Emir Yilmaz

Ammonia (NH3) is a promising carbon-free fuel that offers higher volumetric energy density and easier transport than hydrogen. However, its direct combustion is limited by its slow burning rates and high ignition temperatures. Co-firing with a more reactive fuel, such as ethanol, can overcome these limitations. In this study, the combustion of ammonia/ethanol blends was evaluated in a customized spark-assisted compression ignition (SACI) engine featuring a sub-chamber and a 17.7 compression ratio. The experimental investigation focused on the interaction between boost pressures (5 to 15 kPa) and ammonia energy ratios (50% and 60%) at 1000 rpm and an excess air ratio of 1.2. The results suggest that increasing boost pressure reduced the thermal efficiency and indicated mean effective pressure (IMEP). Accordingly, this resulted in a lower in-cylinder pressure and heat release rate, which slowed down the combustion process. Such unusual behaviour can be attributed to an increase in the latent heat of vaporization of ethanol with pressure, which requires more heat energy for fuel vaporization and effectively cools the combustion chamber. Furthermore, while adopting lower ammonia energy ratios improved engine performance, it resulted in higher NOx emissions due to the lack of unburned NH3 in the exhaust to undergo the thermal DeNOx process, suggesting the need for careful adjustment of the boost pressure and ammonia energy ratio for the practical application of ammonia/ethanol fuels in combustion engines.

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.