Cylinder pressure indexing

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Charge amplifier , spark plug with sensor hole, shoulder and front sealing high pressure sensor (200 bar, 350 ° C, approx. 18 pC / bar)

The cylinder pressure indexing is a metrological method for recording the internal pressure (or indexed pressure) prevailing in the working space of engines with internal combustion and other piston engines as a function of the crankshaft angle or time. The aim is a better understanding of the thermodynamic process management as well as the indicated performance provided.

There are different ways of indexing, whereby the electronic variant with piezoelectric pressure sensors is more common than the mechanical one. The indexing carried out in the cylinder is called high pressure indexing due to the high pressures prevailing there. In addition, the low-pressure indication, i.e. the measurement of the pressures in the inlet and outlet for gas exchange analysis , with piezoresistive pressure sensors for holistic measurement.

During the measurement, a direct reference to the position of the piston or the time is established, which enables an assignment to the respective working phase of the internal combustion engine. The internal engine torque can be calculated from the pressure, (cylinder) bore, (piston) stroke and position of the crankshaft . In addition to the correct recording of the crank angle, which should be as close as possible to the crankshaft throat of the indicated cylinder, a clean measuring chain that is undisturbed by external influences is important for indexing the cylinder pressures. If a simple impedance converter is used instead of a charge amplifier , it is sufficient to touch the cable ends with washed hands and thus to reduce the very high insulation resistance of at least 10 12 Ω required (for simple devices) so that the measurement results are falsified. A charge amplifier is not critical in this regard.  

The first mechanically recorded pressure-volume diagrams , then called indicator diagrams, were created by Nikolaus Otto . These confirmed his diagrams calculated with the help of the gas laws .

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