Improving the fuel/air mixing and combustion process in a low-speed two-stroke engine by the IFA strategy under EGR atmosphere

Abstract

Exhaust Gas Recirculation (EGR) is one of the most effective in-cylinder NOx emission reduction methods to meet the Tier III standards, but the introduction of the exhaust gas is usually accompanied by the expense of fuel economy. To solve the deterioration of fuel economy when using EGR, a method named Injected Fresh Air was firstly proposed to improve the swirl level of in-cylinder flow without sacrificing the scavenging efficiency in this work. The related numerical research of the fuel/air mixing process, combustion performance and emission characteristics on the EX340EF two-stroke low-speed marine engine at 75% load under EGR atmosphere was performed. With the IFA strategy under EGR atmosphere, the swirl ratio could increase by 163%, accelerating the fuel/air mixing process and further optimizing subsequent combustion performance. As a result, compared with the cases without the IFA strategy at the EGR rate of 20%, 25%, and 30%, the duration of CA90 with the IFA strategy was reduced by 47.1%, 49.4% and 50.8%, respectively, and both the NOx emission and the brake specific fuel consumption (BSFC) were decreased. Finally, compared with the base case, the NOx emission was reduced to 2.82 g/kWh with a nearly 75.5% reduction when using the IFA strategy under the EGR rate of 40%, which could meet the Tier III standards (3.4 g/kWh). Moreover, the BSFC was decreased by 2.33 g/kWh with a nearly 1.3% reduction. The IFA strategy successfully achieved the goal of improving the fuel/air mixing and combustion process under EGR atmosphere by enhanced swirl ratio, and the results of this study can be used to develop a new method to satisfy the IMO Tier III requirements and avoid high fuel consumption, has a better application prospect in emissions control and fuel consumption optimization of low-speed two-stroke diesel marine engines.