Question

why is bmep lower at max. power than at max. torque explained

Answer 1

One reason is that friction increases with engine speed and friction at rated power is significantly higher than at peak torque speed. BMEP = IMEP - FMEP therefore if IMEP (measure of work done in the cylinder) were the same at both rated power and at maximum torque (which would require the same volumetric and indicated thermal efficiencies at both conditions) then the higher friction (FMEP) at rated power would result in a lower BMEP.

Answer 2

Hello mate here is your answer.

A second factor is that volumetric efficiency is typically higher at maximum torque than at rated power primarily due to intake and exhaust tuning in a naturally aspirated engine or increased turbocharger efficiency in a turbocharged engine.

Hope it helps you.