The contact area between the cylinder bore and the piston and its rings is of critical importance. The material and surface treatments on these components, as well as their design, affect the lubrication, friction and wear - and, as a consequence, have a direct effect on the output of the engine, not only in its absolute level but also in terms of its consistency over time. Moreover, reliability can be compromised if the wrong materials and surface treatments are chosen.
Many years ago, engines were usually equipped with cast-iron bores, and aluminium cylinder blocks were likewise equipped with cast-iron liners, either of the dry or wet type (referring to whether the outside of the liner is in contact with the water in the cooling system). Nitride-hardened steel or iron liners were used for a while but, more recently, and especially with the widespread use of the linerless aluminium cylinder block in series production roadcars, it has become common to use coated bores. Liners can thus be produced in aluminium, offering a weight saving for race engines still fitted with liners.
There are a number of suitable candidate coatings here, some of which are commonly used in race engines.
The most traditional of these, though by no means the most popular, is 'hard' chromium. As the name suggests, hard chrome is a chrome-plating process which has a very hard surface, in the region of 65-70 Rockwell 'C'. The process is electrolytic, allowing reasonably thick deposits to be applied, certainly much thicker than is common with decorative chrome-plating processes. After plating, the surface is honed to produce the correct surface finish.
There are a number of proprietary variations on the basic chrome-plating process, with some coatings specifically developed for bores to retain oil. These have been in common use for more than 40 years, and were among the first to allow aluminium liners to be used in place of cast iron or steel.
For race purposes, the most common family of coatings are 'composites' based on nickel, with hard particles of ceramic (generally silicon carbide) embedded in the metallic matrix. These were also introduced more than 40 years ago, originally in response to the poor wear characteristics of rotary engines which were more popular at the time for automotive use. The honed surface finish leaves hard particles of ceramic at the surface that help with wear resistance.
Although these processes are electrolytic they can, with appropriate measures, be applied to linerless cylinder blocks for race purposes.
The alternative is to use a sprayed coatings, and in 2008 these were reported in RET magazine (Issue 31) as being used at the time in both Formula One and sportscar racing. Again based on a composite metal coating with ceramic particles in a metal matrix, this process is thought to offer more opportunity to tailor the coating to the exact application.
Fig. 1 - The Bugatti Veyron has plasma-sprayed cylinder bores (Courtesy of Sulzer Metco)
Written by Wayne Ward