Emir Yilmaz

With the decarbonization of internal combustion engines, alternative fuels have gained increasing attention. When using fuels with low combustibility, such as ammonia, detailed analysis of the intake system and in-cylinder flow is essential for improving combustion efficiency. Proper orthogonal decomposition (POD) has been widely used to extract coherent structures in flow fields within internal combustion engines. However, most previous studies have focused on analyzing cycle-to-cycle variations in gasoline engines, while time-resolved analysis within a single cycle of diesel engines has rarely been conducted. In this study, the effect of tangential port opening on in-cylinder flow characteristics was investigated using an optical single-cylinder diesel engine equipped with two intake ports and two exhaust ports. The opening area of the tangential port was varied under five conditions using different gaskets, and in-cylinder velocities were measured using particle image velocimetry. POD was applied to the acquired velocity data to evaluate the flow structures of the higher modes and their correlations with the mean flow and turbulence intensity. The results showed that in POD mode 1, a swirl flow was formed during the compression stroke when the tangential port opening exceeded 25%. Evaluation of the correlation between POD mode 1 and the ensemble-averaged flow using the relevance index revealed a strong correlation during the compression stroke. In POD mode 2, complex flows were observed during the intake stroke, and structures different from the mean flow were also confirmed during the compression stroke. A moderate correlation was observed between POD mode 2 and turbulence intensity under all conditions. Energy contribution analysis indicated that in the early intake stroke, the variation in mode 1 was large, and the flows represented by mode 2 and higher modes were dominant, whereas in the late compression stroke, mode 1 consistently accounted for a higher proportion.

Carbon dioxide (CO2) is the primary contributor to greenhouse gas emissions. Ammonia (NH3) has emerged as a promising alternative fuel due to its high energy density, ease of transportation, and carbon-free molecular structure. However, its practical application is challenged by slow combustion characteristics and high ignition temperatures. This study investigates the combustion behaviour of ethanol-ammonia mixtures using a high-compression-ratio engine (17.7:1) equipped with a sub-chamber. The engine operated at a constant speed of 1000 rpm. Ammonia energy ratios of 40%, 50%, and 60% were tested across ignition timings of 0°, 2°, 4°, 6°, and 8° crank angle (CA) before top dead center (BTDC). Results indicate that advancing the ignition timing increases in-cylinder pressure and heat release rate while reducing combustion duration. Lower ammonia energy ratios yielded higher thermal efficiency. Conversely, higher ammonia content and advanced ignition timings led to increased NOx emissions.