<p>Marine engine in-cylinder combustion greatly affects engine output performance, fuel economy, and emissions. Effective monitoring and assessment of the in-cylinder combustion process enable timely fault detection and are vital for enhancing engine reliability and combustion efficiency, reducing emissions, and avoiding abnormal combustion. In practical engineering applications, the most direct method for monitoring the combustion process is obtaining in-cylinder pressure signals using cylinder pressure sensors. However, traditional metrics for assessing combustion roughness, such as the maximum amplitude of pressure oscillation, require lengthy calibration periods and are vulnerable to data quality issues. To overcome these limitations, this study proposes a new indicator, namely the integral of pressure deviation (IPD), based on measured cylinder pressure data for assessing the in-cylinder combustion process. The IPD is a novel dimensionless metric for the quantitative assessment of marine engine combustion processes, with an emphasis on abnormal phenomena such as misfire and knock. By integrating pressure deviations relative to multicycle averages, the IPD metric overcomes the limitations of traditional parameters by reducing dependency on engine-specific thresholds and enhancing adaptability to dynamic operating conditions. Experimental validation was conducted on a YC6K dual-fuel engine test bench, supported by three-dimensional computational fluid dynamics simulations to generate knock pressure data, thereby avoiding high-cost physical tests. Cross-validation with a CHG622 spark-ignition natural gas engine further demonstrated the adaptability of the method. This work advances combustion diagnostics by providing a universal, threshold-independent tool for monitoring and assessing the performance of marine engines.</p>