The combustion process using two fuels with different reactivity, known as dual-fuel combustion or RCCI is mainly studied to reduce emissions while maintaining thermal efficiency compared to the conventional diesel combustion. Many studies have proven that dual-fuel combustion has a positive prospect in future combustion to achieve ultra-low engine-out emissions with high indicated thermal efficiency. However, a limitation on high-load expansion due to the higher maximum in-cylinder pressure rise rate (mPRR) is a main problem. Thus, it is important to establish the operating strategy and study the effect of in-cylinder flow motion with dual-fuel combustion to achieve a low mPRR and emissions while maintaining high-efficiency.
In this research, the characteristics of gasoline-diesel dual-fuel combustion on different hardware were studied to verify the effect of the in-cylinder flow motion on dual-fuel combustion. To see such an effect, different head types (swirl and tumble) were used with different combustion chamber shapes (conventional vs bathtub).
The higher thermal efficiency with swirl motion on low load combustion was shown with stable combustion due to the faster combustion occurred by air-fuel mixing of diesel fuel by swirl effect. However, under the high load condition, the fast combustion at the first stage of the combustion process by swirl motion causes high pressure rise rate, which could not be fully controlled by EGR rate. High EGR rate makes poor combustion by rich combustion under high load condition due to the limitation of intake pressure. On the other hand, tumble motion gives slow combustion rate in the first stage of the combustion (MFB 05-50), and contributes on second stage of the combustion (MFB 50-90) to be faster which are mostly gasoline fuel. The higher tumble motion increases the latter combustion speed and results in faster overall burn duration. With higher tumble motion, thermal efficiency can be improved with potential of expanding higher load condition.