*2.1. Test Cell and Engine Description*

A single-cylinder diesel engine, representative of commercial truck engines, has been used in this study. The major difference to the standard unit production is the hydraulic variable valve actuation (VVA) system, which allows controlling of the timing, duration, and lift of each valve independently. Detailed specifications of the engine are given in tab:applsci-07-00036-t001.


**Table 1.** Single cylinder engine specifications.

To enable RCCI operation, the engine was equipped with a double injection system, one for each fuel used. This injection hardware enabled to vary the in-cylinder fuel blending ratio and fuel mixture properties according to the engine operating conditions. To inject the diesel fuel, the engine was equipped with a common-rail flexible injection hardware which is able to perform up to five injections per cycle. The main characteristic of this hardware is its capability to amplify common-rail fuel pressure for one of the injection events by means of a hydraulic piston directly installed inside the injector. Concerning the gasoline injection, an additional fuel circuit was in-house built including a reservoir, fuel filter, fuel meter, electrically driven pump, heat exchanger, and commercially available PFI. The mentioned injector was located at the intake manifold and was specified to be able to deliver all the gasoline mass into the cylinder during the intake stroke. Consequently, the gasoline injection timing was fixed at 10 CAD after the intake valve opening (IVO) to allow the fuel to flow along 160 mm length (distance from PFI location to the intake valves seats). Accordingly, this set-up avoided fuel pooling over the intake valve and the undesirable variability introduced by this phenomenon. The main characteristics of the diesel and gasoline injectors are depicted in tab:applsci-07-00036-t002.


**Table 2.** Diesel and gasoline fuel injector characteristics.

To carry out the experimental tests shown in Sections 3.1 and 3.2, commercially available diesel and 98 octane number (ON) gasoline were selected as high and low reactivity fuels (HRF and LRF), respectively. Their main properties are listed in tab:applsci-07-00036-t003. For convenience, the properties of the fuels used in Section 3.3 will be presented there.

> **Table 3.** Physical and chemical properties of the fuels used along the study.


The engine was installed in a fully instrumented test cell, with all the auxiliary facilities required for its operation and control, as it is illustrated in Figure 1.

**Figure 1.** Test cell setup.

To achieve stable intake air conditions, a screw compressor supplied the required boost pressure before passing through an air dryer. The air pressure was adjusted within the intake settling chamber, while the intake temperature was controlled in the intake manifold after mixing with the exhaust gas recirculation (EGR) flow. The exhaust backpressure produced by the turbine in the real engine was replicated by means of a valve placed in the exhaust system, controlling the pressure in the exhaust settling chamber. Low pressure EGR was produced taking exhaust gases from the exhaust settling chamber. The EGR rate was calculated using the experimental measurement of intake and exhaust carbon dioxide (CO2) concentration.

The concentrations of NOx, CO, unburned HC, intake, and exhaust CO2, and oxygen (O2) were analyzed with a five gas Horiba MEXA-7100 DEGR analyzer bench by averaging 40 s after attaining steady state operation. Smoke emissions were measured with an AVL 415S Smoke Meter and averaged between three samples of a 1 liter volume each with paper-saving mode off, providing results directly in FSN (Filter Smoke Number) units. Soot measurements of FSN were transformed into specific emissions (g/kWh) by means of the factory AVL calibration.
