Piston rings – materials

While there is always something new going on with piston design, we tend to see less obvious development with piston rings. Visually it is pretty obvious with a piston that someone has a new concept or idea, and we can see from looking at new pistons at various trade shows that fresh design concepts appear that take advantage of new materials or different manufacturing methods. Piston ring development is very much more incremental though – evolution rather than revolution.

The material choices for piston rings have remained broadly static for many years, although the budgets available in racing allow us the latitude to use more expensive materials and coatings than can be justified for passenger cars. For many years, plain cast-iron rings were the order of the day, having been well-proven in mass production, while ductile iron also plays an important part in the commercial ring market. Steel rings were the next logical step since, for a similar mass, they are much stiffer and so allow us a greater factor of safety against running into a resonant running condition such as ring flutter.

In terms of commercial steel rings, the elastic modulus (a measure its stiffness) is in the region of 200-230 GPa, while for cast iron the range is about 90-150 GPa. Steel materials are also stronger than cast iron in fatigue. In comparing a cast iron and steel ring material of the same static strength (1300 MPa), published results* show that an 18% chromium steel has a 60% higher fatigue strength (>500 MPa) at 10 million cycles than a cast-iron ring material. Tool steels are offered commercially by some racing ring makers. These offer improved static and fatigue properties compared to commercial ring steels, but come at a premium owing to their composition and manufacturing processes.

Another important advantage of using steels, especially those able to resist softening at the moderate temperatures seen by piston rings, is that piston designers can run steel rings higher on the piston than would be possible for the same reliability with a cast-iron ring. This eliminates some crevice volume and therefore improves combustion efficiency, since less unburned fuel can remain in the crevice volume where the flame is quenched.

The higher the tempering temperature of the ring, the higher the temperature it can operate at, and the higher the ring can be run – although the limiting factor may be the piston material or the design of the piston itself, especially valve pockets. The advantages of having the piston rings positioned closer to the piston crown are more than simply reduced crevice volume, as the compression height of the piston can be reduced to give the opportunity to run a lighter piston and a longer con rod.

Both of these design options have their own benefits. The longer rod can benefit the engine in terms of reduced friction owing to lower maximum rod angularity and lower piston thrust forces. The lighter piston means lower inertia forces at top dead centre and bottom dead centre, and these can allow both a lighter-section con rod to be used and a lower-mass crankshaft. All these benefits can flow from using an improved piston ring material.

* http://www.federalmogul.com/korihandbook/en/img_23_400_279.htm

Written by Wayne Ward