Number of measurement cycles and accuracy of engine indication process

The results of measurements of instantaneous pressure in the cylinder are most often inputs tofurther calculations of the characteristic parameters of the working cycle of the engine. Many repetitions of measurements always feature a degree of unrepeatability in registered cycles, making determination of the so called representative round difficult. The paper considers the problem of estimating the minimal number of measurement cycles for the registration of pressure in order to obtain a representative course ofpressure in the cylinder of an compression ignition engine. Maximum pressure was adopted as an indicator of cycle repeatability. After excluding systematic errors, repetition ofmaximum pressures recorded in subseąuent measurement cycles were analyzed for fixed load and rotation speed of the engine, defining the coefficient ofvariation CV. subsequently, for selected states of the engine the representativeness of the average maximum pressure depending on the number of measurement cycles averaged was examined. full evaluation of the accuracy of the indication process in relation to the number of averaged measurement cycles reauires also determination of the so called expanded uncertainty of the measurement's result. Further analysis including determination of the expanded uncertainty of the measurement result revealed that pressure course averaging over 20 consecutive cycles of measurement is sufficient to obtain a representative cycle.