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This study investigates the effect of hydrogen enrichment on combustion stability in a CNG-fueled multi-cylinder spark-ignition engine. Hydrogen was blended into CNG at 0%, 18%, 25%, and 30% by volume, and the blends were tested under constant-speed, full-load conditions. Combustion stability was assessed using the coefficient of variation (CoV) of peak cylinder pressure (Pmax), mass burn fraction at 50% (MBF50), and heat release rate (HRR). A correlation matrix analysis was employed to examine interrelationships between these stability parameters. Results indicated that the 18% HCNG blend provided the best overall improvement, achieving the lowest CoV values for MBF50 and HRR, and significantly reducing CoV_Pmax compared to pure CNG. Increasing hydrogen content to 25% maintained stability but introduced minor irregularities, while 30% hydrogen further improved Pmax consistency yet adversely impacted MBF50 and HRR stability. Correlation analysis highlighted a strong positive relationship between MBF50 and HRR stability, emphasizing the critical role of combustion phasing control. The study concludes that hydrogen enrichment in the range of 18–25% optimally enhances combustion stability without inducing combustion irregularities. These findings offer valuable insights for optimizing HCNG blends to achieve cleaner and more efficient future engine designs.

Recent innovations in engine control and diagnostics are providing room for development of innovative combustion approaches (e.g., low-temperature combustion) able to minimize the creation of pollutants. To ensure the constant fulfillment of the prescribed thermodynamic conditions, however, a fast real-time monitoring of the in-cylinder pressure is needed. To this end, dynamic pressure sensors, flush-mounted on the cylinder head, are commonly used. With this approach, the measurement accuracy is high, but the durability is limited by the harsh working conditions. The installation on the cylinder head is also complex. The development of robust and effective indirect measurement systems could then represent the enabler of a further development of this technology. In the present study, an innovative methodology to measure the in-cylinder pressure has been conceived and extensively tested on a four-stroke single-cylinder engine. The proposed approach is based on the analysis of the mechanical stress on the engine studs by means of a piezoelectric strain washer. This solution allows the user for a rapid and cost-effective sensor installation, described in the paper along with the signal post-processing techniques. Results showed good accuracy and robustness of the methodology, making the results of practical use for engine control.