*2.4. The Test Stand*

The engine was mounted on a test stand with an Eddy-current brake dynamometer with a nominal power of 100 kW. A brake controller has a module for measuring and recording the temperature measured at selected positions. Fuel consumption measurement was conducted using a gravimetric method with electronic registration of instantaneous values of this parameter. Figure 5 shows a general view of the test bench.

**Figure 5.** General view of the test stand of the engine with the additional expansion of exhaust gases; 1—Tested engine, 2—Eddy-current brake dynamometer, 3—Engine managemen<sup>t</sup> system, 4—Engine coolant heat exchangers.

### *2.5. Methodology for Measurement of the In-Cylinder Pressure*

Pressure measurements in the fired cylinder and the additional expansion cylinder were carried out by the use of an optical sensor type Optrand AutoPSI-TC (D822D6-SP) with temperaturecompensated output signal [21]. The pressure sensor with a threaded adapter was fitted to the combustion chamber of the fired cylinder (No. 4) through the drilled hole in its side surface. The hole was machined parallel to the bottom plane of the cylinder head.

Figure 6 shows a photo of the combustion chamber of the fired cylinder with a visible hole for in-cylinder pressure measurement and the adapter for the pressure sensor.

**Figure 6.** Combustion chamber of the fired cylinder (No. 4); 1—Adapter of the combustion pressure sensor, 2—Hole in the combustion chamber wall, 3—Intake Valves, 4—Exhaust valves.

The view of the pressure sensor installed in the fired cylinder of the test engine is shown in Figure 7.

**Figure 7.** View of the combustion pressure sensor installed in the fired cylinder of the research engine (location of the sensor is indicated by the yellow arrow).

The in-cylinder pressure sensor for the cylinders of additional expansion was installed in cylinder No. 3 to the adapter mounted in the place of the spark plug. The sensors were able to measure the combustion pressure up to 10 MPa (with a possible short-term overload of 50%) and sensitivity equal to 385.8 mV/MPa. In order to perform a correlation of the recorded waveform of the in-cylinder pressure with the momentary position of the crankshaft, an incremental encoder was mounted to the test engine. The encoder had a resolution of 360 pulses per 1 revolution with a separate output phase for the Top Dead Center (TDC) marker.

The registration of waveforms of the in-cylinder pressure of the fired and additional expansion cylinders using Optrand sensors was performed using a PC equipped with a multifunctional data acquisition card with a 14-bit resolution of Analog-to-Digital Converter (ADC). The data acquisition process was carried out using a specialized application developed for this purpose in the LabView environment. The results presented in the following parts of this work were obtained by averaging the data from dozens of engine cycles for each measurement point. The study was preceded by an experimental procedure of the encoder settings to TDC of the cylinder No. 1.
