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Design and modeling of a free-piston engine generator

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Abstract

Free-piston engine generators (FPEGs) can be applied as decarbonized range extenders for electric vehicles because of their high thermal efficiency, low friction loss, and ultimate fuel flexibility. In this paper, a parameter-decoupling approach is proposed to model the design of an FPEG. The parameter-decoupling approach first divides the FPEG into three parts: a two-stroke engine, an integrated scavenging pump, and a linear permanent magnet synchronous machine (LPMSM). Then, each of these is designed according to predefined specifications and performance targets. Using this decoupling approach, a numerical model of the FPEG, including the three aforementioned parts, was developed. Empirical equations were adopted to design the engine and scavenging pump, while special considerations were applied for the LPMSM. A finite element model with a multi-objective genetic algorithm was adopted for its design. The finite element model results were fed back to the numerical model to update the LPMSM with increased fidelity. The designed FPEG produced 10.2 kW of electric power with an overall system efficiency of 38.5% in a stable manner. The model provides a solid foundation for the manufacturing of related FPEG prototypes.

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Abbreviations

FPEG:

Free-piston engine generators

LPMSM:

Linear permanent magnet synchronous machine

BDC:

Bottom dead center

CR:

Compression ratio

TDC:

Top dead center

D :

Bore of engine

D sca :

Bore of scavenging pump

F s :

Stoichiometric fuel-air ratio

F mag :

Electromagnetic force of LPMSM

H u :

Calorific value of fuel

k :

Elasticity coefficient

k L :

Generating load coefficient

m fuel :

Injected mass of fuel per cycle

ṁ f :

Mass flow rate of fuel

ṁ a :

Mass flow rate of air

n crank :

Equivalent crankshaft rotating speed

N stroke :

Number of strokes per cycles

P e :

Electrical power

P fuel :

Fuel power

Q fuel :

Energy from fuel in per cycle

S :

Stroke

v p :

Mean speed of piston

x :

Displacement of translator from zero point

x 0 :

Neutral position with a zero spring force

γ :

Specific heat ratio

η c :

Fuel conversion efficiency

η s :

System efficiency

δ :

Equivalence ratio

ρ 0 :

Density

λ v :

Volumetric coefficient

λ p :

Pressure coefficient

λ T :

Temperature coefficient

λ l :

Leakage coefficient

λ T :

Temperature coefficient

λ l :

Leakage coefficient

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Acknowledgments

This project was supported by the Shanghai Science and Technology Commission (No. 19511108500). We would also like to thank the sponsors of this study.

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Authors and Affiliations

  1. Key Laboratory for Power Machinery and Engineering (Ministry of Education), Shanghai Jiao Tong University, Shanghai, 200240, China

    Jinlong Wang, Jin Xiao, Yingdong Cheng & Zhen Huang

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  1. Jinlong Wang
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  2. Jin Xiao
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Correspondence to Jin Xiao.

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Competing interests Zhen HUANG is the Editor-in-chief of Frontiers in Energy, who was excluded from the peer-review process and all editorial decisions related to the acceptance and publication of this article. Peer-review was handled independently by the other editors to minimise bias.

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Wang, J., Xiao, J., Cheng, Y. et al. Design and modeling of a free-piston engine generator. Front. Energy 17, 811–821 (2023). https://doi.org/10.1007/s11708-022-0848-2

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