*2.3. Measurements samples Preparations*

The marine diesel engine investigated in this paper is frequently used to drive a generator, therefore, two engine speed 900 rpm and 1000 rpm are selected in the measurement, and four operating points for each engine speed are measured (see Figure 3). In order to ge<sup>t</sup> the engine running points to be as thermodynamically stable as possible, the operating points at constant speeds were traversed alternatively up and down, as indicated by the arrows.

**Figure 3.** Map of measured points.

When the diesel engine is used for power generation, the engine speed has to be kept constant in order to ensure the constant frequency of current. Due to fixed current frequency *f* used in the marine power generation such as 50 Hz in China and 60 Hz in US, together with the number of poles *P*, the engine speed can be determined by Equation (1).

$$m = \frac{60 \cdot f}{P} \tag{1}$$

Therefore, the 50 Hz and 60 Hz are corresponding to 1000 r/min and 900 r/min when the poles numbers are selected to be 3 and 4 separately.
