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Diagrams: MIP and MEP
quote Mean Effective Pressure when giving data about their
engines. Although Mean Effective Pressure and Mean Indicated
Pressure are similar they are not the same.
Sometimes reference is made to IMEP (Indicated Mean Effective
Pressure) and BMEP (Brake Mean Effective Pressure). This really
confuses the issue, but if it has the word Indicated, then it is
measured in the cylinder, so is the same as MIP (Mean Indicated
Pressure). Similarly BMEP is measured at the Brake or Flywheel.
When using an indicator diagram to calculate the power in the
cylinder, the Mean Indicated Pressure is derived by dividing the
area of the diagram by its length and multiplying the result by
the spring constant. This is then multiplied by the swept volume
of the cylinder (which is a constant for that particular engine)
and the power strokes per second. So if indicator diagrams were
taken for all the engine cylinders, because the speed and the
swept volume are the same, to compare the indicated power for
each cylinder, only the Mean Indicated Pressures need to be
compared. To have the engine perfectly balanced, (i.e. Indicated
Power the same for all cylinders), the Mean Indicated Pressure
would have to be equal for all cylinders
If all the indicated powers for the individual cylinders were
added together this would not equal the power output of the
engine. This is because of the losses through friction
between what is taking place in the cylinders and the flywheel
of the engine where power output is measured. These losses can
be somewhere between 8 and 15%, although a ballpark figure of
10% is not far out.
When the engine is first erected on the testbed, it is coupled
to a dynamometer (more commonly known as a water brake) to
measure the power output.
The words Brake power comes
from the type of dynamometer: often called a water
brake because when put into operation it is trying
to slow the engine down.
The engine power
is calculated from the torque and speed figures
according to the formula: Torque × rpm / 9549, where 9549 is a constant.
brake dynamometers are very popular, due to their
high power capability, controllability, and
relatively low cost compared to other types. They
consist of a fluid coupling where water is the drive
transmission between the engine driven rotor and the
housing which is capable of rotation, but is
restrained by a torque arm connected to a meter.
schematic shows the most common type of
water brake, the variable level type. Water
is added until the engine is held at a
steady rpm against the load. Water is then
kept at that level and replaced by constant
draining and refilling, which is needed to
carry away the heat created by absorbing the
energy (which in itself is a measure of
power output of the engine). The housing
attempts to rotate in response to the torque
produced but is restrained by the scale or
torque metering cell which measures the
When the power for the full load operation has been measured, it
is assumed that the power developed in each cylinder is equal.
Therefore the power developed by each cylinder is total power
divided by the number of cylinders.
If this figure is now divided by the swept volume × power
strokes/second, the Mean Effective Pressure (MEP) should be
For example a 10 cylinder two stroke engine of 960mm bore,
2.5 metre stroke has a swept volume per cylinder of:
p × 0.482 × 2.5 = 1.81m3
If the measured power at the flywheel is 45000kW when the engine
is running at 100rpm (1.67rps), then it can be assumed that each
cylinder is delivering 4,500kW.
Therefore MEP = 4500/(1.81 × 1.67) = 1492 kN/m2 =
If the MIP was measured at 16.6 bar, this would show a 10% loss
through the engine.
MEP/MIP = Mechanical Efficiency