Improving the Reliability of Piston for Increased Power Density by Changing the Shape of Piston Bowl for a Medium Speed Heavy Duty Diesel Engine

Global development trend in diesel engine is to extract more power from the same engine thereby increasing the brake mean effective pressure (BMEP). With increase in the engine BMEP, maintaining reliability of the pistons and piston ring set becomes challenging as mechanical and thermal loading increases simultaneously. Reliability can be maintained by changing the material to higher grade and/or applying different coatings; however this involves significant cost and development time. It is always preferred to keep the same material and improve the reliability of parts. In this work the BMEP of a heavy duty medium speed diesel engine is increased by 10% (from 22.8 bar BMEP to 25 bar BMEP) without change in the piston or piston ring set material. This is achieved by studying the effect of existing piston bowl shape and then changing the shape to improve the reliability of piston and piston ring set. A systematic 6 step methodology is followed. These steps are Theoretical study, CFD analysis of the in-cylinder combustion and heat transfer, Mechanical FEA analysis of the piston to establish factor of safety (FOS), long duration endurance testing, piston-piston ring set component merit rating after endurance testing and comparison with old piston and piston ring set merit rating. After completion of the above 6 steps, it is revealed that the long duration endurance test run parts of 25 bar BMEP engine are better as compared to 22.8 bar BMEP engine. New bowl shape designed with new methodology could strike balance between combustion and reliability performance. This paper describes this new methodology in detail.