A low-speed diesel engine holds a unique position in large ship propulsion usage thanks to its high economic efficiency and superb reliability. However, the grounds of its reputation are not necessarily clear especially for the engineers in the other engine relating fields, since detailed and objective discussion has been rare among engines from high-speed to low-speed. At first, various indexes such as representative speed, mean piston speed and stroke-to-bore ratio are investigated among engines of various sizes. The result shows that the marine low-speed diesels run much more slowly than expected from a similarity law based on the engine size, and the ship propulsion efficiency should be a key of the ultra-low revolution. Detailed comparison between low-speed diesels and high-speed ones are also done to capture the essential differences in the engine type and structure. Finally, the application of EGR (Exhaust Gas Recirculation) to marine diesels is taken up to exemplify the difficulty in dealing the marine fuels of high sulfur content.
Referring some of the experimental and / or analytical studies conducted by late Dr. Yutaro WAKURI, an emeritus professor of Kyushu University, and his collaborators, the lubrication for the elements and lubricants of marine diesel engines have been reviewed to consider the design concepts for engines still important in the future. Some studies on lubrication problems have been reviewed orderly for the cylinder and piston ring system, the lubricating oil and the bearing system. In particular, concerning the piston rings, some design concepts have been considered referring the study results on the characteristics of abnormal or normal wear, the occurrence mechanisms and prevention measures for scuffing failure, the characteristics of oil-film behavior and / or oil consumption. Also regarding the bearings, some key points of design have been also considered referring the study results on the lubrication characteristics or the improvement of load-carrying capacity for crosshead-pin bearings.
The crosshead bearings in large two-stroke diesel engines operate under severe lubrication conditions, because the bearings oscillate within a small angle at a low speed and are subject to a high specific load. Crosshead bearings have several axial oil-grooves on the loaded surface to promote oil film exchange with bearing oscillation. However, because the development of a thick oil film by hydrodynamic action is impaired by these oil grooves, the bearings become prone to damage by seizure and fatigue cracking. This manuscript describes the lubrication characteristics of the crosshead bearings. A decrease in the clearance ratio is generally effective at enhancing the squeeze action on the entirety of the bearing pads. However, this degrades the formation of an adequate wedge film and reduces the load carrying capacity. Incorporating oil-grooves with tapered geometry and an externally pressurized lubrication system promotes a thicker oil film, achieving a significant enhancement of the load carrying capacity. The aluminum-tin bearing coated with a tin-copper overlay improves running-in properties, enhancing the load carrying capacity.
This paper shows the technical trends and future aspects of piston rings for 4-stroke and 2-stroke marine diesel engines. The combination of cast iron and chromium plating is the current standard specification of piston ring for 4-stroke diesel engine. On the other hand, for 2-stroke diesel engine, the combination of cast iron and thermal spray coating, which is applied in accordance with the liner type, is also adopted. In addition, differences in piston ring specification for marine diesel, for example base materials, surface coatings or shape of cross section compared to the small bore 4 stroke diesel engines for trucks, buses and off-high way application are shown. From the viewpoint of technical development, it is assumed that the combination of steel material and PVD coating, likewise heavy duty trucks, will be applied to piston ring for 4-stroke marine diesel engines in order to achieve higher durability in the near future.
The friction regime between the piston ring and the cylinder liner in marine diesel engines changes from fluid film lubrication at mid-stroke to boundary lubrication near dead centers. As the lubrication conditions of large two-stroke engines become more severe with increasing the mean effective pressure and the piston speed, the prevention of severe wear and scuffing has become important issues. The temperature of the cylinder liner also tends to increase, keeping the sulfuric acid above its dew point to reduce corrosive wear. This manuscript describes the lubrication characteristics of the piston rings in marine diesel engines. The effect of the total base number (TBN) of the cylinder oil on the scuffing resistance is evaluated. The measures to prevent abnormal wear and scuffing are also described. The on-line monitoring to detect severe lubrication conditions is effective for enhancing durability and reliability of marine diesel engines.
Internal combustion engines have been strongly required to have greater fuel economy and lower exhaust emissions. The thermal efficiency of an engine can be improved by reducing the mechanical friction loss. The friction loss of piston assembly accounts for about 40% of total friction loss. A theoretical analysis is presented of the mixed lubrication between the cylinder and piston ring pack in a small size 4-stroke cycle diesel engine. The analysis comprises Patir and Cheng’s average flow model and Lee and Ren’s asperity contact model. The former decides the hydrodynamic film pressures and the latter estimates the contact pressure and the real contact pressure. Influencing factors such as surface roughness, lubricant viscosity, ring tension and sliding surface profile of top ring on the characteristics of film thickness, friction and lubricant transport are discussed.
It has been reported that oil film thickness can be increased and rolling fatigue life can be improved by imparting anisotropic roughness pattern (orientation) to the surface or truncating the surface roughness. With respect to roughness pattern, Zhu researched the change of oil film thickness in pure rolling point-EHL condition with gaussian surface roughness. On the other hand, there are few papers that verified the effect of truncating the roughness. In this report, it is shown that non-gaussian surface pattern influences the behavior of lubricant flow, which affects the oil film thickness and the load obtained by numerical simulation using the average flow model. Johnson SU theory was used for a probability density function (skewness -1.5, kurtosis 10) of surface roughness and we estimated the oil thickness and asperity load in pure rolling point contact EHL conditions. Results show that oil film thickness of non-gaussian surface becomes larger than that of gaussian surface as Λvalue becomes lower and asperity load ratio of non-gaussian surface becomes smaller than that of gaussian surface as Λvalue becomes lower. This trend is common to all surface orientations (transverse, isotropic, longitudinal). Therefore, based on the numerical simulations, it is considered there is a possibility that rolling fatigue life can be improved by truncating the surface roughness.
The tribocharge sensor to monitor the rotational speed and the temperature of ball bearing was fabricated and we studied the basic feasibility this tribocharge sensor. The tribocharge sensor was built up by the segmented electrode on rotor, the segmented electrode on stator. Electrode material is Cu of 35µm thick and the material of base film is polyimide of 25µm thick. This sensor was attached on the side on the ball bearing. This tribocharge sensor outputted the pulse voltage from the stator electrode. The output voltage was proportional to the rotational speed of ball bearing. The generated electric power was 0.2µW at 2000 rpm, however, the electric power of 20µW was expected by the reduction of PFTE film thickness. On the other hand, the temperature of ball bearing was estimated by the envelope of negative voltage from the tribocharge sensor, and the estimated temperature approximately agreed with the temperature estimated by the friction force.