*4.3. Indicators of Engine Operation*

## 4.3.1. Thermodynamic Indicators

Using the quantities in Figure 12 and taking into account Equation (1), the thermodynamic indices of an engine fueled by methane and propane were determined. The combustion center (CoC) and start (AI05) and end of combustion (AI90) were determined. The results of these measures are included in Figure 13.

According to data from Figure 13, the combustion of methane and propane in PC at λ = 1.3 and 1.5 results in high operation stability. As the energy delivered to the PC increases, the time of combustion I phase also increases. This is due to the control method of keeping the CoC constant (8 deg aTDC). Such control requires an increase in the ignition advance value, which results in an advance at the start of combustion. At the same time, during the combustion of both fuels, a longer combustion phase II (AI90—CoC) is also observed. This means that the fastest process occurs when small doses of fuel are fed into the prechamber. The combustion of fuels at λ = 1.8 results in large variations in thermodynamic indicators. This is due to the combustion conditions (operating instability) presented previously. Although the CoC value was kept constant, both the start and the end of combustion have a different trend from the previous values of the excess air ratio.

**Figure 13.** Thermodynamics of the combustion process: beginning of combustion (AI05), center of combustion (CoC), and end of combustion (AI90) determined for average pressure curves during methane and propane combustion.
