A team from McMaster University in Canada has published a comprehensive review of recent progress in low-temperature combustion (LTC), alternative fuels (AF), over-expansion Atkinson cycle, and waste heat recovery (WHR) techniques as applied to hybrid-specific engines. Their open-access paper is published in the journal Energy Reports.

As a midterm technology from conventional internal combustion engine vehicles to electric vehicles, hybrid electric vehicles have received wide research attention from industry and academics alike and are sharing an increasing percentage of vehicles in the market. As a crucial component in hybrid powertrains, the internal combustion engine has important impacts on vehicle performance. Recent years have witnessed tremendous effort towards hybrid electric vehicle-specific engine technologies.

… Engines as one of the propulsion components in HEVs significantly affect vehicle fuel economy and emission performance. Advanced combustion regimes, cleaner fuels, efficient operating cycles, and waste energy recovery are four complementary technology pathways to high-performance hybrid-specific engines. Over the years, there have been a large number of studies on applying low temperature combustion, alternative fuels, over-expansion Atkinson cycle, and waste heat recovery on hybrid powertrain platforms. To find the current research status and provide insights on future research opportunities, this paper gives a comprehensive review of these four technological solutions from perspectives of benefits, challenges, and future prospects.

—Wang et al.

Among their findings:

• LTC-HEVs offer the opportunity for fuel economy improvement when further engine downsizing is limited. HEV configurations where engines can be independent of wheels are most suitable for implementing LTC. Vehicle control strategies and driving conditions have direct impacts on LTC-HEV performance. Multi-mode LTC-HEV shows a slight improvement over single-mode LTC-HEV at the expense of higher system complexity. Further research is required on extending LTC operating range and combustion timing control.

• AF-HEVs are a viable strategy for achieving emission reductions. Although a higher capital cost is inevitable, the simplicity or elimination of aftertreatment systems could make up for it to some degree. Future research work should focus on seeking new high-performance, cost-effective alternative fuels.

• Atkinson-cycle engines have been extensively used on HEVs with great fuel-saving performance. Modern Atkinson engines are mostly realized by VVT technology. Range-extended electric vehicle (REEV) architecture enables Atkinson engines to constantly operate at high-efficiency zone, which is favorable in engine downsizing. Future engine designs could consider Otto-Atkinson cycle to improve power density.

• Thermoelectric generators (TEGs) and thermodynamic bottoming cycles are two WHR approaches for HEVs. Hybrid powertrains exclusively possess the capability of directly using TEG power, although the resulting fuel savings are limited. High-performance thermoelectric materials and transient system designs are two areas of future focus. Thermodynamic bottoming cycles present relatively higher efficiency than TEGs while exhibit challenges in working fluid selection and system integration/control, where future research efforts are suggested.

On powertrain level, it is necessary to understand the interaction between the engine and other powertrain components. For hybrid-specific engines, eliminating redundant elements is just as important as adding new engine technologies. In the meantime, fuel economy and emissions are not the only metrics of performance. Hybrid-specific engine design should be a careful balance of all the factors.

—Wang et al.

Resources

• Yue Wang, Atriya Biswas, Romina Rodriguez, Zahra Keshavarz-Motamed, Ali Emadi (2022) “Hybrid electric vehicle specific engines: State-of-the-art review,” Energy Reports, Volume 8, Pages 832-851 doi: 10.1016/j.egyr.2021.11.265