*4.1. Cyclic HC—IMEP Correlation*

As described in the Introduction, one of the aims of this work is to demonstrate the correlation between the cyclic HC production and IMEP (gross) by using a crank-resolved measurement system operating the heavy-duty SCRE in dual-fuel mode.

Our previous efforts [17] demonstrated that despite remarkable improvements in pollutant emissions and fuel consumption reduction possible with dual-fuel combustion at very advanced SOI, the combustion stability is strongly influenced by the PES and the thermodynamic conditions of the air–fuel mixture. As a result, through the analysis of the experimental data acquired during the performed SOI sweeps, to find out the HC–IMEP correlation it was decided to focus the attention on the 80% PES tests described in Figure 4.

The two engine operating points identified to highlight this correlation are characterized by approximately the same IMEP and CA50 but very different COV of IMEP (we discuss this aspect later in the paper). As reported in Figure 6, the spread of the cyclic IMEP and CA50 are seen by analyzing a subsystem of 300 consecutive cycles (same considerations can be extended to the entire dataset) in the two identified engine conditions at low load: "Advanced SOI" (SOI −60 deg aTDC) and "Retarded SOI" (−10 deg aTDC).

**Figure 6.** Comparison between measured cyclic CA50-IMEP gross (300 engine cycles) related to two different engine conditions: SOI −60 deg aTDC called "Advanced SOI", and SOI −10 deg aTDC called "Retarded SOI".

For dual-fuel combustion at very advanced SOI, the ignition process (as discussed previously) is driven more by in-cylinder chemistry, which in turn, is affected by the local air–fuel ratio [26–30,36]. To better clarify this aspect, the cyclic IMEP of two groups of 300 consecutive cycles with different SOIs ("Retarded SOI" and "Advanced SOI") are shown in Figure 7. By using advanced SOI, the HRF ignition delay increases due to the "over leaning" of local air–fuel ratios, and this leads to high COV. The spread of CA50 instead is related to the presence of recovery cycles after "near-misfire" engine cycles. After a very low-efficiency combustion, the residuals are mostly composed by unburned air–fuel mixture (fully premixed, after the entire exhaust and the intake stroke). Such extra chemical power given by the residuals increases the engine load anticipating the CA50 of the following cycle ("recovery cycles") consequently. Such behavior is summarized

in Figure 8, where the two random groups of engine cycles (the group are highlighted through the yellow box) highlighted in Figure 7 are reported. As explained in the previous section of the paper, due to the different thermodynamic conditions when the HRF is injected, the two extreme SOIs are characterized by extremely different cyclic variations and combustion shape. It is important to mention that, using the AC dynamometer with its associated speed controller (Figure 1), it was possible to investigate extremely unstable engine conditions while keeping the engine speed at a constant value (engine speed fluctuations were mitigated by the dynamometer speed controller).

**Figure 7.** Measured IMEP for 300 consecutive engine cycles: (**a**) "Retarded SOI" condition, (**b**) "Advanced SOI" condition.

**Figure 8.** Measured Rate of Heat Released (RoHR) for consecutive cycles in the two different analyzed engine conditions: (**a**) "Retarded SOI", dual-stage combustion, (**b**) "Advanced SOI", "Gaussian" combustion.

As is evident from Figures 7 and 8, due to the different ignition dynamics and consequently combustion RoHR shape and duration, the two analyzed conditions clearly differ in terms of cyclic IMEP behavior: the "Retarded SOI" condition is characterized by very low COV (Figures 7a and 8a) whereas significant IMEP variations are observed for the "Advanced SOI" condition (Figures 7b and 8b).

To demonstrate the impact of the cyclic variations on HC production, cycle-resolved HC emissions were measured using a CAMBUSTION HFR400 fast FID analyzer. By using the fast FID measurement system, which can sample the hydrocarbon emissions at 500 Hz, it was possible to obtain the cycle-resolved HC emissions for each engine cycle. By running

the engine at 1339 rpm, the 500 Hz measurement frequency of the fast FID system led to a crank angle resolution of 16 degrees for the cycle-resolved HC measurements. As a result, since the output of the fast FID measurement system was phased with the data acquisition system, it was possible to link the cylinder pressure trace during the combustion process and the respective cycle's HC emissions.

Figure 9 shows the in-cylinder pressure signal for a single engine cycle and the associated fast FID signal. To improve the quality of the fast HC and fast NOx visualization and analysis, a Butterworth low-pass digital filter (300 Hz) has been applied to the FID output.

**Figure 9.** In-cylinder pressure signal and measured cycle-resolved HC emissions for dual-fuel operation at SOI −10 deg aTDC.

It is important to mention that due to the distance between the FID probe and the exhaust port, the fast FID signal is intrinsically characterized by a time-delay (approximately 0.0125 sec in the analyzed conditions) with respect to the exhaust valve opening. Since the analyzed engine operating conditions were run at a constant RPM and under the hypothesis that the exhaust gases pass through the exhaust manifold at the sound speed (i.e., under choked conditions) during the exhaust blowdown process, which can approximately be considered proportional to the square root of the exhaust gas temperature, the time-delay was compensated, and the fast FID signal was synchronized with the exhaust valve opening (EVO) for each cycle. As it can be seen in Figure 9, the instantaneous HC signal starts rising after EVO and, therefore, the information contained in the fast FID signal can be directly correlated with the analyzed cylinder pressure signal.

By the analysis of the instantaneous HC signal (Figure 9), three regions are noticeable, which provide different information about the HC emissions arising from the engine cycle. The first HC peak, located between −360 and −260 deg aTDC, is related to short-circuiting of natural gas from the intake directly to the exhaust. Despite the overlap window of the engine being very small (15 deg), because of the pressure difference between intake and exhaust, a small quantity of LRF–air mixture flows in the cylinder directly to the exhaust, causing the reported peak. The second region can be considered representative of the actual cyclic HC production (green area highlighted in Figure 9) during the combustion process. This area is characterized by the maximum value of instantaneous HC, which is directly related to the HC production during and after the combustion, and a clear

descending trend. Such a trend can be reasonably explained through the exhaust gases mixing process in the exhaust manifold during the expansion (the HC concentration lowers progressively). As a result, the cyclic HC production can be evaluated as the average of the fast FID signal during the exhaust stroke (green area in Figure 9). This value represents the "effective cyclic HC emissions" for the following engine cycle. Since the aim of this work is to correlate the IMEP cyclic variations with the HC emissions, the effective cyclic HC emissions was evaluated by averaging the FID signal (blue area highlighted in Figure 9) during the compression stroke, when both intake and exhaust valves are closed. Then, the Net Cyclic HC emission (NCHC), defined by Equation (3), was evaluated as the difference between the cyclic HC production (average of the green area in Figure 9) and the effective cyclic emissions for each engine cycle (average of the blue area in Figure 9).

$$\text{Net Cycle HC emission } [ppm] = \text{HC}\_{\text{cyclic}} \text{ production} - \text{HC}\_{\text{effective cyclic emission}} \tag{3}$$

The *NCHC* represents the net *HC* production (if positive), or the net *HC* reduction (if negative), for the analyzed engine cycle compared to the previous one. As a result, through the NCHC evaluation it is possible to obtain an index that can be easily correlated with the cyclic *IMEP* variation. To evaluate the cyclic *IMEP* variations, the Δ*IMEP*, defined in Equation (4), was calculated as the difference between the *IMEP* of two consecutive engine cycles.

$$
\Delta IMEP\left[bar\right] = IMEP\_{n\_{th}cycle} - MEP\_{(n-1)\_{th}\,cycle} \tag{4}
$$

Once *NCHC* and Δ*IMEP* were defined, the cyclic *HC*–Δ*IMEP* correlation was obtained by the comparison between the two engine operating conditions considered in this work: "Retarded SOI" and "Advanced SOI". Figure 10 clearly shows that a net *HC* reduction is related to a more efficient combustion process (positive Δ*IMEP)*. On the other hand, if the combustion decreases in efficiency (negative Δ*IMEP*) the *NCHC* increases. It is interesting to notice that even if the spread of the *IMEP* is extremely different (because of differences in the nature of combustion at the two SOIs as evident from the different RoHR shapes), the correlation between the *NCHC* and the Δ*IMEP* is still valid for both analyzed engine conditions. Therefore, it appears that the obtained linear correlation (R2 = 0.86) gives a reasonable indication about one the main source of *HC* production in RCCI engines and the relation between the cyclic variability and *HC* emissions in diesel–NG dual-fuel combustion.

**Figure 10.** Cyclic HC-IMEP correlation for 300 consecutive cycles in the two different analyzed engine conditions: SOI −60 deg aTDC called "Advanced SOI" and SOI −10 deg aTDC called "Retarded SOI".
