**2. Experimental Setup**

Experimental studies were performed on a single cylinder version of a heavy-duty PACCAR MX11 engine setup at the University of Alabama (UA). The engine specifications are provided in Table 1. The Single-Cylinder Research Engine (SCRE) was coupled to a 393 HP AC dynamometer and a Dyne Systems IL5 controller was used to control the engine speed, while engine load (as determined by the Indicated Mean Effective Pressure (IMEP)) was set by controlling fueling rates.


**Table 1.** Engine technical characteristics.

A common rail direct injection system was used to inject the High-Reactivity Fuel (HRF) and the rail pressure was controlled using a solenoid actuated inlet metering valve

and rail pressure control valve. A National Instruments Direct-injector Control and Measurement device (DCM) interfaced with the Calibration Viewer software from Vieletech and was used to control injection timing and injection duration. The Low-Reactivity Fuel (LRF) was fumigated along with the intake air and the LRF flowrate was controlled using a Swagelok KLF series pressure regulator. The fuel flowrates were measured using Emerson Micromotion Coriolis flowmeters.

The intake air system uses an external air compressor which also conditions the highpressure air (up to 5 bar) using a heatless desiccant dryer. The intake air mass flowrate was measured using a sonic nozzle, for which the pressure and temperature upstream of the nozzle were also measured to compute the air mass flowrate. The upstream pressure was measured using a Setra 206S pressure transducer and the temperature was measured using an Omega K-type thermocouple. A Kistler piezoelectric pressure transducer (6124A) coupled with a charge amplifier (5018) was used to measure the in-cylinder pressure, and the dynamic pressures of both the intake and exhaust were measured using Kistler piezoresistive pressure transducers. The in-cylinder pressure profile was phased using a BEI shaft encoder with 0.1 CAD resolution.

Cycle-resolved HC and NOx were obtained by installing CAMBUSTION HFR400 FID (Fast FID) and CAMBUSTION CLD500 (Fast NOx) measurement systems in the exhaust manifold, respectively (as close as possible to the exhaust port). By using the Fast FID and Fast NOx systems, which can sample the pollutants at 500 Hz, it was possible to obtain the "instantaneous" emissions for each engine cycle. As a result, since the output of such measurement systems were phased with the data acquisition system, it was possible to link the cylinder pressure trace during the combustion process and the respective pollutant production in a given cycle.

Moreover, to compare the present data with the literature, slow-speed (1 Hz) pollutant measurements were also performed by using a NOVA 5466 AK gas analyzer (present in a standard Richmond five-gas emission bench) and a CONTINENTAL UniNOx SNS14 sensor (100 Hz) mounted close to the Fast NOx probe. Figure 1 shows the UA SCRE test cell layout used for the present experimental efforts.

**Figure 1.** A schematic representation of the UA single-cylinder research engine setup used for the present experiments.
