## 3 Results and Discussion
### Comparison of Different Evaluation Modes  
The development phase of the spark duration method revealed that there is a certain distribution of duration that allows identifying the firing TDC by simply acquiring a set of 200 consecutive cycles, finding the median value, and using it as the threshold [37]. More to the point, spark events during compression tend to be shorter (and therefore below the threshold), while 'wasted' sparks are longer (and above the median threshold). A similar approach was implemented for the rpm method, meaning that engine speed tends to be lower during intake-compression, while the contrary is true during expansion-exhaust. Figures 5 and 6 show the results of applying the rpm and spark duration methods in the'median' mode (the points are in-cylinder pressure-ID signal pairs obtained by integrating the two traces during the windows indicated in Figure 3). An evident observation is that in idle conditions, the spark duration method provides much better results. Indeed, the rpm method featured over 24% 'false IDs'. This was actually found to be correlated with the fact that engine speed was not that stable in this condition; COV\({}_{\text{rpm}}\) was close to 0.3%. At first glance this value seems to be relatively low (values as high as 1% are not uncommon for such smalls size power units [43]), but when comparing it to the COV of around 0.15% recorded at the 1.0 kW load, it becomes evident that there is a significant difference. This practically meant that a large portion of the recorded cycles featured an average rpm below the threshold and the corresponding expansion-exhaust strokes were erroneously identified as intake-compression events, given that their rpm was below the median value. Figure 4: Raw signals acquired during two consecutive cycles at the 1.0 kW load. Figure 5: Distribution of the intake/expansion TDC ID signal and in–cylinder pressure for idle and full load conditions when employing the rpm method in median mode. The contrary was found for the full load condition, for which the count of false IDs was 0.1% when using the rpm-based method and 2.1% for the spark duration procedure. This is the result of improved engine speed stability (due to higher combustion repeatability, as well as the fact that engine speed increased from 4000 to 5200 rpm, thus resulting in better distinction between intake-compression and expansion-exhaust strokes), and on the other hand, in more extensive effects of turbulence on spark duration (thus yielding wider dispersion of the spark parameter). Nonetheless, one of the main conclusions is that neither of the two methods can be used as true cycle-to-cycle evaluation methods. Even if its application is relatively straightforward (setting a threshold once the median value is identified), it still requires a set of certain number of cycles to be recorded (the larger the number, the more the threshold is statistically robust) and may not give the intended results in certain operating conditions (e.g., high COV\({}_{\text{rpm}}\) or increased turbulence). For this reason, an alternative approach was considered. Figure 7 shows the intake-compression/expansion-exhaust rpm and spark duration recorded during a sequence of 20 consecutive cycles (please note that at the 1.0 kW load, the actual average engine was slightly over 5150 rpm, but for easier identification, the 5200-rpm denomination was preferred). Two values are shown for each cycle (odd/even), and it is quite evident that for both cases, there is a high/low rpm and spark duration. This is true for all 20 cycles, even if the median threshold (shown as dashed lines) does not clearly divide the two categories, especially at 4000 rpm (further highlighting the reason behind the distribution observed in Figure 5). Slightly wider dispersion of compression sparks is also evident (emphasizing the explanation previously iterated for the results illustrated in Figure 6).