The notion of recovering the energy generated by vehicle suspension systems has attracted increased attention in the automotive industry. This has resulted in the creation of regenerative suspension system prototypes aimed at capturing the linear motion and vibration energy that occurs between the vehicle body and chassis when it travels over road disturbances.
A key aim of such systems is to enhance fuel efficiency by feeding the harvested energy into a variety of components such as batteries or electrical systems like heating, ventilation and air conditioning. What types of approaches are under development? And how well do they perform?
Manufacturers have long been aware that a significant amount of vibration energy is dissipated by the types of oil shock absorbers typically used in the existing vehicle fleet. As awareness of such lost energy grows, various research teams are turning their attention to how best to harness the vibration energy from vehicle suspensions by replacing the current systems with energy-harvesting shock absorbers. This is primarily in an effort to increase fuel efficiency and improve vehicle dynamic variables such as ride comfort and road handling.
This research has resulted in several promising technologies, including linear electromagnetic transducers that can be used to convert kinetic energy into electricity. It has also resulted in designs dedicated to transforming linear motion into rotation that is used to power geared electromagnetic motors.
Another innovative technology, recently created by staff at Virginia Tech in the US, is a mechanical motion rectifier (MMR), which converts the linear oscillatory motion of shock absorbers into unidirectional rotation, in a similar way to electrical voltage rectifiers that convert AC into DC power. The harvested energy can be used to charge a vehicle battery and help to reduce alternator and engine load, or be used for suspension control.
According to Lei Zuo, director of the energy harvesting and mechatronics research lab at Virginia Tech, a key benefit of the system is its high energy-harvesting efficiency. This is achieved by changing the irregular vibrations with time, varying velocity and amplitude into one directional rotation “with relative steady speed,” resulting in better energy conversion efficiency.
Other benefits include better reliability – as a result of what Zuo claims is significantly reduced backlash and impact force in the unidirectional rotation – and “much better vehicle performance”.
“The rotation inertia in the MMR-based energy-harvesting shock absorber can improve vehicle comfort and road handling and reduce the suspension motion stroke,” he says. “It can also enable self-powered semi-active suspension control.”
The Virginia Tech team has carried out road tests of the system in an SUV and demonstrated that it is capable of harvesting 15W of energy from a single shock absorber at 15mph on a smooth road.
“We recently tested the design on a truck and demonstrated 11% reduction of vehicle vibration using the MMR-based energy-harvesting shock absorber shunted with a resistive electrical load. Further performance can be improved by using semi-active control,” says Zuo. At this stage, Zuo says that the key challenge is to create a “highly efficient, reliable and retrofittable energy-harvesting shock absorber that can replace all the functions of the current oil shock absorber, or provide better vehicle dynamics”.
He adds: “So far we have demonstrated it in lab and road tests with prototypes even lighter than the oil shock absorber. The mechanical energy conversion efficiency is up to 70%, much better than the hydroelectric energy-harvesting shock absorber. Another challenge is to reduce the cost. The cost is still several times higher than the oil shock absorbers.”
Provided these challenges can be overcome, Zuo is confident that the future prospects for the MMR system look rosy. He reveals that a commercial arm called Energy Harvesting Technology has been set up, with one eye on a range of preliminary target markets, including trucks, commercial vehicles, off-road vehicles and trains.
Another interesting recent development is a regenerative hydraulic shock absorber system created by Ruichen Wang (pictured below), a researcher in the Institute of Railway Research at the University of Huddersfield. The prototype system uses pressurised fluid to convert the reciprocating motion of a hydraulic cylinder piston into the rotational motion of a hydraulic motor and generator – and stores recovered energy in a battery for later use.
Wang has carried out mathematical modelling of the performance of the system. He has explored the effect of introducing a gas-charged hydraulic accumulator to provide power smoothing in an attempt to provide more stable levels of recoverable power.
He has also built an experimental rig to measure the reaction of the system to variations in motor pressure and shaft speed under different levels of excitation and investigate voltage output and recoverable power at different electrical loads.
By using piston-rod dimensions of 30-50mm, Wang reveals that the system achieves 260W of recoverable power at an efficiency of 40% under sinusoidal excitation of 1Hz frequency and 25mm amplitude. He says: “The simulation of the system and parameter computations have all been realised on the Matlab platform. This provides sufficient flexibility to take into account more factors for analysis and can thus be an effective mathematical tool for further research in this direction, such as the optimisation of the structures, control strategies and system integrations.”
Although the system has not yet been integrated into a vehicle, Wang is seeking funding to carry on the project. And, as part of his work at the institute, he is also attempting to use the technique in a rail vehicle – with a Smart Damper project that began last month.
Ultimately, although a variety of regenerative suspension design concepts have been evaluated, Wang expects that the hydraulic-electromagnetic configuration will be used. That’s largely because of the “better flexibility and reliability of system performance and power regeneration”.
But he admits that various challenges still need to be addressed, not least those relating to management of the trade-off between ride comfort, road handling and power “recoverability” under different driving speeds and road conditions.
He has identified avenues for future research. These include road testing “in an attempt to develop ride comfort and road handling”. Another approach will be to optimise the design structure for specific applications “such as the dimension of shock absorber body, hydraulic accumulator, hydraulic motor, valves and generator”.
Audi redesigns dampers
In recognition of the fact that the recovery of energy via suspension systems is likely to grow in importance, Audi is also now working on a prototype system, known as eROT. It replaces traditional hydraulic dampers with electromechanical rotary dampers to open up “entirely new possibilities for adjusting the suspension” and enable vehicles to harness kinetic energy and convert it into usable energy.
Stefan Knirsch, board member for technical development at Audi, revealed that the 48V electrical system is capable of adapting to both driving style and “irregularities in the road surface”.
As well as boasting a “freely programmable damper,” the eROT system also features a novel lever arm that absorbs the motion of the wheel carrier – and converts it into electricity by transferring the force to an electric motor via a series of gears.
Although the system does not yet feature in new vehicles, its use in future Audi production models is viewed as “plausible” and the preliminary results look promising. According to the company, an average recuperation output of 100 to 150W has been achieved during testing on German roads.
The results ranged “from 3W on a freshly paved freeway to 613W on a rough secondary road” – equating to a reduction in CO2 emissions of up to 3g/km (4.8g/mile) under “customer driving conditions”.
An updated version scheduled for release sometime during 2017 will also be used as the “primary electrical system in a new Audi model”. It will be used to power a high-performance mild hybrid drive, offering “potential fuel savings of up to
0.7 litres/100km”.