Nissan breaks 50% TE barrier with its e-Power series hybrid system

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Nissan has announced a significant breakthrough in IC engine efficiency, reaching 50% thermal efficiency (TE) with its in-development, next-generation e-Power system, a series hybrid that utilizes an onboard gasoline engine to provide electrical energy to the e-powertrain battery pack.

“In pursuit of carbon neutrality across our product lifecycle by 2050, Nissan aims to electrify all new models launched in major markets by the early 2030s,” said Toshihiro Hirai, senior vice president of the company’s powertrain and EV engineering division. “Nissan’s electrification strategy promotes the development of e-powertrains and high-performance batteries for EVs, with e-Power representing another important strategic pillar.”

Nissan notes that conventional ICE vehicles demand power and performance from an engine under a wide range of speeds and loads. This fundamental requirement means conventional engines cannot perform at their optimal efficiency at all times. However, because Nissan’s e-Power system utilizes an onboard engine as a dedicated generator for the e-powertrain, operation of the engine is limited to its most efficient range, appropriately managing the engine’s electricity generation and the amount of electricity stored in the battery.

With this dedicated approach, and the evolution of battery technology and energy management techniques, Nissan says it has been able to improve thermal efficiency beyond current levels. To achieve 50% thermal efficiency, the company developed a concept called STARC, which stands for Strong, Tumble and Appropriately stretched Robust ignition Channel. It says the concept enables improvement of thermal efficiency by strengthening in-cylinder gas flow and ignition, reliably burning a leaner air-fuel mixture at a high compression ratio.

Nissan notes that in a conventional engine, there are restrictions on controlling the dilution level of the air-fuel mixture to respond to changing driving loads, with several trade-offs between various operating conditions, such as in-cylinder gas flow, ignition method and compression ratio, which can sacrifice efficiency for power output. However, a dedicated engine running at an optimal range of speed and load for electrical generation makes it possible to dramatically improve thermal efficiency.

In internal testing, Nissan claims it achieved a thermal efficiency of 43% when using the EGR dilution method and 46% when using lean combustion with a multi-cylinder engine. A level of 50% was achieved by operating the engine at a fixed RPM and load combined with waste heat recovery technologies.

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Lawrence has been covering engineering subjects – with a focus on motorsport technology – since 2007 and has edited and contributed to a variety of international titles. Currently he is responsible for content across UKI Media & Events' portfolio of websites while also writing for the company's print titles.

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