This work is an attempt to divulge the influence of compression ratio (CR) on the behavior of a 4.4 kW, single cylinder, air-cooled, diesel engine operated on upgraded biogas-Karanja methyl ester (UBG-KME) dual fuel. Earlier, an experiment was conducted by the authors to examine the use of UBG-KME-DEE (diethyl ether) in a dual fuel engine, and the results indicated that UBG-KME-DEE port injection functioned well and provided improved performance and lower emissions in comparison to the raw biogas RBG-KME-DEE mode. Nevertheless, the engine produced a lower brake thermal efficiency (BTE) compared to that obtained with diesel operation. Hence, to increase the BTE, experiments were conducted with varied CRs (16.5, 17.5, and 18.5) of the engine, KME was injected at a fixed timing of 24.5 degrees CA bTDC, DEE supply to engine was limited at 6%, and the UBG supply was made constant at 0.9 kg/h. The test results indicated that UBG-KME-DEE operation with CR = 18.5 gave optimum results. An increase in heat release rate of 60 J/degrees CA and shorter ignition delay of 7.8 degrees CA were observed for UBG-KME-DEE operation with CR = 18.5 at full operating load. BTE was increased and brake specific energy consumption was decreased, by about 7% and 6.8%, respectively, for UBG-KME-DEE operation with CR = 18.5 in comparison with KME. About 44, 42, and 42.8% decreases in the emissions of CO, hydrocarbons, and smoke were observed for UBG-KME-DEE at CR = 18.5. However, the emission of NO for UBG-KME-DEE operation with CR = 18.5 was 7.6% higher than with diesel, but 1.2% lower than with KME, at full operating load. The novel findings of this study raise the possibility of lowering the NO-smoke emission trade-off which is a prime challenge in diesel engines. In addition, UBG-KME-DEE diesel engines (renewable nature) can substitute the use of diesel and compressed natural gas.