Research Center for Innovative Combustion Technology

Center Director: Toshihisa Ueda (Professor, Faculty of Science and Technology)

Main Campus: Yagami

The Research Center for Innovative Combustion Technology, within the Keio Advanced Research Centers (KARC), aims to create innovative elemental combustion technologies to improve engine thermal efficiency. It establishes a research hub to carry out this research and technological development swiftly and smoothly by building a close collaborative network among expert researchers from related institutions both within Keio University and externally.

The subject of our research and development is the "research and development of super lean-burn for high-efficiency gasoline engines." We have set the following as priority items: ignition and rapid combustion technology for ultra-lean premixed mixtures to achieve super lean-burn, which is expected to lead to a dramatic improvement in thermal efficiency; technology to reduce cooling loss, a major cause of reduced thermal efficiency; and numerical analysis of fuel and its elementary reactions to avoid the occurrence of knocking.

Specifically, we will proceed with the development of various elemental technologies in an organic and complementary manner, including: developing a stable ignition system to achieve low-temperature lean combustion with an excess air ratio of λ=2.0; analyzing and optimizing strong tumble flow at the 20–50 m/s level; establishing lean combustion promotion technology based on clarifying flame propagation phenomena; optimizing heat transfer reduction technology based on clarifying and modeling the wall heat transfer mechanism; and formulating suppression technology based on clarifying the conditions and mechanisms of knocking. Our goal is to increase the engine's brake thermal efficiency to 50% by the final fiscal year of 2018.

Internal combustion engine, automobile, combustion technology, high efficiency, CO ₂ reduction

The research and development themes are the following fundamental technologies, centered on super lean-burn technology, to raise the current maximum brake thermal efficiency of automotive gasoline engines from 39% to 50%.

■ Activities Continuing from the Previous Fiscal Year: Background, Rationale, and Goals

In conventional engine combustion, thermal loss was large, and thermal efficiency was limited to about 41%. However, by introducing new ignition methods (high flow, extended discharge path, flame discharge ignition) and a new combustion method (Thin Reaction Zone combustion), we successfully developed technology for combustion at lower temperatures than before and achieved a brake thermal efficiency of 47.1% in FY2017. This thermal efficiency is one of the highest values in the world for an SI engine.

In FY2018, we will achieve a brake thermal efficiency of 50% through a thorough reduction of thermal loss. We will do this by challenging λ values exceeding 2.0 with low-temperature combustion, which is realized by a high-energy ignition system and increased flow (higher tumble) resulting from changes in the compression ratio, EGR rate, and S/B ratio (longer stroke).

If this figure is achieved, it will be possible to create automobiles with internal combustion engines that have lower CO2 emissions than electric vehicles (EVs), contributing to the reduction of greenhouse gases. We intend for this to be a landmark year for the research center.

■ New Activity Goals, Content, and Background for FY2018

FY2018 is the final fiscal year for our center, and we will achieve the engine brake thermal efficiency of 50% that we set as our final goal.

Specifically, we will conduct the following research and development to achieve a globally unprecedented 52% indicated thermal efficiency and 50% brake thermal efficiency.

Furthermore, development research related to supercharging, exhaust heat recovery, and friction reduction will be promoted in collaboration with the Loss Reduction Team of the Cabinet Office's SIP "Innovative Combustion Technology."

■ Implementation Details, Research Results, and Degree of Achievement against the Business Plan for the Fiscal Year

In engine tests up to FY2015, we were able to extend the lean limit to λ=1.89 and increase the indicated thermal efficiency (including the portion from the electric supercharger) to 47.6% by using high-energy ignition and high flow. Data also suggested the possibility that the valve duration (opening time) of the initial-design metal engine used at that time was slightly too large to obtain high thermal efficiency. Therefore, in FY2016, we designed a valve timing cam with an optimized exhaust valve duration (opening time) and pursued improvements in thermal efficiency. As a result, the lean combustion limit was extended to λ=1.93, and an indicated thermal efficiency of 48.5% was achieved.

In FY2017, we increased the compression ratio ε, a basic parameter of reciprocating engines, from 13 to 15, and also increased the bore-to-stroke ratio B/S value from 15 to 17 for a longer stroke, aiming to enhance tumble flow and reduce thermal loss. As a result, we achieved an indicated thermal efficiency of 50.1%. Taking into account the results reported in the SIP research by the Loss Reduction Team—a 0.63 percentage point improvement in supercharger efficiency and a 1.11 percentage point reduction in friction loss—the estimated brake thermal efficiency is equivalent to 47.1%. This value exceeds the FY2017 target of 46.1% brake thermal efficiency set by our research center by one percentage point.

In FY2018, we integrated past research results and (1) established the "ultra-lean combustion (super lean-burn)" concept. (2) The challenge: The problem with ultra-lean combustion was that it is difficult to ignite with conventional ignition technology. Even if partially ignited with a large discharge energy, the fluctuation between flame propagation and quenching was large, resulting in unstable combustion. (3) Implementation and results: We clarified the phenomena of high-turbulence, lean combustion by introducing a strong tumble flow (longitudinal vortex) into the ultra-lean combustion field. Based on these results, we developed an ignition technology that enables stable ignition. This allowed us to achieve ultra-lean combustion, which is a low-temperature combustion with low energy loss, and we successfully demonstrated our final goal of 50% brake thermal efficiency.

This achievement is expected to contribute to reducing the environmental impact of automobiles equipped with internal combustion engines, which are predicted to remain mainstream for the next several decades, and to reducing global carbon dioxide emissions.

0 published papers, 4 conference presentations, and 1 journal submission. The breakdown is as follows.

As external events, we disseminated information about this research to the public.