**3. Experimental Methodology**

As widely reported in the literature [26,27] and explained later in this paper, to achieve high combustion efficiencies with dual-fuel combustion, an early Start of Injection (SOI) is needed. With early SOIs, the shape of the apparent rate of heat release (RoHR) curve is "Gaussian" and it generates very low pollutants, especially NOx, and high indicated efficiency. However, dual-fuel combustion at very advanced SOIs leads to the ignition process (dictated by the HRF injection) becoming driven more by in-cylinder chemical reactions that are especially driven by the local air–fuel ratio [28–30]. By using advanced SOIs, the HRF ignition delay increases due to the local air–fuel ratio "over leaning", and it generates engine instability as evident from high values of the coefficient of variation (COV) of indicated mean effective pressure (IMEP).

To better highlight differences in combustion RoHR shapes, efficiency, and pollutant emissions in dual-fuel combustion, two SOI sweeps were performed, operating the SCRE at low loads, where HC emissions are exacerbated. To highlight the link between cyclic HC emissions and combustion instability, highly unstable engine operating conditions were specifically studied. Moreover, to better understand the reason behind the NOx production with dual-fuel combustion, different boost pressures and percent energy substitution (PES) of natural gas were considered. The engine operating conditions presented in this paper are summarized in Tables 2 and 3. The experimental studies were performed at a speed of 1339 RPM, which was identified as the "B-speed" of the original PACCAR MX-11 engine and 1000 cycles of data were collected for each operating point. For the analyzed SOI sweeps, the injection pressure was kept at 500 bar, and the PES was maintained at 80% and 75%, respectively. The PES was computed using Equation (1).

$$\text{PES} = \frac{\dot{m}\_{\text{LRF}} LHV\_{\text{LRF}}}{\dot{m}\_{\text{HRF}} LHV\_{\text{HRF}} + \dot{m}\_{\text{HRF}} LHV\_{\text{HRF}}} \tag{1}$$


**Table 2.** Engine operating conditions that were chosen to study the impact of cyclic variations on HC emissions at Intake Pressure 1.5 bar, Intake Temperature 20 ◦C, IMEP 5 bar and PES 80%.

**Table 3.** Engine operating conditions that were chosen to study the impact of cyclic variations on NOx emissions at Intake Pressure 1.48 bar, Intake Temperature 20 ◦C, IMEP 5 bar and PES 75%.


To study in depth the combustion process at different engine conditions, the RoHR, the cumulative heat release, and the main combustion indexes, such as IMEP, location (CAD) of 50% heat release (CA50), Peak Pressure Rise Rate (PPRR), were calculated starting from Equation (2) (*γ* represents the specific heat ratio, *V* and *p* are the combustion chamber volume and pressure, respectively, and *dV* and *dp* are their derivatives) [31] for each engine cycle:

$$ROHR = \frac{1}{\gamma - 1} \cdot V \cdot \frac{dp}{d\theta} + \frac{\gamma}{\gamma - 1} \cdot p \cdot \frac{dV}{d\theta} \tag{2}$$

The Start of Injection (SOI) was swept from a retarded injection timing of −10 deg aTDC to an advanced injection timing of −50 deg aTDC at constant intake and rail pressures.

Figure 2 shows the impact of the SOI on the average CA50 at a constant IMEP of approximately 5 bar running the sweep described in Table 3. The trend reported in Figure 2 clearly shows the typical behavior during the transition between a typical two-stage profile to a Gaussian profile for dual-fuel combustion in terms of the "switchback" of CA50 direction with respect to the SOI [26,32]. The reason of the SOI-CA50 trend shown in Figure 2 is related to the fact that advancing the angular position of the diesel injection within the cycle increases the ignition delay because of different charge thermodynamic conditions [28,30].

**Figure 2.** Average CA50 and IMEP measured during the SOI sweep at a constant IMEP of 5 bar.

To better explain the SOI-CA50 trend, the difference in combustion RoHR shapes during the SOI sweeps at the same load (5 bar IMEP) are shown in Figure 3. As evident from this figure, starting from SOI −10 to SOI −30 deg aTDC the combustion shows the typical dual-stage RoHR shape: a first peak in RoHR generated by the combustion of a relatively non-homogeneous air–fuel mixture related to the HRF injection then followed by combustion in the more homogeneous natural gas–air mixture. As has been widely documented in the literature [3], such behavior is generated by very short ignition delay of the HRF when injected close to TDC (the air–fuel mixture pressure, temperature, and the charge stratification are high). After the first combustion phase, the RoHR reported in Figure 3a shows the typical second RoHR stage is characterized by smoother and slower energy release process, due to the combustion of the LRF–air mixture far from the stratified zone. Based on the previous explanation, running the engine with late SOIs, from −30 to −10 deg aTDC, CA50 and SOI follow the same movement direction because of the very short HRF ignition delay and favorable charge thermodynamic conditions (Figure 2, right side).

**Figure 3.** (**a**) Rate of Heat Release, and (**b**) Cumulate Heat Released curves testing different SOI at the same load (5 bar IMEP), PES (80%), and boost pressure 1.5 bar.

A further advance in SOI, from −30 to −50 deg aTDC, produces a completely different behavior, characterized by a smoother single-stage combustion RoHR that appears "Gaussian" in shape. On the contrary, running the engine with early SOIs, the CA50 retards when the SOI advances (Figure 2, left side). Due to the extremely unfavorable LRF-air mixture thermodynamic conditions when the HRF is injected at early SOIs, the HRF ignition delay increases, allowing for greater HRF mixing with the LRF–air mixture and avoiding the creation of stratified zones close to the injector tip. This enhanced mixing leads to a relatively slower and smoother combustion process characterized by the single-stage Gaussian RoHR curve. It is important to highlight the presence of the Low–Temperature Heat Release (LTHR), approximately at −16 deg aTDC, which represents a well-known phenomenon [33] often visible in single-stage dual-fuel combustion.

Moreover, from Figure 3a, it is important to observe the presence of remarkable RoHR oscillations during the expansion stroke when the engine was run at a retarded SOI, from −10 to −30 deg aTDC. Such a phenomenon is typically triggered by the pressure waves generated by an impulsive combustion process, such as the first stage combustion in Figure 3a [34]. Such a very short and intense energy release increases the amplitude of the resonance frequencies related to the vibrational mode of the combustion chamber. To better analyze this aspect, further analysis is currently being performed.

As has been well documented by the literature [35], the transition from two-stage to Gaussian single-stage combustion produces remarkable differences both in terms of pollutants and efficiency. Figure 3b, which shows the ensemble-average Cumulated Heat Released (CHR) at different SOIs, confirms that even if the CA50 has been varied, the load has been kept almost constant (maximum CHR close to 1500 J). Figure 4 shows the average Indicated Specific Fuel Consumption (ISFC) in terms of diesel-equivalent ISFC and the main pollutant emissions (total HC, CO and NOx) sampled by the Richmond gas analyzer during each test.

**Figure 4.** ISFC and pollutants (HC, CO and NOx) production during the SOI sweep running the SCRE engine in dual-fuel mode at low load (IMEP 5 bar), 75% PES, and boost pressure 1.48 bar. Note: For ease of comparison and because the nominal load was held constant at IMEP = 5 bar, steady-state emissions are presented in raw units (ppm) instead of the customary brake-specific units.

From Figure 4, typical ISFC and emissions trends are observed. The increase in NOx is linked to the more locally stratified combustion leading to higher local temperatures, while ISFC and HC reductions are linked to higher combustion efficiencies. It is important to highlight that even if the CA50 was around maximum brake torque (MBT) or maximum IFCE location for standard ICEs (i.e., 5–7 deg aTDC at SOI equal to −30 deg aTDC), the two-stage RoHR and the associated stratified combustion (from SOI −10 to −30 deg aTDC) are characterized by high NOx emissions. As a result, despite its benefit in combustion controllability through the injection position, the two-stage RoHR shows limitations in reaching both goals simultaneously.

Since the aim of dual-fuel combustion is to drop both emissions and fuel consumption as much as possible, further advance in SOI generates a Gaussian combustion and it shows the best tradeoff in pollutants and efficiency. The experimental evidence reported in Figure 4 is well-supported by data available in the dual-fuel literature [26,27]. A smooth combustion characterized by CA50 close to MBT produces very low NOx. A further increment in SOI generates a slight rise in ISFC and HC mainly because the CA50 occurs later as the piston is further down the expansion stroke. Furthermore, the CO generation increases mainly because the CA50 is retarded, the combustion duration increases with a direct impact in lowering the exhaust temperature. Figure 5 shows ISFC and pollutants emissions for engine operation at a slightly higher PES of 80% and boost pressure 1.48 bar. As expected, even at slightly higher PES, SOI variations exhibit similar trends.

**Figure 5.** ISFC and pollutants (HC, CO and NOx) production during the SOI sweep running the SCRE engine in dual-fuel mode at low load (IMEP 5 bar), 80% PES, and boost pressure 1.5 bar.

Despite previous efforts in which the authors demonstrated significant reductions in fuel consumption and pollutants by performing a "calibration style" parametric study [17], this work is focused on demonstrating the links between high HC and NOx emissions and the combustion process. Therefore, the focus is specifically not on determining the "best operating point" with dual-fuel combustion.

The following section demonstrates the links between the combustion indices and the information obtained from the high frequency (cycle-resolved) pollutants measurements with low-load dual-fuel combustion.

#### **4. Results and Discussion**

To analyze the impact of the combustion characteristics on cycle-resolved pollutants production, attention was focused on those operating conditions that led to high cycle-tocycle variability (and associated high HC emissions) and to high NOx emissions.
