Tribology Of Conventional Piston Ring And Cylinder Liner Materials

Figure 2 shows a plot of wear rate against temperature for segments of Cr plated piston ring sliding on pieces cut from a grey iron cylinder liner with a conventional 15W40 CE/SF mineral oil based lubricant (fresh).

Wear rates are expressed as the volume of wear divided by the product of sliding distance and normal force. Wear rates for the ring were found to double for every 48K rise in temperature for the lubricant tested in Figure 2. Liner wear rates doubled every 87K.

Typical TRR temperatures for conventional heavy duty diesel engines are in the range 250 -300°C, for which ring wear rates are predicted from Figure 2 to be in the range 5 x 10-12 to 10-11 mm3/mm/N. Top ring wear rates calculated from engine test data (field and dynamometer tests) agree with these numbers.

Further information about lubricant properties may be obtained by increasing the temperature at a constant rate (e.g. 3 K/min) while monitoring the friction coefficient (see figure 3). The temperature at which scuffing starts (where the friction coefficient rises sharply) is found to be a strong function of the rate of supply of the lubricant (Figure 3), and this type of information may be used to determine decomposition kinetics for the lubricant, by plotting log (supply rate) against the reciprocal of scuffing temperature. For the lubricant tested in Figure 3, an activation energy of 74 kJ/mol was calculated for the decomposition process.

Below the scuffing temperature, the friction trace generally provides a 'signature1 of the lubricant. Figure 4 shows a pronounced and very repeatable decrease in friction at 240°C for a fresh 15W40 CE/SF mineral oil based lubricant, whereas the same oil shows no such drop when heavily oxidized and soot-loaded during an engine test. Such transitions may possibly be related to the lubricant chemistry, particularly the composition of the anti-wear additive. Note that the scuffing temperature is the same for the used oil as for the fresh sample.

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