Short Answer:

The piston of an internal combustion engine works under very high temperature, pressure, and friction. Therefore, materials used for pistons must have high strength, low weight, good thermal conductivity, and resistance to wear and corrosion. The piston material should also expand uniformly with temperature changes and must not deform during operation.

In addition, good machinability, fatigue strength, and ability to withstand repeated heating and cooling cycles are necessary. These properties ensure long piston life, efficient sealing, and smooth performance of the engine. Materials like aluminum alloys and cast iron are generally preferred because they fulfill most of these requirements.

Detailed Explanation :

Properties required for piston materials

The piston is one of the most important parts of an internal combustion engine. It moves up and down inside the cylinder and helps convert the energy of expanding gases into useful mechanical work. During operation, the piston experiences extremely high temperatures (up to 300°C on the crown) and very high gas pressures. Hence, the material used for making the piston must possess certain essential properties to perform efficiently and to ensure durability.

Below are the main properties required for piston materials:

High Strength at Elevated Temperature:
The piston must be strong enough to withstand the high pressures generated during combustion without deformation or cracking. Since the combustion gases can raise the temperature of the piston crown up to several hundred degrees Celsius, the material should maintain its strength at these high temperatures. Materials like aluminum-silicon alloys and cast iron retain sufficient strength under these conditions.
Light Weight:
A lightweight piston reduces the reciprocating mass of the engine. This helps in achieving higher engine speeds and smoother operation. Aluminum alloys are preferred because they are much lighter than cast iron and allow faster acceleration and reduced vibration. Lightweight materials also improve the balance of the engine and reduce bearing loads.
Good Thermal Conductivity:
The piston is continuously exposed to hot combustion gases, so it must quickly transfer heat away from its surface to prevent overheating. Good thermal conductivity helps in maintaining uniform temperature across the piston, preventing local thermal stresses and avoiding seizure. Aluminum alloys have excellent heat conductivity compared to other metals, making them ideal for piston construction.
Dimensional Stability:
Pistons expand when heated, and excessive or uneven expansion can lead to engine knocking or seizure. Therefore, piston material must have a low and uniform coefficient of thermal expansion. It should maintain its shape and clearance under operating temperatures. Alloys containing silicon in aluminum reduce the expansion rate, maintaining stable dimensions during operation.
Wear Resistance:
During continuous motion, the piston rubs against the cylinder wall, causing friction and wear. Hence, piston materials should be hard enough to resist wear while maintaining a smooth surface. The use of alloying elements like silicon, copper, and nickel improves the wear resistance of aluminum pistons. In some cases, the piston skirt is coated with a thin layer of graphite or tin to further reduce friction.
Corrosion and Oxidation Resistance:
The piston is exposed to hot gases and sometimes to the products of incomplete combustion that can cause corrosion. The material must resist chemical attack, oxidation, and the effects of fuel impurities. Alloying elements like nickel and chromium enhance corrosion resistance, especially in engines that use high-sulfur fuels.
Fatigue Strength:
The piston undergoes continuous cyclic loading due to repeated combustion strokes. Therefore, it must have high fatigue strength to avoid cracking or failure over time. Aluminum alloys possess good fatigue strength when properly heat-treated, making them reliable for long-term engine operation.
Machinability:
The piston must be easy to manufacture and machine to achieve precise dimensions and smooth surface finishes. Materials like aluminum alloys have excellent machinability, allowing the production of accurate and fine-detailed pistons at lower manufacturing costs.
Low Coefficient of Friction:
A lower coefficient of friction between the piston and the cylinder wall reduces energy losses, wear, and heat generation. This helps in smoother operation and longer life of the piston and cylinder. The use of coatings or surface treatments can help achieve this property.
Ability to Withstand Thermal Shock:
Engines often experience rapid changes in temperature, especially during starting and stopping. The piston material should withstand such sudden temperature variations without cracking or deforming. A combination of good thermal conductivity and moderate expansion coefficient helps resist thermal shock effectively.
Non-seizure Property:
Piston materials should not stick or seize against the cylinder wall under high temperatures and loads. Aluminum-silicon alloys have excellent anti-seizure properties due to their smooth surface and good heat dissipation.
Cost and Availability:
In addition to mechanical and thermal properties, piston materials should be economical and easily available. This makes mass production of pistons feasible for different types of engines.

Common Materials Meeting These Properties:

Aluminum Alloys: Lightweight, good thermal conductivity, corrosion resistance, and machinability make them the most commonly used material for pistons in automotive engines.
Cast Iron: Strong and wear-resistant, used mainly in slow-speed engines where weight is less critical.
Steel Alloys: Used in high-performance or heavy-duty engines where strength and durability are crucial despite higher weight.

Conclusion:

Piston materials must combine light weight, strength, heat resistance, and wear resistance to perform effectively under harsh engine conditions. Aluminum-silicon alloys are the most suitable choice as they offer an ideal balance of these properties. Proper selection of piston material not only ensures smooth and efficient engine operation but also increases the overall lifespan and performance of the engine.