**1. Introduction**

Over the recent years, thanks to a vast improvement in fast sensor and actuator technology and a profound understanding of the phenomena that affect the combustion process in IC engines, the feasibility of implementing cycle-by-cycle-based control to attain optimum engine performance and emissions is growing. With the push towards global decarbonization growing stronger, a combination of advanced combustion strategies and effective next-cycle control could be a pivotal pathway towards meeting the stringent emissions regulations for IC engines. Researchers have demonstrated the abilities of technologies like Homogeneous Charge Compression Ignition (HCCI) [1,2], Premixed Mixture Ignition in the End-gas Region (PREMIER) [3,4], pre-chamber combustion systems [5,6] and Reactivity-Controlled Compression Ignition (RCCI) [7,8] to produce high fuel conversion efficiencies while maintaining low engine-out emissions. Of these, the RCCI/dual-fuel Low-Temperature Combustion (LTC) strategy has proven to be a viable pathway to achieve ultra-low NOx emissions and good fuel conversion efficiencies (FCE) at both high and low

**Citation:** Silvagni, G.; Narayanan, A.; Ravaglioli, V.; Srinivasan, K.K.; Krishnan, S.R.; Collins, N.; Puzinauskas, P.; Ponti, F. Experimental Characterization of Hydrocarbons and Nitrogen Oxides Production in a Heavy-Duty Diesel–Natural Gas Reactivity-Controlled Compression Ignition Engine. *Energies* **2023**, *16*, 5164. https://doi.org/10.3390/ en16135164

Academic Editors: Tomasz Czakiert and Monika Kosowska-Golachowska

Received: 30 May 2023 Revised: 20 June 2023 Accepted: 3 July 2023 Published: 4 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/).

load operations [9–16]. Hariharan et al. [17] experimentally studied the effect of various engine operating parameters like injection timing, intake pressure and injection pressure on emissions and performance of dual-fuel combustion with diesel–natural gas (NG) and polyoxymethylene di-methyl ether (POMDME)–NG as fuel combinations. At low loads (~5 bar IMEP), the authors were able to achieve indicated FCEs of >40% and indicated specific NOx values of ~0.1 g/kWh at injection timings ranging from 40–60 deg bTDC. However, these single-injection operating points were found to have high cyclic variations and high HC emissions. The authors were able to mitigate the negative impact of cyclic variations by adding a second injection closer to TDC and by operating at lower intake pressures. Cyclic variations, which arise due to fluctuations in local air fuel ratios and incylinder thermodynamics, have a profound negative impact on HC emissions and engine performance. Methods to quantify cyclic variations and to identify its cause have been extensively documented in the literature [18–23]. For example, Jha et al. [24] experimentally examined the effect of methane energy fraction at low loads in diesel–methane dual fuel combustion at early injection timings. They observed that increasing the energy fraction of methane from 50% to 90% at −50 deg aTDC start of injection decreased NOx emissions by a factor of 43; increased HC emissions by a factor of 9 and increased cyclic variations. The authors attributed the increase in cyclic variations to the decrease in reactivity of the mixture as methane energy fraction increases. Various diagnostics methods have been investigated to examine cyclic variations in dual fuel combustion. Cheng et al. [25] examined cyclic variations in diesel—methane dual fuel combustion using optical methods and used a proper orthogonal decomposition method to analyze and quantify cyclic variations. The increased fluctuations in the luminosity field were considered to be a marker for increased cyclic variations. They observed delayed onset of cyclic variations as the methane fraction increased at an injection timing of 15 deg bTDC.

The objective of the present work is to correlate the cause of high HC and NOx emissions to the combustion behavior in diesel–NG dual fuel combustion using cyclic emissions data obtained from fast pollutant measurement systems in combination with combustion data.
