**1. Introduction**

The development of society and the economy is inseparable from energy. The demand for energy is increasing constantly in the current society, and the rapid consumption of fossil fuels has caused people to face a serious energy crisis. Therefore, people develop and explore new renewable and efficient fossil fuel alternative energy sources to deal with the increasingly severe energy problem [1–3]. At the same time, the massive burning of fossil fuels has also brought serious pollution problems. Large amounts of CO2 and thermal NOx are produced when the engine burns fossil fuels. NO and NO2 are the two most important harmful pollutants in thermal NOx produced by combustion [4–6]. In order to alleviate the serious environmental damage caused by fossil fuels combustion, various countries have formulated different measures to control the exhaust emissions of diesel engines strictly. The European Parliament plans to limit greenhouse gas emissions to deal with the serious problem of global warming and strive to achieve the goal of reducing more than 80% in 2050 [7]. The British Parliament passed an amendment in 2019 to achieve a net zero emission target by 2050. The French National Assembly had already incorporated a net zero target into law in June 2019. Therefore, the development and utilization of new renewable clean energy is receiving more and more attention.

N-heptane is often selected as a surrogate diesel fuel in previous simulations of diesel engines [8–10]. However, the carbon chain of actual diesel fuel is longer than that of n-heptane fuel, and the number of carbon atoms is about 10 to 25 [11]. The composition of diesel is complicated, and it is difficult to reproduce the combustion chemistry characteristics of diesel under low-temperature combustion through a single component

**Citation:** Wang, S.; Zhang, J.; Yao, L. Effect of Combustion Boundary Conditions and n-Butanol on Surrogate Diesel Fuel HCCI Combustion and Emission Based on Two-Stroke Diesel Engine. *Atmosphere* **2022**, *13*, 303. https://doi.org/10.3390/ atmos13020303

Academic Editors: Duanyang Liu, Kai Qin and Honglei Wang

Received: 11 January 2022 Accepted: 8 February 2022 Published: 10 February 2022

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represented by n-heptane. N-dodecane is considered to be a closer alternative fuel to diesel than n-heptane. Earlier, Luo et al. developed a simplified one-component diesel substitute (pure n-dodecane) to simulate diesel engines [12]. However, the mechanism of n-dodecane lacks alkylbenzene, which is an important component in diesel. It cannot reflect the real combustion of diesel well. Then, Pei et al. developed a diesel substitution mechanism (consisting of 77% n-dodecane and 23% m-xylene) on this basis and compared it with the experimental data of shock tube. Research has found that the mixture of m-xylene and n-dodecane can represent the combustion and emission characteristics of diesel fuel better than pure n-dodecane [13].

Oxygenated fuel can reduce PM and soot emissions significantly in diesel engine combustion [14–19]. At the same time, some oxygenated fuels are renewable. Oxygenated fuel can be used as a single fuel or mixed with diesel in a certain proportion. Therefore, oxyfuel is an ideal substitute for fossil energy. As a representative of the new generation of oxygenated fuel, the development and application of n-butanol (C4H10O) have attracted the extensive attention of scholars [20,21]. N-butanol has less corrosiveness and high safety. As an oxygenated fuel, burning n-butanol can reduce soot emissions effectively. The viscosity of n-butanol is lower than that of diesel, and it has better mutual solubility with diesel. The mixed combustion of n-butanol and diesel can be realized on the diesel engine with small changes to the engine [22–25]. In terms of environmental protection, n-butanol can be produced by biological methods, which can solve the problem of large fossil fuel consumption effectively. The renewability of n-butanol has high research value and wide application prospects.

As a new type of combustion concept, HCCI combustion has low NOx emissions and high combustion heat efficiency compared to traditional gasoline and diesel engines [26–28]. Analyzing the fuel combustion reaction rate through chemical reaction kinetics and controlling the ignition strategy of the engine are the most important contents in HCCI combustion [29]. A single fuel or fuel mixture can be used to control the ignition of different types of engines and the chemical reaction rates of the fuel in HCCI combustion process [30–32]. At present, a variety of fuels including biodiesel, n-butanol and n-heptane have been used in HCCI engines for research [33–37].

Therefore, the purpose of the study is to apply the HCCI combustion method to a marine diesel engine and reduce engine emissions of diesel. The method of reducing engine emissions in diesel surrogate fuel (77% n-dodecane and 23% m-xylene mixture) HCCI combustion by mixing with n-butanol in a two-stroke diesel engine is first proposed in the research.
