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Judging by engine makers' graphs showing curves of brake mean
Page 49
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effective pressure and torque, maximum bmep is produced at the same speed as maximum torque. What is the explanation of this?
AA torque value is definitive in that it can be
measured on a dynamometer during a laboratory test. It is indicative of the turning effort applied to the crankshaft which is a function of the pressure in the cylinders. But it does not necessarily vary according to the maximum pressure in the cylinders which can also be measured and recorded.
A high maximum pressure that is maintained over a very short period produces less power than a lower pressure, that extends over a larger crank angle and in assessing the value of cylinder pressure it is necessary to base the calculation on the average pressure over the combustion cycle that is effective in producing torque. The pressure in a cylinder before the piston reaches top dead centre is doing negative work and the pressure that is obtained when the piston is at top dead centre is "noneffective".
Brake horsepower is obtained by measuring torque and speed and applying a formula in which bhp is related to torque and speed values. Brake mean effective pressure is calculated by using another formula in which it is related to bhp and rpm. Bmep is therefore directly proportional to torque at a given speed. It is calculated: it cannot be measured.