MAN Copenhagen Reduces Residual Stresses in Diesel Engine Components

For MAN Diesel and Turbo (MDT), Copenhagen, understanding and controlling residual stress levels in their engine components is a crucial factor for an economical production. To successfully handle this task, MDT employs a sophisticated, effective and professional quality management system, ensuring part and component quality. An insight into the engineering, production and quality management of their diesel engines is presented in the following article.

MAN Diesel Technology (MDT) engines have been renowned for their quality for decades. In 2014, around 48 000 ships transported more than half of the world’s commercial goods across the oceans propelled by MDT two- and four-stroke diesel engines, with a power output of up to 87 MW. The component list of a diesel engine includes a large number and variety of castings such as liners, bearing caps and cylinder frames. Roughly 30% of the mass of a two-stroke engine is made of cast components, and on the smaller bore four-stroke engines, the entire frame structure is cast in one piece. Materials range from steel to CGI, providing the required material properties such as sufficient strength and good heat transfer. Many of the cast parts are classified as a “controlled component”. This marks them as being known to carry an important function and to present manufacturing challenges, often with regard to castability and durability. To ensure longevity, MDT is always looking to minimize residual stresses in the component. In some cases and under certain conditions, residual stresses can sum up to problematic amounts and result in a shortened engine lifetime. This is a problem that MDT quality management is eager to avoid before it occurs, as the fallout is very hard to deal with.

An example for this are the cylinder frames for the two-strike-engines. These frames are sand cast in GJL-250 gray iron or GJS-500 ductile iron and weigh up to 40 tons. The functionality dictates the geometry and requires changing wall-thicknesses, sometimes leading to crucial stress situations. To identify the endangered areas and validate design and process changes, MDT uses MAGMASOFT®.

Cylinder frame on top of the frame box, supporting the cylinder liner

In order to ensure that the residual stress results from MAGMASOFT® give the correct picture of the stress situation, MDT initially performed residual stress measurements on actual cylinder frames using the ring core measurement method. This approach is well suited for materials with coarse structures, such as cast iron. In this context, MDT and MAGMA worked together to verify new capabilities of a visco-elasto-plastic material model for MAGMAstress. This was a key to success, as MDT found that the stress levels actually observed were predictable with MAGMASOFT® when considering creep effects occurring during the slow cooling of the casting in the mold. As a result of this and further successful validation efforts, this new time-dependent plasticity model will be made publicly available in MAGMA5 Rel. 5.3.

To obtain the required material properties for certain engine types in production, design changes were investigated to make the cylinder frame easier to cast. In addition, there was also a desire to optimize the design to minimize residual stresses. Based on the good fit, MAGMASOFT® was used to identify the optimal design modifications. The main change was a significant reduction of the wall-thickness in the top of the cylinder frames, while some of the thinner sections were thickened slightly. Larger fillets were also introduced to reduce the heat impact on the cores.

Residual stress levels before (left) and after (right) design changes

The final design was the result of several MAGMASOFT® iterations. The predicted local stresses in the critical areas were reduced by 50%. Also, the new design resulted in a 1,000 kg weight savings per casting - a considerable reduction of about 10% with corresponding savings in material and energy.

As this example shows, the manufacture and assembly of MDT’s engines is a huge challenge. This is further complicated by the fact that a centralized production would neither be economically nor ecologically sound. Therefore, MDT’s parts are produced and assembled by associates and licensees near local shipyards all around the world. To globally ensure part and engine quality, MDT has set high standards for standardization, licensing and troubleshooting. All steps from pre-engineering through production to support rely on concise documentation. Strictly defined requirements must be met by every vendor or licensee worldwide. This procedure involves three key factors: analysis, communication and documentation, all reliable and accountable. Here, MAGMASOFT® is a key tool for MDT. The software allows the identification of quality inspection areas, the validation of casting process recommendations, and helps the licensees to understand where to be careful. Furthermore, MDT uses MAGMASOFT® as a standard tool to illustrate the difficulties that can be experienced during the casting process, as the software provides an excellent platform for technical discussions.

As a result of the successful application of MAGMASOFT® on a wide range of occasions since 2006, MDT has gradually integrated the software more deeply into the company’s production and business processes. With many more tasks at hand, the new functionalities of MAGMA5 Rel. 5.3 are eagerly awaited. The new capabilities for carrying out virtual experimental trials will allow the evaluation of wider sets of potentially eligible process and design variations in less time, and the assessment and interpretation of complex results will be easier than before.

Building MAN Diesel & Turbo SE

MAN Diesel & Turbo SE, based in Augsburg, Germany, is the world’s leading provider of large-bore diesel engines and turbomachinery for marine and stationary applications. It designs two-stroke and four-stroke engines with power outputs from 47 kW to 87 MW. MDT also designs and manufactures gas turbines up to 50 MW, steam turbines and compressors. The product range is rounded off by turbochargers, propellers, gas engines and chemical reactors. MDT’s range of goods includes complete marine propulsion systems, turbomachinery units for the oil & gas as well as the process industries, and complete power plant solutions. Customers receive worldwide after-sales services marketed under the MAN PrimeServ brand. Reliability, efficiency and longevity are all trademarks of MDT’s products.

 

* Text and images courtesy of MAN Diesel & Turbo SE, Denmark

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