DESIGN OF A NEW ALLOY FOR INTERNAL COMBUSTION ENGINES …

[Pages:12]Proceedings of the 7th International Conference on Mechanics and Materials in Design Albufeira/Portugal 11-15 June 2017. Editors J.F. Silva Gomes and S.A. Meguid. Publ. INEGI/FEUP (2017)

PAPER REF: 6729

DESIGN OF A NEW ALLOY FOR INTERNAL COMBUSTION ENGINES PISTONS

Antoni Jankowski, Miroslaw Kowalski(*) Air Force Institute of Technology, Warsaw, Poland (*)Email: miroslaw.kowalski@itwl.pl

ABSTRACT

The results of the work on the development of a new composite alloy pistons designed for internal combustion engines are presented in the article. Tests of alloy crystallization, metallographic tests, tensile strength and hardness tests, coefficient of thermal expansion and tests for seizure resistance as well as the engine tests were performed. Tests of dimensional stability of the pistons in the thermal chamber was carried out and found that permanent deformations of the piston diameter do not exceed 2 ?m. The engine with new pistons showed a lower consumption of lubricating oil, lower levels of exhaust emissions and lower noise levels in comparison to the standard engine.

Keywords: combustion engines, engine pistons composite aluminum alloys, engine piston testing, material testing.

INTRODUCTION

The design and materials used for the pistons exert a significant influence on the value of the clearance of the piston-cylinder assembly. Clearances have an impact on the lubricating oil consumption, blow-up into the crankcase, exhaust gases toxicity and noise of engine operation [Tinaut et al.].

Hysteresis of pistons dimensions due to changes in temperature distribution and stress has a major impact on the value of the clearances. In the most internal combustion engines, the pistons are made of alloys based on aluminum, whereas the cylinder liners are made of cast iron, which has different expansion coefficient than the aluminum alloys expansion. Therefore, the attempts of the steel pistons application are investigated [Birch].

However, these pistons have a much larger mass, which increases the load of the crank mechanism and reduces the nominal speed of the engine. Moreover, the implementation of steel pistons requires new and expensive production methods. Therefore, the use of composite pistons of the aluminum is a better solution. It allows to increase the strength of the material at elevated temperature and to reduce the changes in the piston dimensions. Furthermore, the use of a new alloy allows the use of smaller assembling clearances between the piston and the cylinder.

The composite alloy is obtained by adding to the base material (aluminum) of the refractory elements such as chromium, molybdenum, nickel, tungsten. These elements form the intercrystalline compounds, which are located on the grain boundaries. Moreover, the fibres, such as silicon carbide are added, which strengthens the structure of the alloy. As a result, strength of the alloy and dimensional stability is increased, and the deformations of the pistons are reduced [Jankowski Jankowska, Slawinski].

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Topic-D: Composite and Advanced Materials

OPERATING CONDITIONS OF THE COMBUSTION ENGINE PISTONS

Pistons in internal combustion engines operate under permanent changes in their temperature and structural stresses distribution, due to changes in engine external loads and engine speed. The materials of the pistons must therefore have appropriate properties not only at the room temperature but also at the operating temperature of the pistons. The maximum temperature of the pistons, which are present on the piston crown, can reach 320-350oC. In the lower part of the piston, at the piston skirt, the temperature reaches a value of 100-140oC. Very important is the temperature in the region of the piston ring grooves, which is determined and limited by the properties of the lubricating oil, it should be a maximum of 230-240oC. If the temperature in this area exceeds this value, the piston must be internally cooled. Large temperature differences in the piston cause the occurrence of high thermal stresses, which superimpose to the mechanical stresses. Large differences in the piston stresses and temperature gradients cause deformation of the pistons, which should disappear after the withdrawal of stresses and temperature. The presence of pistons permanent dimensional changes, because of pistons permanent heating and cooling, known as hysteresis, is a serious problem that needs to be taken into account in the pistons design process. The value of these distortions, determine in fact the dimension of the clearance in a piston-cylinder liner assembly, which affect on piston seizure, lubricating oil consumption, exhaust gases blowby into the crankcase and harmful emissions, mainly unburned hydrocarbons. The value of the pistons hysteresis can be limited, primarily by proper selection of the pistons chemical composition and by selection of the appropriate piston manufacturing processes, including the piston heat treatment processes. The long experience in the pistons manufacturing and testing led to the selection of different aluminum alloys, which is used in the pistons mass production. Specific designs and the usage of the engines, however, require the introduction of the additional requirements that result, that it is necessary to correct the chemical composition of the piston alloys. The most commonly used piston aluminum alloys are Al-Si alloys, containing about 12% Si. They are near eutectic alloys, further more comprising a number of alloying additives.

THE SCOPE OF THE TESTING

The work aim was to replace pistons of the military applications engines, which were produced by forging a wrought aluminum alloy PA12. The pistons produced by casting a near eutectic Al-Si alloy should be characterized by similar strength properties and improved functional properties, which are primarily enabled by reducing the clearances between the piston and the cylinder.

Table 1- Chemical composition of near eutectic alloys for engine piston, at % wt

Si Cu Mg Ni Fe Ti Cr Mo W V Mn Zn Other

1

10.512.5

0.51.5

0.81.8

0.51.5

0.7

0.1

-

-

-

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