Abstract. 1 Introduction

Transcription

1 Ship propulsion control system design as a way of torque transient restriction Z. Domachowski, W. Prochnicki Department ofship Automation and Turbine Propulsion, Faculty ofship Technology and Ocean Engineering, Technical Abstract in the ship propulsion systems the engine - torque is through high-flexible coupling on the main gear transmitted. The high flexibility of the coupling is demanded because of the required correct collaboration of the flexibly supported engine and the main gear. Such a system is characterized by its considerable sensitivity to the periodic, harmonic or random disturbances. Particularly dangerous are the signals created by the irregularity of engine running In this case exists the possibility of appearance of the propulsion system resonance with some harmonic components of the periodic (or random) disturbances. From the exploitation experience are known the emergencies of considerable torque overshoot of the coupling or other propulsion system element, appearing at dvnamic transient. It is possible to limit the above mentioned torque overshoots by an appropriate ship propulsion control system design. Basing on the ship propulsion system dynamics analysis the paper deals with the control system design aiming to improve the ship power transmission dynamic characteristics. 1 Introduction A two-engine ship propulsion system has been considered, see Fig.l. Transients in such a system may result in significant overshoots, among others of torque of couplings. These overshoots can be harmful and even dangerous as well to the couplings as to the gear. There are different causes of transients in the ship propulsion dynamic system. Particularly dangerous are those ones resulted from an irregularity of engine running. This is the case, for example, when one or more engine cylinders are not acting (not firing). A periodical disturbance is then occuring Consecutively an eventual resonance between some disturbance

2 456 Marine Technology and Transportation components and the propulsion system natural fequency is to be taken into account, e.g Jenzer [2], Parker [3], Prochnicki & Puhaczewski [5]. Any design of the above mentioned dynamic system has to make to avoid dangerous transients. It turns out that one of possible ways to do it is to appropriately adapt the structure and parameters of the engine speed control system Such a way can be fruitful because of that a dynamic behaviour of the torque, and consecutively of mechanical stresses, is more or less influenced bv the structure and parameters of the engine speed control system. This paper deals with such an influence. CO * R1 ^it:jjft& FC2 Figure 1: Diagrammatic arrangement of twoengine ship propulsion: E -engine, FC - flexiblecoupling, G - gear, P - propeller, R - Governor, Z - disturbance, co, co - angular speed, set value of a.s., o)p - a.s. of propeller. Several simulation investigations on the engine speed control system optimization have been carried out, viewing the torque dynamic characteristics. Basing on the ship propulsion system mathematical model: i) an influence of proportional-differential engine speed governor parameters on the high-flexible-coupling torque transients have been analyzed, ii) a modification of the engine speed control system has been suggested to improve the high-flexible-coupling torque transients. Advantages of a modified engine speed control system have been illustrated. 2 Mathematical model of a ship propulsion system The torque is from both engines transmitted through high-flexiblecouplings and main gear to a propeller, e.g. Hiller [1], Prochnicki & Puhaczewski & Krzeminski [6]. Fig. 1 represents a diagram of the discussed ship propulsion system. Its structure as well as constructional parameters have been based on a real system of a ferry boat. A linear assumption has been done. In such circumstances Fig. 2 represents a simplified block diagram of the considered ship propulsion control system, e.g. Prdchnicki & Puhaczewski [5], Puhaczewski & Pr6chnicki & Krzeminski [6]. The rotation part of the system, from the engine rotor up to the propeller, has been modelled with inertia, stiffness, and dumping elements.

3 Marine Technology and Transportation 457 Figure 2: Simplified block diagram of twoengine propulsion system: EI 2 -engine, h^- fuel lever, i - transmission ratio, M^,, 2 - flexible coupling torque, P - propeller, P^ - propeller shaft R^governor, T, 4- tranducer, Z, 2,?- disturbance, o>i 24>- angular speed hi Fig. 3 represents a scheme of an i-th section of such a model. A relationship between an engine fuel lever and an engine rotational torque has been modelled with the 1-st order differential equation. The engine speed governor has been represented by the following transfer function 1 +Ts Figure 3: Diagrammatic scheme of an i-th section of mathematical model, B, D - coefficient of outer, and inner damping, k - stiffness coefficient, J- moment of inertia, <p - torsional deflexion Assuming possible disturbances it has been supposed that one of cylinders of one engine would not be acting. The resulting periodical disturbance signal, see Fig.4, has been modelled by 32 harmonic components corresponding to frequencies 0,5 (0%, 1,0 CD,,; 1,5 co^,... 16x co^, where co is the rated value of the engine rotor angular velocity A response of highflexible-coupling torque to such a disturbance signal has been registered and analyzed.

4 458 Marine Technology and Transportation 0,6-r - Figure 4; Periodical disturbance signal of_one cylinder not firing: Z = Z: Mn, relative value of disturbance. t is: 3 Engine speed control system preliminary investigations Role of engine model accuracy Every considered engine is composed of 8 cylinders. Therefore the engine mathematical model contains 8 sections, e.g. Pr6chnicki & Puhaczewski [5], Puhaczewski & Prochnicki & Krzemmski [6], For a simplification purpose a two-section engine model has also been investigated 3,2 Figure 5: Response to periodical signal Z of flexible coupling torque MS i, of engine Nol a) - two-section model, b) eight-section model: M,, = M^ : M,,- relative torque.

5 Marine Technology and Transportation 459 It has turned out that such a model is not adequate. There have been significant differences in simulation investigations of both above mentioned models. Fig 5 compares high-flexible-coupling torque in the engine in which a disturbance signal has occurred, in both considered models. It is worth to remark that the more flexible is the engine rotor the more significative is the role of the engine model accuracy in the considered sense. In all farther simulations an eight-section engine model has been adopted Role of engine speed governor parameters The engine speed governor amplification coefficient, K%, influences insignificantly the torque dynamics of high-flexible-coupling, of both engines. Its increase provokes a very slight decrease of the torque amplitude. It doesn't influence the torque setting time It is worth to remember that an engine speed governor amplification coefficient is related to an imposed static error of engine speed. This is in particular important when electric generators are connected to the main gear The engine speed governor inertia time, T, influences slightly the high-flexible-coupling torque in the engine in which a disturbance occurs For example, increasing T from 0,1s up to 0,5s results in increasing of the torque overshoot from about 1,7 up to about 1,8. On the other hand the torque oscillations amplitude of the other engine higt-flexible- coupling increases of about 40%. The torque setting time is not influenced in neither of both engines. 150 Figure 6: Step function ZL response of flexible-coupling torque of engine Nol and No 2, b)- T<, -0,1s " 1.2 t[s] 2,0 2,4 ZB The differentiation time of speed governor, Tj, influences mostly the torque setting time which diminishes considerably when Ty increases, see Fig.6. In the same time the overshoot of high-flexible-coupling torque decreases, especially in that engine in which the disturbance doesn't occur, see Fig.6. Unfortunately the increasing of the differentiation time, T,

6 460 Marine Technology and Transportation should be limited. It should be less that the inertia time, T The reason is a deterioration of an engine speed stabilization on one hand, and an amplification of noises on the other hand 4 Control system design attempt to improve torque dynamics Assume an engine speed governor structure It is then possible to influence the torque transients by governor parameters. Anyway such an influence is limited. On the other hand a significant improvement of torque transients is attainable by an appropriate modification of engine speed controller. In the frame of some modification of the engine speed control system, from the point of view of high-flexible- coupling torque dynamics, an auxiliary control signal added to the PD governor has been considered. As a first step of investigations an auxiliary control signal proportional to the angularar velocity difference between both sides of high- flexible-coupling has been assumed. Fig. 7 represents a block diagram of such a modified engine speed controller. Figure 7: Block diagram of a modified engine speed controller; K% 2 - factor of amplification of correction loop, mi 2- fuel mass flow ratio, OR 1,2 governors of engine 1,2. CO] In spite of a very simple modification of engine speed controller the result in coupling torque dynamics is very advantageous, see Fig. 8 and Fig.9. Examples shown in Fig.8 and Fig.9 are corresponding to modified engine speed control system parameters found as optimal in simulation investigations. In dynamic systems of both engines the coupling torque transients have been considerably improved. The improvement has been especially significant in the path of engine No 2 while the disturbance signal has been arisen in the engine No 1. When modyfying the engine control system a PD speed governor has been assumed. The differentiating part of engine speed governor is then required as eliminating, or decreasing at last, a slight deterioration of control transient of engine speed. In the discussed modification of an engine speed control system the angular velocity difference before and behind the high-flexible-coupling is

7 Marine Technology and Transportation 461 measured. That signal is clearly smaller than the engine speed (angular velocity) control error. Therefore it needs a measure device of higher exactitude Figure 8: Step function response of the flexible - coupling torque of engine Nol modified control system, M^,, to the disturbance in engine Nol, Z,; K Kl^K2=KK - factor of amplification of correction loop The aim of the suggested engine speed control system modification is to limit torque transients resulting from important disturbances. In the case of small disturbances that modification is not required. Therefore, for to avoid unnecessary engine inputs, a dead zone in the engine control correction loop should be entered 150 Figure 9: Step function response of the flexible - coupling torque of engine No2 modified control system, M^, to the disturbance in engine Nol, Zj; K K1~K-K2=K _ factor of amplification of correction loop. Simulation investgations have been focused on transients of torque. Nevertheless in the same time engine speed control transients have been registered for to satisfy all corresponding requirements.

8 462 Marine Technology and Transportation Responses to the engine driving torque disturbances have been analyzed. They are not, of course, the only disturbances in the ship propulsion control system Nevertheless they are most important and by the same the most dangerous ones. All above mentioned engine speed control system modifications, improving high-flexible- coupling torque responses to an engine driving torque disturbances, improve also those responses, e.g. that one to a propeller load torque disturbance. Conclusions Simulation investigations have ilustrated a possibility to influence the torque transients with the aid of structure and parameters adaptation of engine speed control system. The attention has been focused on the highflexible -coupling torque Among others, transients of that torque can be harmful and even dangerous to the instalation. Assuming a proportional - differential engine speed governor it is possible to improve high-flexible-coupling torque dynamics, however not enough. A very significant improvement of the mentioned torque transients is available by a specific modification of the engine speed control system. In this frame eg. the application of an auxiliary control signal acting on the input of engine speed controller seems to be very advantageous. References 1. Hiller R. Parallelbetrieb von Mehrmotorert - A.nlagen (Parrnllel-Moiion of Multi-Engine Propulsion System), Hansa, 1977, 9 2. Jenzer J. Vibration analysis for modern ship machinery'. Sulzer Parker G.E., Gorvey D.C. Steady-State Speed Oscillation of Internal Combustion Engines, Transactions'pf'ASMb, Passens M.G., Wu J.Y. Limits cycles in diesel controllable pitch propulsion system using load control International shipbuilding progress, Nov Pr6chnicki W, Puhaczewski Z Analyse des dynamischen Verhaltens einer Zweimotoren - Schiffsantriebsanlagen mit Drehzahlregelung (Analysis of Two Engine Ship Propulsion System Dynamic Behaviours), Schiffbauforschung Wissenschaftlich - Technische Mitlg., Sonderheft Internationales Rostocker Schiffstechmsches Symposium, Puhaczewski Z., Prochnicki W, Krzemiriski A. The algorithm of calculation the dynamic processes of a two - engine ship propulsion system with rotational speed regulation. Marine Technology Transactions, Vol. 4, 1993, Gdansk