piston material-Physics

| June 19, 2015

Material requirements for a car engine piston

Piston is an important part of a car engine as it converts the heat energy into mechanical energy (Nunney, 2006). In order to accomplish its function, the piston reciprocates within the combustion chamber through which the power is transmitted into the crankshaft for motion to occur (Nunney, 2006). Material consideration in manufacture of the pistons is therefore important for effective performance of the engine (Modelenginews, 2005).

The material for construction of piston should be light in weight. Since the piston continuously reciprocates within the cylinder bore (Modelenginews, 2005), it must be light to reduce the inertia forces. Inertia forces tend to oppose the change in position (Philip, 2005) and therefore it would interfere with the operation of the engine. High inertia forces would mean more power consumption for the car since a considerable amount of work is expended for the motion of the individual weight of the piston (Philip, 2005). Therefore light material ensures efficient fuel consumption.

The material should have high resistance to wear (Loser, 2007). As illustrated earlier, piston is exposed to high frictional forces against the walls of the cylinder bore during operation of the engine (Loser, 2007). According to Philip (2005), this amounts to a considerable amount of material loss. Indeed piston replacement is done after sometimes due to wear. Material used for its manufacture therefore should have high wear characteristics to reduce replacement rates and hence low cost of maintenance of the car.

According to Nunney (2006), piston material should be able to withstand high heat conditions. The combustion of fuel and gases takes place in the combustion chamber which is just above the top of the piston and it forms some parts of the engine piston (Loser, 2007).  It therefore implies that the cylinder bore operates at very high temperatures. The piston material should have low expansion at high temperatures in order to ensure that the cylinder bore and the size of the piston are not compromised. High expansion properties of the material would result into increase in the size of the piston against the bore and hence interference with the operation.

Hardness is another important property of the piston. Material used for manufacture should guarantee sufficient hardness. The hardness should be independent of the temperature variations inside the cylinder bore. This property reduces chances of the piston breakage during reciprocation.

Fatigue strength of the piston should be high in order to withstand the cyclic loading nature of the car. The material selected for manufacture of the piston should therefore have high fatigue strength; able to withstand cyclic loading (Modelenginews, 2005).

Toughness is an important property for operation of the pistons. It is the ability of the material to absorb shock energy due to the operational nature of the pistons. As a result a tough material should be used for piston manufacture.

Finally, for the purpose of manufacture, the material for piston construction should be easy to machine with the available machines. This ensures low cost of piston production and hence overall reduction in the cost of the car (Modelenginews, 2005).

Suitable materials for piston manufacture

Cast Iron

Cast iron is easy to machine with the common workshop machines such as lathe, honing machine and grinders (Modelenginews, 2005). Gray cast iron is hard enough to withstand the high operative combustion chamber temperatures. The production of fine finish for cast iron is the biggest challenge, however a variety of methods of honing have proved successful for superior finish. Cast iron has good wear properties in high temperatures and therefore applicable for piston manufacture.

Steel

This is another material which has been used in manufacture of engine pistons. Steel is an elastic material whose behavior is governed by Hooke’s law (Satish & Santha, 2010). Its hardness is low compared to cast iron, however the elastic behavior boosts its application in piston manufacture; it does not fail abruptly. It is easy to machine and manufacture. It has high fatigue strength and good corrosion properties at high temperatures. Generally, steel is stable at high temperatures.

Aluminum-silicon alloy

15cc 4 stroke engine series is constructed with these materials. Aluminum material is the second lightest metal after magnesium. Silicon increases the wear strength properties and the overall strength of the piston (Modelenginews, 2005). Silicon reduces the expansion rates of the material during the piston operation. This material offers the ideal properties of piston and is the most applicable for piston manufacture.

Differences between pistons produced by casting and metal working

Forging is the most common metal working which has been used in manufacture of pistons. According to Magnesium.com (2011) casting results into light- weight pistons which is the ideal property for pistons whereas due to its nature, forging (metal working) results into relatively heavier pistons which may lead to inefficient fuel consumption (Magnesium.com, 2011).

Metal working methods such as forging result into more ductile products. This implies that the piston can give ‘a warning’ before failure (Magnesium.com, 2011). This is an important property for piston operation as the signs of failure can be diagnosed easily before causing an expensive damage to the engine. Casting on the other hand results into a brittle piston which fails abruptly without a warning.

Thermal properties of forged pistons are greatly affected by temperature changes (Magnesium.com, 2011). For instance the linear expansion fro a typical forged material increases by more than 3×106 per degree Celsius within a temperature rise of 150°C, whereas cast pistons are generally stable and temperature effect to properties of these pistons are insignificant. As a result forged pistons expand while in operation and its chances of failure are therefore high.

Cast materials are harder than metal worked especially cold working (Magnesium.com, 2011). Pistons manufactured from forged materials therefore are likely to undergo indentation on their surfaces more than the cast pistons. Casting improves the surface strength (hardness) and hence increases resistance to deformation.

Heat treatment processes for pistons

Annealing process is carried out to a piston material in order to improve its ductility (Nabertherm, 2009). In this process, heat is raised so that atom diffusion in the microstructure of the material occurs. Diffusion redistributes and causes destruction to the metal dislocations. The result is a more easily deformable material hence increased ductility.

Tempering is another heat treatment process mostly carried out on steel pistons in order to make them tougher (Nabertherm, 2009). This process aims at microstructure transformation of the steel by conversion of the bainite (brittle material) into cementite and ferrite microstructure. This heat treatment process increases the energy absorption properties of the piston and therefore improves its fatigue strength.

Quenching is another possible heat treatment process in which the piston is cooled rapidly to achieve certain properties of the material (Nabertherm, 2009). In this case, the piston is cooled at the eutectoid point of the material in order to destabilize austenite in the material microstructure.  Quenching increases hardness and surface resistance to wear of the pistons.

References

Loser, D. (2007). Thermo process. Thermproscee-symbosium: ALD vacuum Technologies             GmbH

Magnesium.com.(2011).  Properties of Forged Pistons made of vmd10 Commercial Magnesium Alloy, Retrieved on February 26, 2011 from          http://www.magnesium.com/w3/data-bank/article.php?mgw=120&magnesium=346

Modelenginews. (2005). Cylinder/Piston Material Selection for Model Engines. January 29,           2005. Retrieved on February 26, 2011 from          http://modelenginenews.org/techniques/materials1.html

Nabertherm (2009). Heat Treatment Annealing, Hardening, Brazing, Forging, Nitriding                . Retrieved on February 26, 2011 from http://www.abellcombustion.com/Foundry.pdf

Nunney, A. (2006). The Reciprocating Piston Petrol Engine. Retrieved on February 26, 2011        from             http://v5.books.elsevier.com/bookscat/samples/9780750680370/9780750680370.PDF

Philip, M. (2005). 2006-2011 World Outlook for Manufacturing & Rebuilding Gasoline Motor Vehicle Engines & Engine Parts. Manufacturing & Rebuilding, 1-200.

Satish, K & Santha, K. (2010). Mechanical Properties of Steel. Design of Steel Structures,

 

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