**2. Experimental Set-Up**

The experimental facility already detailed in previous studies [28,29] was used to obtain the required data for the CFD model validation. Since the engine specifications and test cell features are widely described in the literature, only a brief description of them are included in this paper. Nonetheless, full details about the experimental facility can be found in the following documents [16,30].

A research version of a 2-stroke HSDI CI engine was used as baseline engine. The main specifications of the engine and the injector configuration are summarized in Table 1.


**Table 1.** Main engine specifications of the engine.

The control of the poppet valves relies on a hydraulic cam-driven Variable Valve Timing (VVT) system that allows to modify the opening/closing valve timings. Thanks to its flexibility to adjust the overlap among intake and exhaust periods, and the effective compression-expansion ratios, this system ensures an adequate in-cylinder flow pattern and optimizes the scavenging of burnt gases by reducing the short-circuit losses.

The equipped fuel injection system grants a maximum rail pressure of 110 MPa when operates with RON95 gasoline fuel. This is a prototype Delphi injection system which comes from a common-rail DFI 1.5 system.

The engine was installed in a completely instrumented test cell which supplies all required fluids for the engine operation through multiple auxiliary devices. The installation have independent water and oil cooling circuits and a low EGR system to supply arbitrary levels of cooled exhaust gas in any operating condition.

The in-cylinder pressure was measured with a Kistler 6061B pressure transducer flush-mounted between the intake and exhaust valves. A different piezo-resistive pressure sensor located at the cylinder liner close to Bottom Dead Center (BDC) was used to reference this pressure signal. Instantaneous signals, such as in-cylinder pressure, were sampled using a state-of-the-art acquisition system and recorded during 100 engine cycles for each operation condition.
