*2.2. In-Cylinder Pressure Signal Analysis*

The combustion analysis was performed with an in-house developed one-zone model named CALMEC. This combustion diagnosis tool uses the in-cylinder pressure signal and the mean variables recorded during the experiments (engine speed, coolant, oil, inlet and exhaust temperatures, air, EGR, and fuel mass flow) as its main inputs. The full description of the model can be found in [29], and the main hypotheses are enumerated next:


During the experiments, the in-cylinder pressure was measured with a resolution of 0.2 CAD using a Kistler 61215C pressure transducer coupled with a Kistler 5011B10 charge amplifier. The pressure traces from 150 consecutive engine cycles were recorded in order to compensate the cycle-to-cycle variation during engine operation. Then, the individual pressure data of each engine cycle was smoothed using a Fourier series low-pass filter. Once filtered, the collected cycles were ensemble averaged to yield a representative cylinder pressure trace, which was used to perform the analysis. The first law of thermodynamics was applied between intake valve closing (IVC) and exhaust valve opening (EVO), considering the combustion chamber as an open system because of the blow-by and fuel injection.

The main result of the model used in this work was the Rate of Heat Release (RoHR), which is calculated as stated in Equation (1).

$$RoHR = m\_{cyl} \cdot \Delta u\_{cyl} + \Delta Q\_W + p \cdot \Delta V - \left(\overline{h}\_{f,my} - \mu\_{f,\text{g}}\right) \cdot \Delta m\_{f,rup} + R\_{cyl} \cdot T\_{cyl} \cdot \Delta m\_{bb} \tag{1}$$

The different terms found in the equation are explained below:


the possibility of fuel impinged in the wall. The instantaneous heat transfer coefficient between the gas and the different surfaces is based on Woschni [32] with some improvements detailed in Payri et al. [33]. For the calculation of the different wall temperatures, a nodal heat transfer model was implemented [34].


Once the RoHR is obtained, the start of combustion (SOC) is defined as the crank angle position in which the cumulated heat release reached a value of 5% (CA5) and the combustion phasing is defined as the crank angle position of 50% fuel mass fraction burned (CA50). Combustion duration was calculated as the difference between CA90 and SOC.

The ideal gas equation of state was used to calculate the mean gas temperature in the chamber. In addition, the in-cylinder pressure signal allowed obtaining the gas thermodynamic conditions in the chamber to feed the convective and radiative heat transfer models, as well as the filling and emptying model that provided the fluid-dynamic conditions in the ports, and the heat transfer flows in these elements. The convective and radiative models are linked to a lumped conductance model to calculate the wall temperatures [35].
