DE102011120160A1 - Method for converting time-dependently measured cylinder pressure signal for calculating cylinder power of internal combustion engine, involves converting cylinder pressure signal into angle-dependent pressure values in top dead center - Google Patents

Abstract

The method involves converting the time-dependently measured cylinder pressure signal into angle-dependent cylinder pressure values in top dead center, based on crank-angle measured value. The quality of the measurement and calculation of pressure values is monitored. The cylinder pressure values are set independent of speed fluctuation of motor. The pressure pattern in the cylinder is reproduced over several operating periods.

Description

State of the art:

Modern internal combustion engines have been constantly evolving in recent years. The maximum pressures and the specific benefits increased as a result. Thus, the component loads have increased enormously. The basic setting of the machine and the control of these settings during operation is thus becoming increasingly important. The evaluation of the quality of the settings and the diagnosis of internal combustion engines is often performed by measuring the internal pressure history in the cylinders of the engine. Fixed systems but also mobile systems are sometimes used here. In the case of mobile systems, the assignment of the recorded measured values to the angle of rotation of the crankshaft can not be realized or can only be implemented by additional encoders. A calculation of the indicated cylinder power is only approximately possible with these systems, since for the calculation the assignment of the measured values to the angle of the crankshaft is decisive for the accuracy of the power calculation. One degree of deviation can cause an error of up to 10%.

Object of the invention:

The aim of the invention is the conversion of the time-dependent recorded measured values of a cylinder pressure measurement to an exact crank angle assignment with a predetermined number of measured values per working cycle. This is to create the conditions for an accurate calculation of the indicated cylinder power for mobile devices. Furthermore, to account for the previously required donors for the crankshaft position with this invention. Current machines do not have to be stopped first for encoder mounting in order to carry out a measurement. The devices are thus easier to handle and are similar in accuracy as fixed systems, where the measurement is controlled by a rotary encoder on the crankshaft.

Solution:

To convert the time-dependent recorded cylinder pressure signal, the individual measured values are first converted into pressure values in relation to the ambient pressure. It should be noted that the charge air pressure in dynamic sensors is taken into account when calculating the measured values. As a result, there is a series of measured values that represent the pressure curve in the cylinder over several working cycles.

For the conversion into angle-dependent values, a theoretical compression pressure curve with a mean estimated polytropic exponent is first calculated with the aid of the engine parameters. From this calculated course, the crank angles are determined for several pressure stages in the rising phase of this pressure curve.

Thereafter, the intersections of the measured time course are determined with the specified pressure levels in the respective rising phases. In each of these phases, one obtains a measured value number for each pressure stage. The length of the respective working cycle can be calculated from the differences of the measured value numbers. The number of measured values must be approximately the same for each pressure stage of a working cycle. Larger deviations indicate errors z. B. in the measured value recording out. The number of measured values for a working cycle can still be used for calculating the speed. There is a speed available for each working cycle. This speed deviations can be detected in the measured value recording and the evaluation can be canceled if necessary.

From the knowledge of the intersections of the pressure levels, the length of each cycle and the angle values of the theoretical pressure curve can then be the starting point for each working cycle z. B. 180 ° before TDC can be determined. Again, the quality of the calculation can be monitored by using multiple pressure levels. If the calculation is correct, approximately the same starting points for the respective work cycles must be determined for all pressure stages.

From the previous steps, the starting points and the length of the work cycles are known. With the help of suitable interpolation z. B. cubic splines, each individual work game can be converted to an appropriate angle resolution for further calculation. As a result of this calculation, one obtains a rotation angle-dependent pressure curve for several cycles with the same angular resolution, the only a small angular offset to the correct position of the top dead center has. This offset can then be eliminated by means of a dynamic angle correction, which is not the subject of this invention.

Embodiment:

α₀

... Winkel bei Beginn der Kompression (Ladeluftdruck)

p₀ = p_zyl(α₀) For the determination of the length of the individual working cycles and the starting value for the working cycles, a reference pressure curve of the compression for the engine is calculated. For this purpose, the angle at the beginning of the compression of the timing of the charge exchange organs is determined. The cylinder pressure at this angle is set equal to the charge air pressure.

α ₀

... angle at the beginning of the compression (charge air pressure)

p ₀ = p _zyl (α ₀ )

Thereafter, for several angles (α ₁ , α ₂ and α ₃ ) in the compression phase (α ₀ ... α _OT ), the theoretical cylinder pressure is calculated with the constructive engine parameters and a mean pole exponent κ ( ).

d_z

Bohrungsdurchmesser

h

Kolbenhub

λ

Schubstangenverhältnis

ε

Verdichtunsverhältnis

For this purpose, the cylinder volume is first determined for the selected angle.

d _z

Bore diameter

H

stroke

λ

Pushrod ratio

ε

Verdichtunsverhältnis

Thereafter, the measured value idx are determined at which the measured values cross the calculated cylinder pressure values ( ).
idx _ik for which we have p _m (idx - 1) <p _zi and p _m (idx + 1) ≥ p _zi

From the determined measured value numbers, the number of measured values for each individual working cycle can be determined ( ). mw _{i, k} = idx _{ik + 1} - idx _ik

Ideally, the number of readings per cycle must be the same for all pressure thresholds. In the case of large deviations, this indicates errors in the measured value acquisition. By checking the differences in the number of measurements, erroneous measurements can be detected and discarded.

From the number of measured values per working cycle, the angular increment dα _k and with the aid of the sampling frequency t _ADC the rotational speed n _k can be calculated for each individual working cycle.

If the calculated speeds of the individual work cycles differ greatly from one another, this indicates strong fluctuations in the speed of the measurement. The further calculation can be faulty in the case of strong speed fluctuations. By evaluating the speed fluctuations, measurements with large speed differences can be detected and discarded.

To convert the time-dependent pressure values to the crank angle, it is necessary to calculate the correct assignment to the crank angle in addition to the angular increment. For this purpose, the starting point for the first working cycle is determined from the angles α ₁ , α ₂ , α ₃ and the measured value numbers idx ₁₁ , idx ₁₂ and idx ₁₃ ( ).

From the starting values for the individual pressure stages, a mean starting value can be calculated.

With the middle starting value s and the numbers of measured values of the individual work cycles then the individual work cycles can be cut out of the measured data.

For a further calculation, it is favorable to convert the individual work cycles, which can have different numbers of values and thus different angular resolutions, to equal angular step sizes. For this purpose, suitable interpolation methods such as cubic interpolation splines or similar methods can be used.

As a result of the calculations, there are then several pressure profiles of single work cycles with the same angular increment for the further calculations.

Claims (4)

A method for converting time-dependently measured cylinder pressure curves to measured values relating to crank angles is characterized in that pressure measuring devices recorded over time are converted into rotational angle-dependent pressure values with approximately correct position to top dead center. The method according to claim 1 is characterized in that for determining the correct position to top dead center, a theoretical compression pressure curve is used, which is calculated from engine parameters. The method according to claim 1 is characterized in that the time-dependent recorded cylinder pressure values of the individual working cycles are converted to equal angular distances independent of small speed fluctuations. The method according to claim 1 is characterized in that a monitoring of the quality of the measurement and the calculation is possible during the individual conversion steps.

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