Speed irregularity characteristic of low speed, two-stroke marine diesel engine applied as vessel's main propulsion

The piston engine irregularity is a consequence of the uneven power supply from the engine units. As such, it contains valuable information about engines units working process. Unfortunately, it is strongly influenced by torsional vibration, resonance, and propeller behaviour. In order to better understand the speed irregularity phenomena the instantaneous speed was recorded and its characteristic against engine mean speed was developed. Some statistical parameters: standard error and deviation as well as variance were determined. The results were compared with propulsion plant torsional vibration. The presented results show that the irregularity of the long stroke engine's speed is strongly influenced by torsional vibration phenomena. The analyzed engine has a bared speed range 37-43 rpm. Exactly around that range, the speed irregularity reaches its maximum values of 18% of mean speed. The analysis can be considered as qualitative look into the phenomena. However even based on the presented analysis one can observe some practical inferences. The common type of electronic indicators is used to simplify the measurement by assuming that the shaft speed is equal in between the pickup pulses. In order to increase the accuracy of the indicated power determination the use of such instrument should be avoided in the speed range, which is a multiple number of the barred speed. Due to the fact that the shaft speed is irregular especially in the ranges where torsional resonance may occur the piston positioning (for example for TDC determination) purpose procedure should take it into account.