Piston and Rings for Diesel Engines

Abstract

The piston is at the heart of an engine inside the cylinder bore, forming a moving boundary of the combustion chamber. The pistons are designed to have high strength, low weight, stability at high temperature, wear and corrosion resistance and thermal conductivity apart from easiness to cast and machine. This chapter is divided into three parts: Aluminium piston, piston rings and Cast-Iron pistons. (1) Today, the most popular material is hypereutectic aluminium satisfying the needs. The properties of aluminium are enhanced by alloying with silicon, copper, nickel, zinc, lead, titanium, phosphorus, zirconium and vanadium in optimum proportion. However, the ring groove wear in diesel is solved by a cast iron ring carrier inserted in the aluminium piston. In this chapter, the typical dimensions of the piston and their proportions with respect to the nominal diameter are given to initiate the design. The classical formulae for the sizing of pistons and pins for strength are provided. A detailed study of the piston expansion and movement, rings flutter, flow of oil and gases past the piston assembly though can be made with the help of a computer using both the time-honoured calculus and importantly by using the finite element method. The phenomena are explained using several examples. Pistons are manufactured by forging or casting process depending upon the strength, weight and cost. The casting process results in major or minor defects. Under major, air entrapment at ring zone, sludge at ring zone, eddy current rejections, ring insert variation, cooling gallery tilt, cold shut, bonding mistake or corrosion could arise. Minor defects are miss-handling, air entrapment at pin bore, in-gate broken, impression of glass fibre filter, etc. Systematic analysis of rejections due to defects can be carried out by creating a cause and effect diagram and actions to control or improve the process are taken. An example is provided. (2) Rings are proportioned similar to the pistons. They are made of cast iron mostly and when strength demands either nodular cast iron or steel is used. With higher strength materials, the additional advantage is the ability to design narrow faces to scrape oil better at the same maintain lower tangential load of the rings which helps in reducing the friction power and hence fuel consumption. The formulae useful for the layout of rings is given. (3) In some new generation engines, with the increasing cylinder pressure in the downsized engines to achieve higher power to weight ratio or in engines with high EGR, aluminium is not able to contain the creep and is replaced by forged steel. The highly loaded steel pistons are invariably gallery cooled by oil. The gallery is created by either laser welding the top crown or by friction welding. One to one replacement of aluminium to forged steel would bring in unbearable penalty of weight; however, clever use of the strength of steel as well its conductivity enabled radically new shape of pistons that have relatively lower compression height as well as weight equal to an aluminium piston. The simulation and calculation of the steel piston remains the same as that for aluminium piston but for the change in properties of the material.