Question

On which of the following does brake mean effective pressure (BMEP) in an engine depend upon?

(a) Speed and torque

(b) Speed and power

(c) Torque

(d) Speed

This question was posed to me in examination.

This intriguing question originated from Measurements and Testing in chapter Automotive Electronics, Engine Performance Parameter & Testing of Automobile Engines of Automobile Engineering

Answer 1

Correct option is (b) Speed and power

To explain I would say: Brake power = 2πNT / 60000 where N =engine speed in RPM and T = torque in Nm. The dynamometers give the engine torque from which the brake power can be calculated. Thus the brake mean effective pressure in an engine depends on torque only. Also, Brake power = Pmb*L*A*n / 60000. Therefore, Pmb = kT where k = 2π / L*A and k = 4π / L*A where L = stroke length, A = cross-sectional area of the cylinder, n = no. of working strokes per minute and, Pmb = brake mean effective pressure.