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This paper presents an experimental study of a diesel engine retrofitted to operate in spark ignition mode and powered with different fuels. The investigation is explicitly focused on two extreme operating points, namely idle and full load, chosen to represent boundary conditions that are critical for practical engine operation and assessment. These operating points are examined to capture fuel-dependent effects under conditions representative of minimum load operation and maximum in-cylinder stress. The experimental approach enables a comparative evaluation of combustion behavior and engine-out emissions associated with different fuel properties when applied to a modified compression-ignition platform. The analysis is intended to provide insight into how alternative fuels interact with a spark-ignition retrofitted diesel architecture under these operating conditions, which are often decisive for real-world applicability. The overall objective of the study is to support the applicability of retrofitting solutions aimed at improving brake thermal efficiency while significantly reducing carbon dioxide emissions. The results contribute to the understanding of fuel effects in spark-ignition retrofitted diesel engines and support their evaluation in the context of alternative fuel utilization