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To correspond to the social requirements such as climate change, air pollution, and energy security, enhancing the engine thermal efficiency is strongly required in these days. As for the specific engine technologies to improve the engine thermal efficiency, Atkinson cycle, cooled EGR (Exhaust Gas Recirculation), and low friction technologies have been developed [1–4]. In regard to combustion technology, lean boosted concept has a potential to reduce CO2 emission because lean boosted concept is expected to enhance the engine thermal efficiency. Although expanding lean combustion limit is important for both increasing the engine thermal efficiency and reducing NOx emission, there is a limitation to realize stable lean combustion with SI (Spark Ignition) gasoline engine. In this study, fuel effects on the combustion characteristics from the viewpoint of chemical reaction capability are focused on.

Thermal efficiency can be improved with a super lean-burn. In a super lean-burn engine, combustion takes place at lower temperatures, meaning lower energy losses and much greater thermal efficiency. In Part I (presented at PF&L 2019) [1], we studied the effects of various fuels on the lean limit in super lean-burn conditions. We found that the lean limit could be greatly extended and thermal efficiency improved with the right combination of engine technology and fuel technology. We also found that the lean limit closely linked to the duration from start-of-spark discharge to CA10, and that substances which shorten this duration extends the lean limit. In this study, we evaluated the effects of hydrocarbons closer in composition to commercial gasoline on the lean limit and found that at the specific components, the lean limit could be much higher than that with commercial gasoline. We also studied the lean limit extension mechanism by focusing on autoignition.

To reduce CO2 emission from the vehicles equipped with internal combustion engine, it is important to combine novel fuel characteristics with combustion technology for high thermal efficiency engine. Lean boosted technology has been studied for a long time because of its potential to increase engine thermal efficiency. Super lean-burn whose excess air ratio λ is higher than 2.0 is expected to achieve both high thermal efficiency and low NOx emissions. In such high λ condition, the chemical reaction effect of fuel components becomes more important. However, it is not evaluated enough due to the difficulty of achieving super lean condition. Therefore, in this paper the effects of fuel on combustion speed, knocking, emission and thermal efficiency are analyzed with super lean burn engine.

In this research, we investigated the improvement of combustion and anti-knocking properties as factors that affect the lean limit in order to reflect in fuel design. First, as a basic study, characteristics such as the Laminar burning velocity and Ignition energy of hydrocarbons, which are highly effective in improving combustion speed, were examined. In addition, using the knowledge obtained in the basic study, several concept fuels were created by blending the blend- stocks of the refinery aiming to meet or exceed the current standards in Japan. Their lean limits, thermal efficiencies, and effects on CO2 emission were investigated.