larger, heavier equipment, which has been limited due to insufficient and inefficient power sources. This article explores how hydraulic components, electrical machines and vehicle controls are coming together to optimise machine performance, and how they are positioned to meet future challenges.
No complete replacement for hydraulics
The amount of money invested in new research and electric technologies is unprecedented. Global automakers are planning to spend more than half a trillion dollars on electric vehicles and batteries through 2030, according to a Reuters analysis, weaning car buyers away from fossil fuels and meeting increasingly tough decarbonisation targets; but hydraulics for work functions is not going away any time soon. There will likely always be some applications that are best served by hydraulics.
Hydraulics is unique in power transmission because it has power density. It is the most compact method of mechanical actuation when considering power-to-size ratio. Hydraulics is not replaceable, but adding electrics can improve performance and decrease noise levels. The most promising technologies combine the best of hydraulics with the advantages of electric.
Manufacturers are leveraging opportunities to recover energy through the hydraulic system. The right technology can help store energy in an accumulator or in the battery system through the electric motor that drives the hydraulic pump, enabling the motor to effectively run as a generator when travelling downhill or during deceleration. This is already common in hybrid consumer vehicles, where electric motors recover energy as the car brakes. However, better hydraulic and electric component integration is necessary to achieve the greatest efficiencies. Hydraulic pumps and the electric motors that power them must work together in harmony; smarter hydraulic components that can sense load requirements and systems for energy recovery are needed to achieve efficiency gains. Additional effort is required to ensure all components and subsystems contribute to a holistically smarter, more efficient machine.
The marriage of hydraulic and electric designs
The intersection between the domains of hydraulics and electrification is where today’s innovations are taking place. The goal is to bring together the best of hydraulics, such as robustness, with the controllability and precision that comes from an electrical approach. Machine builders must balance the need to deliver enough torque to drive the machine against the need to make the most economical use of electric power.
As with the drivetrain, engineering the drive elements for the maximum load needed is a critical calculation. A full size excavator must dig and move thousands of kilograms of soil or rocks all day long. It is typically hydraulically driven, drawing its pressure and flow from a pump powered by the mobile machine’s diesel engine. In an all-electric system a heavy-duty electric motor powers a ball screw or other linear component to do the work. However, the size of the actuator and the drain on the battery are substantial, even for a hybrid electric platform.
A more effective solution combines electric power and hydraulic power. It takes power to move a load – whether it’s voltage and current with an electric motor, or flow and pressure with a hydraulic cylinder − to generate the torque needed to do the work. Rather than powering the hydraulics from the diesel motor, the motor runs a generator which then supplies power to an electric motor driving the open-loop hydraulic pump for the cylinders powering the excavator’s tools. This leverages the power density of hydraulics from the combustion engine platform to electric power.
Challenges remain on the road to electrification
Electric power transmission is clean, efficient and accurate at levels that hydraulics cannot match. However barriers still exist. While breakthrough technologies mean batteries charge faster, last longer and are lighter in weight, some will not be commercialised for years. The reality is that a large percentage of batteries on the market today are still heavy, expensive and unable to meet demand. But with work done by companies like Tesla, historic battery costs of $140/kWh will soon realistically be down to $50.
Despite impressive developments, neither batteries nor fuel cell technologies are ready to meet the very high power requirements needed for the harsh conditions to which many heavy-duty vehicles are exposed. Another challenge is energy transmission. It takes 45 seconds to fuel up that 20 litre diesel tank. This compares with 33 minutes to ‘fuel up’ a similarly sized battery pack at 25 kW charging capacity. Significant research is also being undertaken to identify new materials and heat-resistant power electronics to help minimise heat, which is key to improving the overall safety of the vehicle and maximising battery charging speed, longevity and overall lifetime. Thermal runaway is a real fear for batteries. The key driver is to minimise the energy storage size, which will reduce installed cost as operating hours increase between charging.
Hybrid is today’s reality
Hybrid electric systems are well positioned as an interim solution. Hybrid electric equipment combines the power density of a diesel engine with the emissions-cutting capabilities of a battery. A smaller diesel engine alongside a rechargeable battery and electric motor is ideal for both high-intensity jobs and locations requiring low emissions.
The battery in a hybrid system can be relatively small and function as a buffer that allows the pump and engine to work at higher speeds where the efficiency is best. The engine is not loaded at idle, which reduces fuel consumption, emissions and noise. The pump size can be reduced by 40% because its speed is not tied to the engine’s idle condition. This produces several advantages in terms of cost, weight and installation.
Hybrid drives still burn diesel fuel, but they can recover and reuse energy that would otherwise be wasted as heat. This can allow downsizing if the engine can be sized for average loads. If there is a clutch between the engine and drivetrain, the vehicle can be temporarily operated in a purely electric mode, depending on energy storage capacity. The hybrid system also allows OEMs to design certain auxiliary functions as ‘power on demand’, using electric motors that are not drawing from the engine all the time – for example an electric fan used in lieu of a belt-driven fan.
Another version of hybridisation is the series hybrid, where the hydrostatic drivetrain is replaced with two electrics: one on the engine operating primarily as the generator, and the traction drive operating primarily as a motor. It also has energy storage so there is the possibility for recovering energy, operating in pure electric mode, productivity improvements, and peak shaving. Hybrid systems have effectively been used to power propel functions on large trucks and buses and offer significant benefits. In some applications, they have reduced fuel consumption by up to 50%.
It is not just about batteries
There are other sustainable options. There are currently four true zero-emission technologies to power vehicles, of which battery electric vehicles is one. The other zero-emission technologies are hydrogen fuel cells, hydrogen internal combustion engines, and biofuel internal combustion engines. The use of internal combustion engines with fuels such as biomethane and green hydrogen offer the greatest potential in the short term. Fuel cells work like batteries, but they do not run down or need recharging. They produce electricity and heat as long as fuel is supplied. Hydrogen fuel cells can refuel rapidly and carry heavy cargo due to the system’s efficiency in storing electrons. In addition, fuel cell electric vehicles have a much higher energy density by weight, allowing them to overcome the range and weight challenges of battery electric vehicles. Hydrogen tanks are also more compact and lighter than an array of fully charged batteries.
The future is electric
Driven by successes in the automotive industry, interest in electric machines is at an all-time high. The reduction of battery costs has been a huge enabler, making battery technologies more affordable and attractive. Digitalisation has made a huge impact on system operations, performance monitoring and predictive maintenance. Greater connectivity allows for in-depth analysis of how well the entire system is operating and can ward off problems. As part of the effort to fast-track electrification-enabling technologies, there is an unprecedented level of collaboration taking shape. OEMs, suppliers and industry leaders have come together to share their knowledge in the hopes of identifying smarter solutions.
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