A comparative study of three additives, such as Diethyl Ether (DEE), Dimethyl Ether (DME), and Methyl Tert-Butyl Ether (MTBE) on the performance and emissions of a B20 Simarouba Methyl Ester mix in a direct injection (DI) diesel engine. The results show that DEE improves brake thermal efficiency (BTE) by about 5.5%, and also improves exhaust gas temperatures (EGT) about 10%. The reason is the high cetane number in the additive and combustion characteristics. The effect of DME, which reduces emissions of hydrocarbons (HC) up to 25% and carbon monoxide (CO) up to 30%, because of high oxygen content and efficient combustion properties, makes it the most effective additive. The DME additive also reduces smoke emissions in the range of 20% due to higher combustion efficiency. But the DEE additive which leads in reduction of nitrogen oxide (NOx) in the range of 25% due to lower peak combustion temperatures. The comparative of MTBE gives certain benefits that are consistently less effective compared with DEE and DME for the emission and performance metrics. The comparative results of three additives, the DME, are advised for minimizing the CO, HC, and smoke emissions; the DEE is the supreme additive for improving engine performance and lowering NOx. © 2025 Elsevier B.V., All rights reserved.