(a) Thermal stresses are induced in components like cylinder heads, liners, and pistons due to temperature gradients, where one side of the component is exposed to intense heat while the other remains cooler. This temperature difference results in differential expansion and contraction within the material.

• The hot side (exposed to combustion heat) tries to expand but is restricted, causing compressive stress.
• The cold side (cooled by water or oil) develops tensile stress to balance the compressive stress on the hot side.
• When tensile stresses from thermal gradients combine with tensile stresses from cylinder pressure, it increases the overall stress on the component, leading to fatigue cracks that can grow over time.

Thermal stressing can lead to component failure, especially in the form of cracks and wear in the cylinder heads, liners, and pistons. It can further cause reduced engine efficiency, component overheating, and mechanical breakdown.

Causes of Thermal Stress:

• Cooling water failure causes components to overheat due to insufficient heat removal.
• Low temperature of cooling medium leads to higher temperature gradients and increased thermal stress.
• Low temperature of charge air reduces component temperature, increasing the gradient with the hot combustion chamber.
• Failure of lubrication or insufficient lubrication raises surface temperatures, increasing wear and thermal stress.

Hoop stress in the cylinder liner is represented as: (σ = PD / 2t)

where P = gas pressure, D = liner diameter, and t = liner thickness.

• With an increase in cylinder bore (liner diameter), the hoop stress increases unless the liner thickness is also increased.
• A thicker liner can handle the added hoop stress but introduces a greater temperature gradient across the liner wall, leading to higher thermal stress.
• A thicker liner also elevates the surface temperature, reducing material strength and leading to oil film burning. This results in more wear and elevated thermal stressing, particularly in large cylinder bores.

• Modern engines have low cooling in cylinder liner and even in some cylinder heads to bring the cooling water as close as possible to heat surface to reduce thermal stress.
• Engines should be warmed up gradually before starting to minimize thermal stress during operation.
• Proper treatment, such as nitrite treatment, helps prevent scale and corrosion, maintaining efficient cooling performance.
• Lubricating and piston cooling oil temperatures should be adequately maintained.
• Ensuring complete combustion prevents excessive deposits on pistons
• Cleaning the liner and piston cooling spaces when the liner is withdrawn improves heat transfer, which reduces thermal stress on these components.