【作者】

Francisco Jimenez Espadafor

【摘要】

该论文已在芬兰赫尔辛基举行的第28届CIMAC大会上发表。论文的版权归CIMAC所有。Low speed two stroke diesel engines operate with a superb efficiency being integrated by many cylinders, which pistons are linked to a flexible large crankshaft. These components together can be considered a torsional system that can be simulated by a lumped-mass model. This work shows the development and operation of a torsional model of a 10 cylinders two stroke low speed diesel engine that currently is implemented in a diesel power plant. 1. Torsional system model The nature of the torsional system makes it be highly non-lineal. A lumped-mass non-lineal torsional system model with sixteen degrees of freedom (dof) is formulated where excitation comes from the combustion pressure in each cylinder. 2. System identification Data provided by the engine maker do not fulfill the real engine parameters and therefore the model needs a system identification analyses. For this purpose combustion pressure on the ten cylinders and the instantaneous mechanical torque are measured. An optimization procedure allows the fitting between the measured torque and the torque provided by the model, which is fed with the combustion pressure. 2.1. Torque measurement The real mechanical torque delivered by the engine is measured from strain gages located in the engine-generator shaft. Because of generator efficiency has been measured and the electrical power is known the mechanical torque is correctly calibrated. The torque curve along an engine cycle is far to be flat, being maximum torque about 50% of the mean torque. 3. Model application 3.1. Acute indicated engine power measurement Indicator diagrams from combustion pressure measurement allow the evaluation of indicated engine power and the correct balancing of the cylinders. If the crankshaft dynamic torsion is not considered, the combustion chamber volume is poorly evaluated and therefore the cylinder power is overvalued. In a two stroke ten cylinders low speed diesel engine, the maximum error in the evaluation of the power of one cylinder has been of 5%, giving an error in the engine power higher than 10%. This error affects not only the engine efficiency calculation but also the cylinders power balancing. 3.2. Estimation of Combustion Pressure The lumped-mass non-lineal torsional system model allows the estimation of the combustion pressure of each cylinder. From the measured pressure at plenum intake and considering polytrophic compression and expansion processes, a functional law of two parameters is formulated for combustion pressure description. In the case of a ten cylinder engine this structure supposes 20 unknowns that are evaluated from an optimization procedure. Although the combustion pressure provided is not as accurate as the measured one, it is enough for diagnostic. 3.3. Identification of crankshaft cracks The torsional model fitted to the engine at section 2 is sensitive to changes in the mechanical engine parameters. Mass and inertia are invariant but in the case of the existence of a crankshaft crack, there is a reduction of the dof stiffness that grows with the crack progress. From a simulated combustion pressure curve that accounts the different engine load, the torsional model is excited and the crack on each crank (modeled as stiffness reduction) are free parameters to be estimated from an optimization procedure. 4. Conclusions A precise model of the torsional system applied to current engine data it is a valuable tool for diesel engine management that it has important applications: • Acute indicated engine power measurement • Estimation of Combustion Pressure Curve per cylinder • Identification of crankshaft cracks The system is successfully in operation on three diesel generators of 15 MW at Balearic Islands in Spain.

【会议名称】

第28届CIMAC会议

【会议地点】

芬兰 赫尔辛基

【下载次数】

2

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