Citation: Halis, S.; Solmaz, H.; Polat, S.; Yücesu, H.S. Numerical Investigation of a Reactivity- Controlled Compression Ignition Engine Fueled with N-Heptane and Iso-Octane. Sustainability 2023, 15, 10406. https://doi.org/10.3390/ su151310406 Academic Editor: Ali Bahadori- Jahromi Received: 11 June 2023 Revised: 22 June 2023 Accepted: 29 June 2023 Published: 1 July 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/). sustainability Article Numerical Investigation of a Reactivity-Controlled Compression Ignition Engine Fueled with N-Heptane and Iso-Octane Serdar Halis 1, * , Hamit Solmaz 2 , Seyfi Polat 3 and H. Serdar Yücesu 2 1 Department of Automotive Engineering, Faculty of Technology, Pamukkale University, Denizli 20160, Turkey 2 Department of Automotive Engineering, Faculty of Technology, Gazi University, Ankara 06500, Turkey; hsolmaz@gazi.edu.tr (H.S.); yucesu@gazi.edu.tr (H.S.Y.) 3 Department of Mechanical Engineering, Faculty of Engineering, Hitit University, Corum 19040, Turkey; seyfipolat@hitit.edu.tr * Correspondence: shalis@pau.edu.tr Abstract: In this numerical study, the effects of the premixed ratio, intake manifold pressure and intake air temperature on a four-cylinder, four-stroke, direct injection, low-compression-ratio gasoline engine, operated in reactivity-controlled compression ignition (RCCI) combustion mode at a constant engine speed of 1000 rpm, were investigated using Converge CFD software. The results of numerical analyses showed that the maximum in-cylinder pressure and heat release rate (HRR) increased and the combustion phase advanced depending on the rise in both intake manifold pressure and intake air temperature. The CA50 shifted by 18.5 ◦ CA with an increment in the intake air temperature from 60 ◦ C to 100 ◦ C. It was observed that the combustion duration dropped from 44 ◦ CA to 38 ◦ CA upon boosting the intake manifold pressure from 103 kPa to 140 kPa. Moreover, a delay in the combustion phase occurred at a constant intake air temperature with an increasing premixed ratio. The maximum value of in-cylinder pressure was recorded as 36.15 bar (at 11 ◦ CA aTDC) with the use of PRF20. Additionally, as the content of iso-octane in the fuel mixture was increased, combustion delay occurred, and the maximum value of in-cylinder temperature obtained was 11 ◦ CA aTDC using PRF20 fuel at the earliest point. While HC and CO emissions reached the highest values at a 60 ◦ C intake air temperature, NO x and soot emission values were detected at quite low levels at this temperature. The values of all these emissions increased with rising intake manifold pressure and reached their highest values at 140 kPa. In addition, while the highest HC and CO emission values were observed with the use of PRF60 fuel, the results revealed that the control of the combustion phase in the RCCI strategy is notably affected by the premixed ratio, intake manifold pressure and intake air temperature. Keywords: RCCI; combustion; intake air temperature; intake manifold pressure; premixed fuel ratio; CFD; emission 1. Introduction Recently, there has been an increase in the cost of oil resources due to a reduction in the amount of them. Moreover, exhaust emissions have caused serious environmental pollution due to the increase in the use of vehicles. Scientists have focused on alternative combustion modes with high efficiency and low emissions in the face of these negative conditions [1–3]. Although spark ignition (SI) and compression ignition (CI) internal combustion engines have high power density and performance, in SI engines, a compression ratio (CR) higher than 14 cannot be applied due to the knock limit. In addition, SI engines are known to have low energy conversion efficiency at high loads and high CO emissions [4,5]. CI engines also have high soot emission, which may occur due to the heterogeneity of the mixture and high nitrogen oxide (NO x ) emission values due to high temperatures [6]. The use of after-treatment systems to prevent these emissions is mandatory and costly. These problems expose disadvantages in the use of SI and CI engines. Considering these negative Sustainability 2023, 15, 10406. https://doi.org/10.3390/su151310406 https://www.mdpi.com/journal/sustainability