Citation: Doppalapudi, A.T.; Azad, A.K.; Khan, M.M.K. Analysis of Improved In-Cylinder Combustion Characteristics with Chamber Modiﬁcations of the Diesel Engine. Energies 2023, 16, 2586. https:// doi.org/10.3390/en16062586 Academic Editors: Roberta De Robbio and Maria Cristina Cameretti Received: 1 February 2023 Revised: 2 March 2023 Accepted: 6 March 2023 Published: 9 March 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). energies Article Analysis of Improved In-Cylinder Combustion Characteristics with Chamber Modiﬁcations of the Diesel Engine Arun Teja Doppalapudi 1 , Abul Kalam Azad 1, * and Mohammad Masud Kamal Khan 2 1 School of Engineering and Technology, Central Queensland University, Melbourne Campus, 120 Spencer Street, Melbourne, VIC 3000, Australia 2 School of Engineering, Computer and Mathematical Sciences, Auckland University of Technology, Auckland 1010, New Zealand * Correspondence: azad.cqu@gmail.com or a.k.azad@cqu.edu.au Abstract: This study numerically analyses the effects of chamber modiﬁcations to investigate the im- provement of in-cylinder combustion characteristics of the diesel engine using a computational ﬂuid dynamics (CFD) approach. Five different modiﬁed chambers, namely, the double swirl combustion chamber (DSCC), bathtub combustion chamber (BTCC), double toroidal re-entrant combustion cham- ber (DTRCC), shallow depth combustion chamber (SCC), and stepped bowl combustion chamber (SBCC) were developed and compared with a reference ﬂat combustion chamber (FCC). The effects of chamber modiﬁcations on temperature formation, velocity distribution, injection proﬁles, and in-cylinder turbulent motions (swirl and tumble ratio) were investigated. During the compression stroke, near top dead centre, the SCC showed a peak temperature of 970 K, followed by the FCC (968 K), SBCC (967 K), and DTRCC (748 K to 815 K). The DSCC and the SCC showed a high swirl ratio above 0.6, whereas the DTRCC and the BTCC showed a high tumble ratio of approximately 0.4. This study found that the SCC, BTCC, and DSCC have better combustion rates than the FCC in terms of temperature, heat release rate, and velocity distribution. However, the DTRCC showed poor temperature formation rates and rapid heat release rates (approx. 150 J/ ◦ CA), which can lead to rapid combustion and knocking tendencies. In conclusion, the DSCC and the SCC showed better combustion rates than the other chambers. In addition, turbulent motions inside the chambers avoided combustion in crevice regions. This study recommends avoiding chambers with wider bowls in order to prevent uneven combustion across the cylinder. Furthermore, split bowls such as the DSCC, along with adjusted injection rates, can provide better results in terms of combustion. Keywords: combustion chamber modiﬁcation; combustion simulation; heat release rate; cylinder temperature; CFD analysis 1. Introduction Air–fuel mixture formation inside the engine cylinder is mainly responsible for en- gine performance and emissions [1]. With the improper mixture, fuel accumulation can occur inside the chamber due to an uneven distribution of air and fuel, which can cause incomplete combustion. To address these issues, engine modiﬁcations such as nozzle modiﬁcations [2] and port ﬂow design have been investigated during recent decades to improve the overall in-cylinder combustion [3–5]. In addition, injection modiﬁcations [6] and piston bowl modiﬁcations [7] have been experimentally investigated to improve the combustion behaviour of diesel engines. Among these, the piston bowl modiﬁcation tech- nique is well-documented in the literature as one good active technique that directly affects air–fuel mixture formation. Piston bowl modiﬁcations have gained signiﬁcant attention in the last decade due to their active interactions inside the cylinder during the air–fuel mixture formation [8]. The piston bowl inﬂuences the air–fuel mixture through turbulence, which is governed by Energies 2023, 16, 2586. https://doi.org/10.3390/en16062586 https://www.mdpi.com/journal/energies