|Cross-section of the FPLA. Click to enlarge.|
Researchers from Shanghai Jiaotong University in China have developed a new and more accurate computer model of the alternator component of the free-piston linear alternator (FPLA)—a two-stroke, free-piston engine combined with a linear alternator to generate electricity, with potential application in a hybrid electric vehicle. A paper on their work is scheduled for the 17 Sept. issue of the journal Energy & Fuels.
The free-piston engine offers a number of benefits, including optimization of the combustion process through variable compression ratios, making multifuel operation possible; reducing frictional losses because of fewer moving parts; and allowing homogeneous charge compression ignition (HCCI) to be more easily achieved. A linear alternator can use the linear piston force without requiring additional mechanical components necessary in a rotary configuration.
The concept has thus stimulated a good deal of research, including that by Dr. Clark at West Virginia University (WVU), Dr. Peter Van Blarigan at Sandia National Laboratory (SNL) and Dr. Fredriksson at Chalmers University of Technology (earlier post). Much of this work has been focused on the combustion process, however, and not on the alternator component, according to Qingfeng Li and his colleagues.
The alternator, which is another main part of the FPLA is ignored. The commonly used modeling method is using different thrust force profiles to simulate the applied alternator loads. Considering that the free-piston motion is only dependent upon the acting forces, the used profiles could not represent the real working process of the alternator. This paper uses the finite element method (FEM) to establish the alternator model.
By this method, the actual thrust force is integrated to the final model of FPLA. Besides, the final model consists of another two main parts, the piston dynamic model and the chemical kinetic model. The piston dynamic model is solved by Matlab/Simulink. The cylinder kinetic process is simulated in Senkin, which is a subroutine of Chemkin. By integrating these three models, the more accurate model for FPLA is successfully established. It can help us to obtain a further understanding of this engine.—Qingfeng Li et al. (2008)
There are three types of free-piston engines, according to the researchers:
Double piston (dual piston and opposed piston); and
Four pistons (dual piston, opposed piston, and complex piston configuration).
They chose to work with the double piston configuration because of the benefits in the compact structure and higher power. The free-piston linear engine in the study, which uses HCCI combustion, contains dual-opposed pistons, connected by a connecting rod and two relevant cylinders. Three intake ports and one exhaust port are placed around the circumference at the bottom end of each cylinder. The fuel is supplied to the cylinder though the injector arranged in the intake port to form the homogeneous mixture.
The linear alternator is used as the start device at the very beginning. After the engine exceeds a certain reciprocation frequency, auto-ignition occurs alternately in each cylinder, making the connecting rod move back and forth. Then, the magnetic flux linked with circular wound coil changes periodically to produce the current.
Among the conclusions drawn by the modeling work are:
During most of the piston strokes, the velocity of FPLA keeps constant. It reverses to the maximum values in the opposite direction just before the TDC. This makes the acceleration at least 3 times larger than the traditional ICE. In addition, it avoids the longer residence time at TDC, which seems a better way to reduce the peak temperature.
The TDC of FPLA can change with the external loads as well as other parameters. It can make sure that the ignition occurs right around TDC. This allows more energy used in the expansion process. Using lean mixture to acquire a higher compression ratio can improve the indicated thermal efficiency effectively.
The FPLA has the shorter combustion duration under a high compression ratio. The peak temperature and pressure are lower than the traditional ICE. This could reduce the temperature- dependent emissions. It is an environmentally friendly power source for the future.
Qingfeng Li, Jin Xiao, and Zhen Huang (2008) Simulation of a Two-Stroke Free-Piston Engine for Electrical Power Generation. ASAP Energy Fuels doi: 10.1021/ef800217k