4.2.2. Each Cylinder Pressure Signals

The combustion fitting results discussed in the previous sections are based on the four-cylinder average pressure signals measurement in order to eliminate the measurement fluctuation effect. In fact, for the 4-cylinder diesel engine, four cylinders behave different performance to some extent in particular for the pressure signals but not too much due to engine operating balance demand. Table 6 shows the fitting results of the four cylinders respectively. The Seiliger parameters *a* and *b* are slightly different for the four cylinders, referring to the heat input ratio, there is maximum 2.22% difference for Seiliger parameter *a* (cylinder 1 and cylinder 2) and maximum 1.24% difference for Seiliger parameter *b* (cylinder 1 and cylinder 4).


**Table 6.** Results of advanced Seiliger process fit in 4 cylinders.

As to Seiliger parameter *c* and *nexp*, there is much discrepancy such as 3.97% of Seiliger parameter *c* and 5.2% of Seiliger parameter *nexp*. This does not apply to cylinder 3, which has irregular cylinder fitting results. However, the fitting results of averaged in-cylinder pressure signals is not much affected by that of cylinder 3 and closed to the result of cylinders 1, 2 and 4. Although there are irregular cylinder fitting results, which are probably caused by the engine operating in reality or a numerical calculation procedure, the averaged in-cylinder pressure signals are relatively reliable to obtain the fitting results in comparison with reality.

### *4.3. The Combustion Fit Results of Engine Running with Generator Conditions*

After obtaining the Seiliger parameters fitting results of one engine operating point, the overall measurements are calculated. Figure 11 shows the trend of Seiliger parameters *a*, *b*, *c* and *nexp* as functions of effective power *Pe* when engine running at 900 r/min and 1000 r/min respectively.

**Figure 11.** Seiliger parameters at engine constant speed: (**a**) 900 r/min; (**b**) 1000 r/min.

Seiliger parameter *a* indicates the premixed combustion phase and a large value of *a* is associated with more premixed combustion. At each engine speed, the value of *a* increases at low load and reaches a maximum, then decreases when going to higher load. As to the effect of engine speed on the Seiliger parameter *a*, it can be observed that for a certain load point parameter *a* is lower for a higher engine speed. At a certain engine speed, *b* goes up with increasing load. The engine speed seems to have less effect on *b*, i.e., there is hardly any differences for different engine speeds at the same load.

Seiliger parameter *c* represents the isothermal combustion stage, to some extent, the large value causing late engine combustion. The value of *c* goes up with the engine power increasing, which means, for this engine, when the engine is running at higher load, the later combustion occurs more frequently. Seiliger parameter *nexp* indicates the very late combustion phase during expansion. Due to the polytrophic expansion exponent, the value of *nexp* varies in a relatively small range (around 1.2 to 1.35) and decreased with engine power going up, representing less heat input during the very late combustion stage.
