Browsing by Author "Mshoro, Idrissa Bilali"

Now showing 1 - 1 of 1
  • No Thumbnail Available
    Item
    An improved computer aided engineering method for the design analysis of diesel engine pistons
    (University of Dar es Salaam, 1996) Mshoro, Idrissa Bilali
    Considering the high performance targets imposed on modern reciprocating Internal Combustion Engines (ICE), critical assessment of piston life has been a major problem area. Currently, transient piston temperature and strain for use in the fatigue assessment are predicted by the finite elements method. However, the Heat Transfer Coefficients (HTC) required in the computation of temperature are determined empirically, which affects the prediction accuracy during conceptual design. From the sensitivity analysis, the HTC on the piston crown and between the piston and liner were found to have the greatest influence on temperatures and are highly influenced by piston geometry, and hence required more accurate modelling. In the present study, the former was computed from in-cylinder fluid flow analysis using Finite Volume (FV) formulations. For this purpose, a parametric FV model generator was developed to optimise the mesh, and the blending factor in the convective flux discretisation. The HTC between the piston and liner was predicted from the analysis of clearances between them. With these approaches, less than 2.1 % deviations between predicted and measured temperatures were observed, as compared to 5 - 20 % when employing existing methods. The computation efficiency was also enhanced. Currently, it could take 25 man weeks to obtain accurate temperatures on a model with 3000 elements, whilst the same can be obtained in 10 - 12 man weeks (4 man days for a repeat analysis) with the improved method. As a test case, piston temperatures and stresses were predicted for a load cycle from 156 kW at 2500 rev/min to no load at 650 rov/nzin. The bowl lip experienced the highest temperature and stress gradients of 300° C/min and 73 MPn/mint, respectively. Subsequently, a piston life of 244292 hours was predicted. By employing existing methods in the computation of thermal loading, Char (1993) and Sivarajah (1993) predicted 208321 and 199027 hours, respectively. Although the design analysis method for pistons has been improved significantly, further work is still required to allow analysis of engine with more than two ports per cylinder under fired engine conditions. Interactive analysis of HTC and clearances between the piston and liner is also required since both are temperature-dependent. Due to computer memory limitations, the same was not performed in the present study