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Engine Assessment

Indicator Diagrams: MIP and MEP
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Engine manufacturers 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.

The water 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.

 The 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 torque. 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 obtained.

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 = 14.92 bar.

If the MIP was measured at 16.6 bar, this would show a 10% loss through the  engine.

MEP/MIP = Mechanical Efficiency

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