Research Center for Innovative Combustion Technology

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

Base 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 for the prompt and smooth execution of this research and technological development by building a close collaborative network among expert researchers from related institutions within Keio University and externally.

The subject of research and development is "Super Lean-Burn R&D for High-Efficiency Gasoline Engines." The key focus areas are ignition and rapid combustion technologies 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 decreased thermal efficiency; and numerical analysis of fuel and its elementary reactions to avoid knocking.

Specifically, we will develop a stable ignition system to achieve low-temperature lean combustion with an excess air ratio of λ=2.0, analyze and optimize strong tumble flows in the 20–50 m/s range, establish lean combustion promotion technology based on clarifying flame propagation phenomena, optimize heat transfer reduction technology based on clarifying and modeling wall heat transfer mechanisms, and formulate suppression technology based on clarifying the conditions and mechanisms of knocking. We will advance the R&D of these various elemental technologies in an organic and complementary manner, with the goal of increasing the engine's net thermal efficiency to 50% by the final fiscal year of 2018.

Internal combustion engines, automobiles, combustion technology, high efficiency, CO ₂ reduction

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

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

In conventional engine combustion, thermal efficiency was limited to about 41% due to significant heat loss. 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, achieving a net thermal efficiency of 47.1% in fiscal year 2017. This thermal efficiency is among the highest in the world for an SI engine.

In fiscal year 2018, we will challenge low-temperature combustion with λ > 2.0, achieved through enhanced flow (higher tumble) by modifying the compression ratio, EGR rate, and S/B ratio (longer stroke), and by using a powerful ignition system. We will achieve 50% net thermal efficiency through thorough heat loss reduction.

If this figure is achieved, it will be possible to create internal combustion engine vehicles with lower CO2 emissions than EVs, contributing to the reduction of greenhouse gases. We aim for this to be a landmark year for the research center.

■New Activity Goals, Content, and Background for Fiscal Year 2018

Fiscal year 2018 is the final year for this center, and we will achieve our ultimate goal of 50% net engine thermal efficiency.

Specifically, we will conduct the following research and development to achieve an indicated thermal efficiency of 52% and a net thermal efficiency of 50%, figures yet to be reached anywhere in the world.

Furthermore, R&D 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" program.

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

In engine tests up to fiscal year 2015, we expanded the lean limit to λ=1.89 and increased the indicated thermal efficiency (including the portion from the electric supercharger) to 47.6% by using powerful ignition and high flow. Data from the initial-design metal engine used at that time suggested that the valve operating angle (opening duration) might be slightly too large to obtain high thermal efficiency. Therefore, in fiscal year 2016, we designed a valve timing cam with an optimized exhaust valve operating angle (opening duration) 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 fiscal year 2017, 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 from 15 to 17 for a longer stroke, aiming to enhance tumble flow and reduce heat loss. As a result, we achieved an indicated thermal efficiency of 50.1%. Taking into account the 0.63 pt improvement in supercharger efficiency and the 1.11 pt reduction in friction loss reported by the Loss Reduction Team in the SIP research, the estimated net thermal efficiency is equivalent to 47.1%. This value is one point higher than the net thermal efficiency target of 46.1% set by our research center for fiscal year 2017.

In fiscal year 2018, 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 partial ignition was achieved by applying a large amount of discharge energy, the combustion was unstable due to large fluctuations between flame propagation and flame extinction. (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 realized ultra-lean combustion, which is a low-temperature combustion with low energy loss, and we successfully demonstrated the final target of 50% net thermal efficiency.

This achievement is expected to contribute to reducing the environmental impact of vehicles 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.