Toyota R&D continues work on free piston linear generators for EVs; novel resonant pendulum control method
In 2014, a team at Toyota Central R&D Labs Inc. published two SAE papers on their work in developing a prototype 10 kW Free Piston Engine Linear Generator (FPEG) for B/C segment electric vehicles. (Earlier post.) The FPEG consists of a two-stroke combustion system, a linear generator, and a gas spring chamber; the unit offers potential for compact build, high efficiency and high fuel flexibility.
Now, the Toyota researchers report on a new FPEG control method to realize stable and flexible piston motion control for efficient electric power generation. They presented their work in a paper at the recent 2016 SAE World Congress.
There are some technical challenges in ensuring an FPEG can achieve continuous operation over a long period, including lubrication, cooling, and piston motion control. Among these technical challenges, the piston motion control is the most significant factor in improving the robustness and efficiency of the FPEG because the combustion characteristics depend strongly on the piston motion, which is controlled by the linear generator. This paper describes a novel linear generator control method which realizes the simple harmonic oscillation governed by the piston mass and the air spring pressure.—Moriya et al.
Control of the linear generator is a key to stable operation of the FPEG over long period of time because the combustion depends on the piston motion—controlled by the force of the linear generator. Linear generator control is also important to the generating efficiency.
The Toyota researchers earlier had developed a target positon feedback control method. Although this method was successful for control of the linear generator at low-power operation points, it was not able to maintain the piston swing motion when the output power of the FPEG was increased.
To address this issue, the team developed the new method, called “resonant pendulum” control.
In their description of the study, the team noted that one of the major problems in the practical application of FPEGs is the vibrations arising as a result of the piston motion. To solve this problem, they said, two sets of FPEGs can be installed in opposing positions on board and controlled such that their pistons are synchronized. Before working on that, however, they needed first to establish a method for precise piston control; as a result, they used a single FPEG in the current study.
Position sensors—gap sensors on the cylinder block and scale lines on the piston—detect the position of the piston. The resolution of the position sensor is 0.55mm/count, and the count value from the sensor is interpolated by software.
The load of the FPEG can be manipulated with the amount of the fuel which is injected by the fuel injector. The air flow rate is adjusted responding to the fuel amount to keep the targeted A/F.
Resonant Pendulum control. The resonant pendulum control method is based on speed control; its purpose is to adjust the positions of top dead center (TDC) or bottom dead center (BDC) to set positions by manipulating speed control commands.
The piston moves freely following the principle of simple harmonic oscillation governed by the piston mass and the air spring pressure during the remainder of the piston stroke. The current of the linear generator is set to zero during this period.
In general, the linear generator has to drive the piston just before the TDC and BDC to adjust the positions of the TDC and BDC to the commands. When both the positions of both the TDC and BDC exceed the commands, the amplitude of the piston swing is decreased. Similarly, when both positions are insufficient to meet the commands, the amplitude is increased.
Conversely, when the position of the TDC exceeds the command and that of the BDC is insufficient to meet the command, the piston swing is shifted in the negative direction, thus decreasing the offset. Similarly, when the position of the TDC is insufficient to meet the command and that of the BDC exceeds the command, the piston swing is shifted in the positive direction, thus increasing the offset.
The new values of the amplitude and offset for the next cycle can be calculated from the previous values of ETDC and EBDC because the piston is considered to periodically repeat the same swing motion.
During motoring operation, the piston is accelerated by the linear generator to approach the speed command values. In this case, the linear generator works as a linear motor and uses electricity from the battery. During firing operation, when the piston speed exceeds the speed command, the linear generator decelerates the piston and generates electricity, which charges the battery. There is no need to switch between the motoring and firing operation modes, as these operations change automatically based on the piston speed.
The team also devised and tested a method of improving the generating efficiency by manipulating the pressure of the air spring chamber; basically, the air spring chamber can function as an energy buffer to suppress the peak of the generated electric power and distribute it to the compression stroke.
The Toyota team investigated the new control method both via simulation and experimentally.
Among the findings reported in the study:
Resonant pendulum control delivers reliable and stable operations in all the modes of start-up, motoring and firing.
The resonant pendulum method controlled the positions of TDC and BDC within 1mm difference in both motoring and firing modes.
Requiring that the generating action occurs only in the range of high-speed piston motion enables high-efficiency generation.
Using simple harmonic oscillation realized generation in both the expansion and compression strokes. The ratio of the generated powers in both strokes changes depending on the air spring chamber pressure. When the generated power during the expansion stroke is larger than that during the compression stroke, the maximum DC power is generated.
Moriya, K., Goto, S., Akita, T., Kosaka, H. et al. (2016) “Development of Free Piston Engine Linear Generator System Part 3—Novel Control Method of Linear Generator for to Improve Efficiency and Stability,” SAE Technical Paper 2016-01-0685 doi: 10.4271/2016-01-0685