**5. Conclusions**

This work highlights the correlation between combustion characteristics and pollutants emissions, especially unburned hydrocarbons (HC) and oxides of nitrogen (NOx) based on fast FID and fast NOx measurements acquired cycle-by-cycle. The experimental investigation was performed on a Single-Cylinder Research engine run in dual-fuel diesel–natural gas mode at a fixed speed and a fixed low load of 5 bar IMEP gross and 1339 rpm, respectively. The natural gas PES was kept constant at 80% and the diesel was injected at an injection pressure of 500 bar, while the HRF SOI was varied from −10 to −60 deg aTDC.

The instantaneous pollutants emissions were acquired using CAMBUSTION HFR400 and CLD 500 with sampling frequency of 500 Hz able to obtain cycle-resolved data. To further validate the values coming from the CAMBUSTION instrumentations, a standard five-gas emission bench was used to compare the outputs in terms of average (steady-state) pollutants concentrations from different sources. Once compensated for the time-delay of the fast emission measurement systems with respect to the actual engine cycle (due to the distance between the exhaust valve and the sampling probe), the analysis of the acquired data allowed to define two correlations aimed at demonstrating the sources of each pollutants produced.

The correlation between cycle-to-cycle variability and the HC production were identified by comparing two different engine operating conditions of −10 deg aTDC and −60 deg aTDC, called "Retarded SOI" and "Advanced SOI", respectively. Introducing the Net Cyclic Hydrocarbon (NCHC) emissions as representative of the net HC production (if positive), or net HC reduction (if negative), for the analyzed engine cycle compared to the previous one, the cyclic variability was defined through the calculation of the IMEP difference between of two consecutive engine cycles (ΔIMEP). Finally, a linear experimental correlation (R2 = 0.86) between NCHC and ΔIMEP was obtained, providing a reasonable physical explanation for high HC emission running diesel–NG dual-fuel combustion at low load with advanced injection timing.

Following the same approach, a physical explanation for high NOx emissions in diesel– NG dual-fuel combustion was proposed. Through the analysis of the ensemble-averaged RoHR data and slow-speed measurements, a potential link between the combustion shape (i.e., the existence of a distinct first stage of RoHR, summarized by the RoHRmax) and NOx emissions was identified. The analysis of the cycle-to-cycle combustion data and crank-resolved NOx measurements confirmed the existence of a clear trend through the identification of a linear correlation (R2 = 0.82) between the maximum value of the instantaneous NOx and the calculated RoHRmax. Since the analysis performed in this paper consider only few SOIs at a constant load (low load), further activities are currently being performed to verify the obtained correlation at different engine operating conditions of RPM, load, intake pressure, rail pressure and number of HRF injections.

**Author Contributions:** Conceptualization, G.S., K.K.S. and S.R.K.; methodology, G.S. and A.N. and F.P.; investigation, G.S., A.N., N.C. and S.R.K.; resources, G.S. and A.N.; data curation, G.S.; writing—original draft preparation, G.S. and V.R. writing—review and editing, K.K.S., S.R.K., F.P. and P.P.; visualization, G.S. and V.R.; supervision, F.P., S.R.K. and K.K.S.; project administration, V.R., F.P., S.R.K. and K.K.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This paper was developed based on partial funding from the Alliance for Sustainable Energy, LLC, Managing and Operating Contractor for the National Renewable Energy Laboratory for the U.S. Department of Energy (Prime Award #DE-AC36-08GO28308; Subaward #NHQ-9-82305-01). Additional financial support was provided by the Department of Mechanical Engineering of The University of Alabama, and the University of Bologna.

**Data Availability Statement:** The data presented in this paper will be provided by the authors on request.

**Acknowledgments:** The authors acknowledge funding from the Alliance for Sustainable Energy, LLC, Managing and Operating Contractor for the National Renewable Energy Laboratory for the U.S. Department of Energy as well as from the University of Bologna and The University of Alabama.

**Conflicts of Interest:** The authors declare no conflict of interest.
