This work presents the exergy balances of biogas dual fuel combustion modes using two different pilot fuels in a single-cylinder diesel engine. The main focus is on the use of biodiesel (BD) as a pilot in place of fossil diesel fuel. The results reveal that the important exergy loss recovery sources are the exhaust gas and cooling water exergy. By accessing these losses, there is an increase of 28–30% in work exergy for dual fuel modes as compared to diesel mode. Use of BD, in place of diesel as a pilot, reduces the maximum exergy efficiency roughly by 2%.