Analysis of processing methods for combustion pressure measurement in a diesel engine

Abstract

This paper analyzes combustion chamber pressure data processing methods related to the number of cycles averaged, top dead center referencing and pressure referencing (pegging). A total of 1000 consecutive engine cycles were measured in a four-cylinder diesel engine. The number of cycles that minimizes the influence of cycle-to-cycle oscillations depends on engine operating conditions and the parameters under analysis. The top dead center (TDC) referencing, using the motored curve, revealed that the thermodynamic loss shifts the peak pressure − 0.4 °CA from TDC. Four pegging methods were compared-least-squares, fixed-point, three-point and two-point-introducing as main novelty the fact they have not been previously investigated on the same baseline conditions. The least-squares based method showed the lowest sensitivity to random noise, but with longer processing time, and the fixed-point method presented higher dispersion in the heat release analysis. The three-point referencing method considers a variable polytropic coefficient, but suffers from noise sensitivity, and the two-point referencing method presented close values and higher dispersion in comparison with the least-squares method. The choice of which method to use depends on the type of analysis, signal quality and processing time available.

A novel method for in-cylinder pressure prediction using the engine instantaneous crankshaft torque

“…where a = 0:1 representing the desired percentage variation from the healthy conditions, wheras U(0, 1) denotes the standard uniform distribution from 0 to 1, and therefore x is a vector of the same size as B that contains uniformly distributed numbers from 0.95 to 1.05 in accordance to equation (26). The unknown parameters vector B is multiplied by a diagonal matrix containing the vector x in accordance by equation (27), which results to the degraded engine parameters vector B d .…”

“…24 Moreover, having measured the in-cylinder pressure, the non-trivial task of signal correction is required, in order to remove the noise and perform averaging over several cycles, and most importantly to remove offsets by calibrating the acquired signal with respect to the engine crank angle 25 and the cylinder pressure. 26 As a result, methodologies have been developed utilising alternative engine measurements to predict important features that belong to, or can be directly derived from the in-cylinder pressure diagrams in a reliable and low-cost manner. In specific, for automotive industry applications, the peak pressure position has been predicted using accelerometers mounted on every cylinder, 27,28 as well as an ICT sensor.…”

Determination of a most representative cycle from cylinder pressure ensembles via statistical method using distribution skewness

“…However, this is a time consuming and inefficient process. An attempt was made to correct for this problem by introducing a socalled ''characteristic variation'' (C i ) weighting factor, as shown in equation (6).…”

“…The conclusion was that 50 cycles were sufficient for calculating a stable value of IMEP and its COV for this set of data. da Silva et al 6 explored the effect of ensemble length in the COVs of IMEP and Peak Cylinder Pressure (PCP) for ensembles presenting cycle-to-cycle variability. This study demonstrated that that the sufficient number of cycles that minimizes cycle-to-cycle variation effects on calculated parameters from cylinder pressure and heat release was dependent on engine operating conditions and the analyzed parameter, ranging from 250 to 850 cycles.…”

The Flex-OeCoS—a Novel Optically Accessible Test Rig for the Investigation of Advanced Combustion Processes under Engine-Like Conditions

“…Piezo-electric pressure sensors, as are normally used to measure cylinder pressures, must be tied to a known absolute pressure to obtain the effective in-cylinder pressure. The most common pegging methods are either based on a thermodynamic calculation assuming a fixed polytropic exponent within a part of the engine cycle, or based on the assumption that the cylinder pressure is equal to the intake manifold pressure at a certain point during the engine cycle [26]. In the case of the Flex-OeCoS, the first method is difficult to apply due to the different wall-heat transfer regimes in the engine cylinder, the optically accessible combustion chamber and the connecting "neck" in-between.…”