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Abstract The objective of the present investigation is to explore the influence of ternary (diesel‐biodiesel‐alcohol) fuel blends on the combustion and vibration parameters of a variable compression ratio compression ignition engine. Kusum seed oil biodiesel and isobutyl alcohol were blended with conventional diesel to make a ternary fuel. The blend of biodiesel‐diesel (B20) was made in proportion of 20% and 80% by volume, respectively. Furthermore, isobutyl alcohol was added at diverse proportions (ie, 0.25%, 0.5%, 0.75%, and 1%). The investigation was carried at different compression ratios (CRs), namely, 16.5, 17.5, and 18.5. The study shows that, upon the addition of isobutyl alcohol, the combustion characteristics (ie, cylinder pressure [CP] and net heat release rate [NHRR]) are significantly enhanced, whereas the mechanical vibration level maintains stable. The CP and NHRR of B20+ 1% isobutyl alcohol are greatly improved for corresponding values of 4.55% and 8.39%, respectively, except for the conventional diesel at CR 18.5. Furthermore, the root mean square velocity (vibration) decreases when increasing the quantity of isobutyl alcohol and attained a minimum with B20+ 1% isobutyl alcohol. The experimental results show that the isobutyl alcohol up to 1% with diesel‐biodiesel blends can be used in the CI engine with no further adjustments.