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Researchers propose optimization strategy for Atkinson cycle engines in hybrids

A team from the Beijing Institute of Technology and Beiqi Foton Motor are proposing a performance optimization strategy for the Atkinson cycle gasoline engine, which is playing a key role in the development of hybrid electric vehicles due to its greater fuel economy than a conventional Otto cycle engine. A paper on their work is published in the journal Fuel.

Because the traditional gasoline engine has some shortcomings such as large pumping loss and poor fuel economy at part load, it can no longer meet the demand of HEV for economy and emission. The Atkinson cycle engine has become the main choice for the development of HEV because of its own advantages. Related research points out that 20%–30% of the fuel economy of HEV is attributed to the development and application of Atkinson cycle engine. At present, it is generally believed that Atkinson cycle engine is an ideal engine type for hybrid power system.

… The purpose of this paper is to introduce a novel performance optimization strategy for the Atkinson cycle engines and investigate the fuel-saving mechanism of the Atkinson cycle engines in great detail at low speed and part load.

—Niu et al.

The team focused on the compression ratio range according to cycle efficiency at small pressure rise ratio and the optimization of the high compression ratio and the late intake valve closure (LIVC) under the compression pressure constraint.

Atkinson cycle has the significant advantage of cycle efficiency at all pressure rise ratios, especially at high pressure rise ratio. The pressure rise ratio and the ratio of expansion ratio to effective compression ratio have significant influence on Atkinson cycle. At a fixed pressure rise ratio, the Atkinson efficiency first increases to a maximum value and then decreases with the increase of εec.

The range of the compression ratio was preliminarily determined according to the Atkinson cycle efficiency at small pressure rise ratio through thermodynamic analysis; the optimization of the IVC at compression ratios was presented under the compression pressure constraint and the compression ratio and other valve timing are optimized through one-dimensional simulation.

The main results of the study included:

  • The team achieved significant improvements in pumping losses and fuel economy, verifying the effectiveness of the optimization strategy. The increase of mechanical efficiency due to the PMEP reduction is the main reason for the fuel economy improvement at low- and medium-load, while the increase of indicated thermal efficiency is the main reason for the fuel economy improvement at high-load.

  • The BSFC of the Atkinson cycle engine was improved by 9% at 2000 rpm@2 bar and 8% at 3000 rpm@3 bar. The increase in intake pressure is the main reason for the decrease in PMEP. The Atkinson cycle engine can achieve more obvious constant volume combustion and the combustion quality was greatly improved.

Further research will be carried out in the following aspects: the power performance, emission performance and the effectively use of intake reflux effect will be studied by exploring the working process of multi-cylinder Atkinson cycle engine.

—Niu et al.

Resources

  • Qingyu Niu, Baigang Sun, Dongsheng Zhang, Qinghe Luo (2020) “Research on performance optimization and fuel-saving mechanism of an Atkinson cycle gasoline engine at low speed and part load,” Fuel, Volume 265, doi: 10.1016/j.fuel.2020.117010.

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