With increasing levels of agricultural mechanization operations in China, the overall quantity of agricultural machinery has also been increasing. By the beginning of 2022, the total power consumption of agricultural machinery in China was 107,768.02 million kilowatts, the total number of medium and large tractors was 477,737, and the number of small tractors was 17,275,995 [
1]. The vast majority of these tractors source their power from diesel engines. Compared to gasoline engines, diesel soot (soot), and nitrogen oxide, (NOx) emissions from diesel engines are significantly increased, which is a serious hazard in respect of human health and the environment [
2]. In China, in order to solve this problem, the Ministry of Environmental Protection introduced emission regulations for non-road mobile machinery (the “National IV” emission standards) on 1 December 2022, but the current emission levels of agricultural machinery in most regions are still dominated by the National II and National III emission standards. In the face of increasingly stringent emission regulations and serious energy and environmental issues, reducing the emission of pollutants from agricultural machinery is of great significance to the stable development of China’s economy and society and to environmental protection [
3,
4].
There are three mainstream methods used to reduce pollutants in diesel engines: Pre-treatment technology (oil technology), in-cylinder technology (in-engine purification technology), and post-treatment technology (off-engine exhaust after-treatment technology) [
5]. Internal optimization, such as combustion chamber system optimization, can effectively improve the combustion efficiency of the engine and reduce pollutant emissions [
6]. As the main container for fuel mist formation and combustion, the combustion chamber plays a key role in mixture formation and combustion. Different combustion chamber structure designs can change the oil–gas mixture state in the cylinder and affect the fluid motion state and combustion process in the cylinder [
7]. An appropriate combustion chamber design can enhance the interaction between vortex and turbulence in the cylinder, accelerate the formation of the oil–gas mixture, and distribute it well in the cylinder, thus improving the combustion efficiency of the whole engine and effectively reducing the generation of harmful emission gases. In this regard, a significant amount of research has been conducted in China and abroad. The team of Su Wanhua of Tianjin University designed a BUMP-type combustion chamber based on the phenomenon of convex ring stripping wall jets [
8,
9,
10], where a finite flow edge is set on the inner wall of the combustion chamber so that a secondary jet is formed in the combustion chamber after the oil beam touches the wall, which accelerates the mixture formation rate and improves the overall combustion efficiency while reducing soot and NOx emissions. Rakopulos et al. [
11] investigated the effect of combustion chamber structure on diesel engine performance and emissions based on a high-speed direct injection diesel engine and found that combustion chamber structure, fuel injection, and airflow motion are the key factors affecting diesel engine performance and emissions. Li et al. [
12] investigated the effect of different combustion chamber crater depths on diesel engine performance and emissions, and the results showed that indented combustion chambers exhibit better oil–gas mixing and open combustion chambers exhibit better combustion performance at low-speed operating conditions. Abdul et al. [
13] investigated the effect of the swirl ratio on engine performance and emissions under different combustion chambers and showed that a lower indentation rate leads to an increase in NOx emissions and a decrease in soot emissions. Jafarmadar et al. [
14] investigated the effect of the piston combustion chamber structure on the swirl number and uniformity index and found that a larger crater diameter generally has a higher swirl number and causes lower soot emissions, while a smaller crater depth results in a stronger squeeze flow and output power. Shahanwaz et al. [
15] studied the effects of different combustion chamber sizes and injection cone angles on engine emissions and found that an annular combustion chamber structure with different injection cone angle couplings can enable oil and gas to mix more fully and reduce carbon soot emission. At present, scholars at both domestic and international levels mainly focus on two methods for combustion chamber design and optimization. One involves designing and optimizing the traditional combustion chamber shape, and the other involves developing new combustion chambers based on new concepts. However, the common purposes of both are to improve the quality of the oil–gas mixture in the combustion chamber, improve the combustion process, and reduce pollutant emissions. The main research approaches include empirical formulations [
16], numerical simulations [
17], and complete engine experiments [
18,
19]. However, most of the methods have long working cycles, high costs, and high accuracy requirements in respect of the experimental models. In recent years, the rise in popular computer technologies, such as machine learning, has provided a new research idea for the rapid prediction of combustion chamber emissions and the mining of new methods and new information [
20].
Based on the above, in this study, we first established a diesel engine combustion chamber model based on a YTO Company (China, Luoyang) agricultural tractor diesel engine for simulation, and the simulation model was then tested to enable validation and analysis. The in-cylinder pressure and heat release values obtained from the simulation of the combustion chamber model were within 6% error of the test data. The overall trend of change was basically consistent. The simulation model can simulate the working conditions of the test engine relatively well. Next, an artificial neural network was established as an agent model based on the indentation rate, tab depth, and combustion chamber depth as the input, and NOx and diesel soot emitted from the engine as the output. Subsequently, based on the general particle swarm optimization (PSO) algorithm, an improved PSO algorithm was proposed, which continuously adjusts the inertia factor with the help of a skip line function during the optimization process, so that the inertia factor adapts to different rates and adjusts the magnitude of the corresponding values in different periods, thus facilitating the PSO in switching gradually from local optimization seeking to global optimization seeking. The improved PSO algorithm was used to optimize the optimal input parameter matching of the proxy model, and the new combustion chamber structure was formed and imported into CONVERGE CFD (Convergent Science Company, Madison, WI, USA) software for comparison with the original combustion chamber. It was found that the new combustion chamber structure can effectively reduce diesel engine emission soot and NOx, which provides suggestions and references for the development of relevant engine combustion chamber systems.