Kistler - Determining TDC Shift

Determining TDC Shift

Combustion analysis values such as the maximum combustion chamber pressure and its crank angle position, the 50 % mass fraction burn and particularly the indicated mean effective pressure represent important aids in modern engine applications. The results achieved for most of these parameters and for methods of calculating heat, combustion curves, etc., are heavily dependent on correct relation of the combustion chamber pressure curve to the crank angle of the ICE, with even the smallest errors potentially leading to unacceptable discrepancies.

These pulses are supplemented with a trigger signal output once per revolution to relate the angle signals to piston top dead center. However, rather than at the actual time of piston top dead center, the trigger signal is output at an angle relative to TDC that is established arbitrarily by assembly factors.

The pressure signals cannot be properly related to the crank angle until this angular difference is known. The two important methods of defining the TDC shift are determination with a capacitive TDC sensor, and from the pressure curve in the unfired mode.

When working from the voltage signal of a capacitive TDC sensor (such as the Kistler Type 2629B) the difference between the determined angle and the angle of the trigger signal can be used directly for the shift in the measurement data for correct TDC. Looking at the pressure curve, the offset determined must be corrected by the thermodynamic loss angle. As a result of leakage and heat losses through the walls the maximum of the pressure curve is shifted before TDC by the thermodynamic loss angle.

The relatively simple practicability of this method makes determination of TDC shift from the pressure curve also suitable for onboard analysis of combustion chamber pressure in the vehicle, thereby obviating involved application of a capacitive TDC sensor, which is not always possible in any case.

Inclusion of combustion cycles must be reliably ruled out from the determination of TDC shift. Depending on the traffic situation this is not always possible in a vehicle. Superimposing the measured cycles on a graph allows combustion cycles to be clearly distinguished as curves deviating from the compression curve

The quality and reliability of the determination of TDC shift can be increased by removing these combustion cycles from the group of valid cycles. The pressure curves shown in the Figure represent two typical curves of combustion cycles in the lower load range.