<p>Today, changes in energy policies fluctuate unpredictably. Therefore, researchers are focusing on finding different sources and working on the efficient use of these sources. Parallel to developments in engine technology, studies are also being conducted on combustion in diesel engines. In recent years, research on engines with RCCI combustion strategies has gained momentum. In this study, energy, exergy, and sustainability analyses were performed through experiments conducted on a single-cylinder, water-cooled RCCI engine using diesel + LPG fuel at a constant speed of 1500&#xa0;rpm and under different engine loads. Diesel was used as the high-reactivity fuel, while diesel–LPG mixtures containing LPG as the low-reactivity fuel were used in the engine. The experimental results showed that fuel consumption decreased with an increase in the LPG ratio. The highest fuel consumption was achieved with pure diesel fuel at 2.05&#xa0;kg h^–1. This indicates that fuel consumption is reduced with LPG use and more efficient combustion is achieved. According to the energy analysis results, the maximum thermal efficiency was calculated as 30.895% at 100% engine load and with a D + Medium LPG mixture. In the exergy analysis, the exergy destruction and exergy efficiency in the system were determined; it was found that the exergy associated with heat transfer and the cooling water exergy showed an increasing trend with engine load. The increase in exhaust gas exergy from 0.511&#xa0;kW to 2.058&#xa0;kW as the engine load increased revealed the significant effect of the load on exhaust losses. The sustainability index was determined using exergy efficiency. In the sustainability analysis, the index value was found to be above 1 for all fuel mixtures, with the lowest sustainability index obtained for diesel fuel and the highest value obtained for the D + Medium LPG mixture.</p>