Steel cylinder liners
On first inspection, steel seems like an unlikely material to use for a cylinder liners. Its density is almost three times that of aluminium, its thermal expansion coefficient is much lower than that of the aluminium piston that runs inside it, and it has low thermal conductivity. However, many people use it for liners, and it can offer the lowest total engine mass, despite its density. The real advantages of steel are its strength and stiffness.
Aluminium cylinder liners are very popular, as they conduct heat well and the similar coefficient of expansion between the liner and piston means the cold clearances can be closer to ideal than with a steel liner. Compared to steels though, the allowable stresses are low, so the material’s thickness increases in order to keep stress within allowable limits. That means the water jacket and the material surrounding it are larger, and the other obvious implication is that, for a given bore size, the bore centres are further apart than would be the case for steel.
The obvious effects are that the engine block is heavier, and the crankshaft is longer and heavier. Less obvious effects are that the cylinder head fasteners are forced further from the combustion chamber, increasing bending stresses. Despite the aluminium cylinder liners themselves usually being lighter than their steel counterparts, it is often the case that, for a new engine design of given bore and stroke, with aluminium liners the engine is heavier overall.
Aluminium liners, where they are used in water-cooled blocks and in direct contact with cooling water, can also suffer from cavitation damage owing to the low strength of the material, and it can be necessary to put a coating on the exterior of the liner to make the surface more resistant to cavitation. Steel liners don’t suffer from this problem, although they may require surface treatments to prevent excessive corrosion.
The thermal conductivity of a cylinder liner isn’t as bad as the comparison of the coefficients of thermal conductivity would first appear. The fact that a steel cylinder liner can be much thinner than its aluminium counterpart to match either displacement or fatigue stresses means that the rate of heat transfer can be almost as high for a steel component as for an aluminium part.
However, there is nothing about the strength of steel that can help offset the fact that the coefficient of thermal expansion is much lower than that of aluminium. Where a ‘standard’ grade of steel is used, we simply have to accept that we need a much larger cold clearance between liner and piston; there is at least one grade of steel though that has been developed specifically to match the coefficient of thermal expansion of aluminium. Although not developed for use as a cylinder liner material, it has been used for cylinder liners. The material is not widely available, nor is it easily sourced in suitable sizes for liners.
The ‘problem’ with the grade of high-expansion steel I am familiar with is that it does not respond to thermal strengthening, although it does work-harden. However, it is not easy to work-harden the large bar or tube sizes that are required for liners, and the low strength of the non-work hardened material may be too low for many applications.
Written by Wayne Ward