Engine combustion chamber tightness diagnostics

The energy transformation efficiency of piston combustion engines depends, among others, on each cylinder’s combustion chamber tightness. The combustion chamber tightness measurement is therefore one of the most frequent diagnostic tasks (BALOG 1999). The compression pressure is a common indicator of the combustion chamber tightness. The compression pressure is an important complementary indicator of the vehicle engine’s technical state and together with the engine’s output composes the main set of parameters characterizing the engine technical state. The common technique of direct compression pressure measurements is very suitable for maintenance, however, for periodical or continual diagnostics or for so called on-board diagnostics is inappropriate. For these purposes, the completely disassembly-free diagnostic technique, detecting the combustion chamber tightness by means of measuring the instantaneous starter current during the engine’s cranking, seems to be very suitable. This method is based on the fact, that the instantaneous cranking current is proportional to instantaneous mechanical resistance of the cranked engine and that the engine’s mechanical resistance is proportional to individual cylinders’ combustion chamber tightness (BALOG 1999). Periodical variations of the cranked engine mechanical resistances then determine the basic (characteristic) frequency of starting current variations (POSTA, PAVLICEK 2002). Besides this basic characteristic frequency, other variations of various frequencies participate on the measured signal waveform, these frequencies superpose on the basic waveform and thus participate on the resulting signal (HAVLIK 2002). The measured signal waveform can be affected, for instance, by the variable mechanical resistance of engaging individual starter pinion teeth to mesh with flywheel gear ring, the starter shaft runout, the brushes passing the commutator bars or by the electrical failures of armature windings (POSTA et al. 2002). The display of the measured waveform can be also affected by the selected sample rate. It is necessary to eliminate these disruptive factors or to distinguish their influence to the diagnostic signal ( POSTA et al. 2000). For the chosen indicator “instantaneous starting current”, the dependence of variations and the sensitivity on the variations of the single cylinder’s combustion chamber tightness of four-cylinder engine was examined experimentally.