Secondary Motion of the Piston in Large Crosshead Diesel Engines

2008 AP4: Secondary Motion of the Piston in Large Crosshead Diesel Engines

Translated from Journal of the JIME Vol.43,No.3 ○C 2008(Original Japanese)

Secondary Motion of the Piston in Large Crosshead Diesel Engines

Masaki Tanaka*1

*1 Mitsui Engineering & Shipbuilding Co., ltd., 3-1-1, Tama, Tamano-city, Okayama, Japan, E-mail [email protected]

Secondary motion of the piston in large crosshead diesel engines was measured. The piston moves in the

cylinder to Cam - Exhaust direction, and also to Fore - After direction. The factors affecting the motion are discussed 1. Preface Secondary motion of the piston in reciprocating internal combustion engines is defined as a piston movement except in its reciprocating direction. Many measurements and analyses of this subject are found regarding trunk piston type engines. However, there is little study of this subject for crosshead engines. This could be because thrust force and piston inclination are too small for there to be much interest. However, observing actual crosshead engines, the following phenomena are found. 1) Wear of the cylinder liner is sometimes not equal in the circumference. 2) Cylinder liner temperature is sometimes different in the circumference. 3) Deposit formation on the piston crown is sometimes different in the circumference. Fuel injection or scavenging air flow has usually been discussed as the cause of uneven distribution of these phenomena. But the author considers the piston secondary motion as one of the factors affecting these phenomena. The author has previously conducted investigation of cylinder liner deformation. 1) Cylinder liner deformation was measured by non-contact eddy current type proximity sensors that measure the distance from the piston to cylinder liner wall in real time held under the piston to 4 directions. Analyzing the data, we can obtain piston secondary motion together with the cylinder liner deformation. Though modern crosshead engines are mainly used as marine propulsion engines, measuring the piston motion was conducted in a generator engine on land for the convenience of the work. Consequently, the following properties of the engine were different from marine engines. 1) Revolution speed of the marine engine varies according to the required ship's speed but the rotation speed of generator engine is constant except idling. 2) Stroke of generator engine is shorter than marine engine to obtain high revolution speed which contributes to the size reduction of the generator. 3) Generator engine is put horizontally on the land and does not incline as does a marine engine. 4) Service output of the marine engine is usually lower than its maximum output. On the other hand, generator engine is usually operated at 100% load. 5) Generator engine rotates only to single direction. Marine engine sometimes run in reversing direction. Further, the following special situation was recognized. 6) The measured cylinder liner was re-machined fully to 0.6 mm oversize of the standard. As a result, measuring the secondary motion was easy. However this kind of machining is not usual.



Because of these differences, the measurement results are not directly applicable to marine propulsion engine. However they could show qualitative tendency. It is noted that the measured maximum displacement of the piston was over 1 mm. It can be because of oversized cylinder liner, worn piston skirt and thermal expansion of the cylinder liner. 2. Structure of the crosshead Structure of the crosshead assembly could affect piston movement. Fig. 1 shows the structure. The piston rod is fixed on the crosshead pin by bolts. The guide shoes and the connecting rod are jointed on the crosshead pin and are able to rotate. Since the piston is guided by the cylinder liner, it inclines little, usually less than 1/1000.

Fig. 1 Structure of crosshead assembly

3. Measuring system 3.1. Arrangement of the measuring system Properties of the measured engine are shown in Table 1. Measured cylinder was number 6. Fig. 2 shows the arrangement of the measuring system on the engine. The names of directions are the as same as used on a marine engine. Output direction is Aft and the opposite is Fore. According to the cam and exhaust system arrangement, transversal directions are called as Cam and Exh as shown in Fig. 2.

Table 1 Properties of measured engine Type K80MC-GI-S Shaft output kW/Cyl. 3610Revolution speed rpm 104.2

Revolution direction

Clockwise, view from the

generatorStroke mm 2300Cylinder diameter mm 800Number of cylinder 12



Fig. 2 Engine arrangement top view

As described in reference 1, eddy current type proximity sensors were put under the piston skirt on Fore, Aft, Cam and Exh direction. They measured real time distances from the cylinder liner wall to the piston. The signals were transmitted with FM telemeters. Piston position was detected by the crank angle measured by rotary encoder. Since the proximity sensors were put under the piston, they went out from the cylinder liner at BDC so it was not possible to measure the distance. When the sensors passed on the ports, it was also impossible to measure the distance because there was no target. 3.2. Signal transmission The four sensor's outputs are transmitted in parallel by different wave frequency. Since transmission was not always perfect, the complete signal was not obtained. However, at least one of the opposite two sensor signals was obtained, and it was possible to estimate the displacement of the piston. The data was sorted depending on the signal quality. 4. Measurement results Measurement results are described for the following cases. (1) 100% load operation (2) Turning (forward and reverse) (3) Idling (51 rpm and 103 rpm) 4.1. 100% load operation Piston movement at 100% load operation is shown in Fig. 3. Fig. 3a shows the displacement of Cam-Exh. direction and Fig. 3b shows the displacement of Fore-Aft direction. Fig. 3c shows the locus of the piston view from the top. Height and reciprocating direction of the piston are marked along the locus. (Ex: "1500 Down" means the piston height is 1500 mm from BDC and the reciprocating direction is downward) The broken line shows the limit of piston movement. Due to the drift of the signal, the limit circle is not coaxial to the axis of the signals. A fine wave can be caused by noise of the signal or vibration of the measuring parts. In this case, piston places on Exh side at BDC. After half of the stroke up, the piston starts moving to Cam side along the Aft side wall as spiral. The piston reaches Cam side at TDC. The piston keeps cam side with small motion to Fore side near TDC. From half down of the stroke, piston returns to Exh side through the center of the cylinder.



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Fig. 3a 100% load, Exh-Cam displacement

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103 RPM 100% Load

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Aft

Fig. 3b 100% load, Fore-Aft displacement

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Fore

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TDC

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Fig. 3c 100% load, locus view from the top

(number shows the piston height from BDC and up/down shows the reciprocating direction)



4.2. The case of turning As the case of turning, cylinder pressure is zero and the inertia force is negligible. Piston movement depends on its gravity and friction force. Fig. 4 shows piston movement at forward turning. Piston jumps from Exh to Cam side. Piston places on Exh side at BDC and moves to Cam side as it goes up. During the downward stroke, the piston moves to Exh side at first but once goes back to the center and finally returns to Exh side. Fig. 5 shows the case of reverse turning. Fundamentally, the movement is symmetrical to forward turning. However, the timing of the motion is different. As the reproductivity of the motion is confirmed by repeated rotation, the difference of fore and reverse turning is not a kind of scatter. There must be certain reasons, for example, the position of the crank shaft or asymmetrical characteristics of the crosshead bearing due to asymmetrical wear.

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Data No. 4 ExhUp

Data No. 4 ExhDown

Turning ForewardExh

Cam

BDC TDC

Fig. 4a Forward turning, Cam-Exh displacement

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Data No. 4 ForeDownTurning ForewardFore

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Fig. 4b Forward turning, Fore-Aft displacement



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Turning Foreward

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Fig. 4c Forward turning, locus view from the top

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Data No. 10 UpTurning ReverseExh

Cam

BDC TDC

Fig. 5a Reverse turning, Cam-Exh displacement

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Data No. 10 Down

Data No. 10 UpFore

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Turning Reverse

BDC TDC

Fig. 5b Reverse turning, Fore-Aft displacement



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Turning Reverse

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TDC

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Fig. 5c Reverse turning, locus view from the top

4.3. Idling In idling condition, the piston is pushed by cylinder pressure, less than 100% load, and by inertia force. Fig.6 shows the case of 51 rpm and Fig. 7 shows the case of 103 rpm. At 51 rpm, the piston movement is like as the case of turning but moves circumferential near TDC. At 103 rpm, the piston rounds in the cylinder. Almost moves two rounds for one stroke.

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Fig. 6a Idling (51 rpm) Cam-Exh displacement

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Fig. 6b Idling (51 rpm) Fore-Aft displacement



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Fig. 6c Idling (51 rpm), locus view from the top

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Fig. 7a Idling (103 rpm) Cam-Exh displacement

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Fig. 7b Idling (103 rpm) Fore-Aft displacement



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Fig. 7c Idling (103 rpm), locus view from the top

5. Discussion 5.1. Piston motion to Fore-Aft direction Piston motion to Fore-Aft direction is an interesting phenomenon. The author postulates the following: 1) Alignment change of the cylinder axis due to the deformation of the engine frame. 2) Deformation of the crank shaft and the connecting rod. As a result, crank pin, crosshead pin and piston are inclined. 3) Twisted position of the crosshead pin in the crosshead bearing. As the pin axis twists against the bearing axis then the piston may be inclined to Fore-Aft. direction. 4) As described later, piston ring moves a little when making contact with the piston. And it applies a friction force to the piston being pushed by high cylinder pressure. Since hydrodynamic lubrication can not be perfect, friction coefficient may be high. Piston is possibly moved by the friction force between piston and piston ring. 5.2. About piston ring rotation Piston ring rotation is the fact because we can not find its gap on fixed position. However, the drive force of it is not certain. Fig. 8 shows the piston ring gap movement by turning. Observed engine is not the same engine as measured engine. The piston ring is painted and the engine is turned forward one revolution. Then the edge of the painted line moved about 1 mm to the left. Cylinder liner wear was about 1 mm in diameter in the upper part of the cylinder liner. Lower part was worn little. As a result, the piston ring expands at the upper part and is drawn at the lower part. By this kind of motion, a part of the piston ring moves to circumferential direction. Since the friction force may not be constant, the piston ring moves slightly and does not return to the original position. Consequently, the piston ring might rotate by piston reciprocation. During turning the engine, friction force between the piston ring and the piston ring groove is small. On the contrary, the friction force is large at when the engine is running because high cylinder pressure pushes the piston ring on the piston ring groove. Author considers that the friction force between the piston ring and piston ring groove should be counted in the factor which drives the piston. Though, the magnitude of the force is not certain, let us estimate the friction force as below.



Assuming that a single piston ring is sealing cylinder pressure, cylinder pressure is 10 MPa, cylinder diameter is 800 mm, radial thickness of the piston ring is 25 mm and the pressure is induced on the half of its thickness. Then the piston ring is pushed down by the force of Fn. It is Fn = 10 [MPa] * π/4 (800 [mm2] - 750 [mm2]) /2= 310,000 [N] = 30 [tons] Assuming the friction coefficient as 0.1, which can be reasonable magnitude in boundary lubrication, then, friction force between the piston and piston ring becomes 3 tons. This is large enough for the piston not to be pushed by thrust force. Consequently, the friction force which is generated by relative motion of piston to piston ring and of piston ring to cylinder liner could affect the motion of piston together with the motion of piston ring.

Before turning

After turning

Fig. 8 Piston ring rotation by turning (Piston ring is hidden on the left and on the right by port ribs)

5.3. Effect on uneven wear of cylinder liner Cylinder liner wear is not uniform in all directions. For example, wear pattern at the upper part is different from the lower part. It is usually considered that the wear is different on Cam direction to Exh direction. Fuel oil injection pattern or scavenging air flow have been considered as the cause of uneven wear. But the secondary motion of the piston can also possibly have an affect on the wear. 5.4. Repeatability of piston secondary motion Consider that if the piston secondary motion is always the same. Fig. 9 shows piston movement of downward stroke at full load. The lines show the records of every 10minutes. The lines are similar but having different patterns. The piston motion is roughly same but it changes in detail.



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Fore

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Fore-Aft direction, downward stroke

Fig. 9 Change of piston motion 5.5. Generality of piston motion The question is that if all engines and all cylinders have same piston motion. By observation of deposit formation on the piston crown, piston seems on Cam side at TDC every time. However, the factors affect on the piston motion is not yet clear. Furthermore, author has no data of other cylinders nor of other engines. Author has not enough reason to mention the generality of our results.

6. Closing Piston movement in 2 cycle crosshead engine was measured and discussed. This kind of data might not been published before. Observing the results, author had many doubts and also surprising. A piston movement seems to be beyond our consideration. Although author supposes that theoretical calculation is much difficult, author is also interested in the calculated results based on an ideal condition. Author hope someone who is enough brave tries this calculation. 7 Acknowledgement This paper is written based on the results of the project SR801, "Study of improving reliability and fuel economy of large bore crosshead engines". The members of the project were Mitsubishi Heavy Industries, Nippon Yusen Kaisha, Mitsui O.S.K. Lines, Kawasaki Kisen Kaisha, Nippon Kaiji Kyokai, Japan Internal Combustion Engine Federation and Mitsui Engineering & Shipbuilding. Author thank to all the members. Author also express great thank to Mr. Otani, the organizer of SR801 committee, and to Mr. Murakami, the secretary of SR801 committee. Reference 1) M. Tanaka and T. Kuwada, JIME, 43-2(2008-2), 84-89

Documents

Secondary Motion of the Piston in Large Crosshead Diesel Engines