Determination of TDC in internal combustion engines by a newly developed thermodynamic approach

In-cylinder pressure analysis is nowadays an indispensable tool in internal combustion engine research & development. It allows the measure of some important performance related parameters, such as indicated mean effective pressure (IMEP), mean friction pressure, indicated fuel consumption, heat release rate, mass fraction burned, etc. Moreover, future automotive engine will probably be equipped with in-cylinder pressure sensors for continuous combustion monitoring and control, in order to fulfil the more and more strict emission limits. For these reasons, in-cylinder pressure analysis must be carried out with maximum accuracy, in order to minimize the effects of its characteristic measurement errors. The exact determination of crank position when the piston is at top dead centre (TDC) is of vital importance, since a 1° degrees error can cause up to a 10% evaluation error on IMEP and 25% error on the heat released by the combustion: the position of the crank shaft (and hence the volume inside the cylinder) should be known with the precision of at least 0.1 crank angle degrees, which is not an easy task, even if the engine dimensions are well known: it corresponds to a piston movement in the order of one tenth of micron, which is very difficult to estimate. A good determination of the TDC position can be pursued by means of a dedicated capacitive TDC sensor, which allows a dynamic measurement (i.e. while engine is running) within the required 0.1° precision 1], 2]. Such a sensor has a substantial cost and its use is not really fast, since it must be fitted in the spark plug or injector hole of the cylinder. A different approach can be followed using a thermodynamic method, whose input is in-cylinder pressure sampled during the compression and expansion strokes: some of these methods, more or less valid, can be found in literature 3], 4], 5], 6], 7], 8]. This paper will discuss a new thermodynamic approach to the problem of the right determination of the TDC position. The base theory of the method proposed is presented in the first part, while the second part deals with the assessment of the method and its robustness to the most common in-cylinder pressure measurement errors.