As a solution to this problem, the increased and decreased amounts of the measured values were marked with - and + respectively to represent them in percentage.
Figure 8 shows relative percentages of piston wear amounts at a front-rear spot, where the wear amount of the piston skirt part without EGR was defined as 100%. It can be seen that the skirt part showed nearly identical wear rates regardless of EGR application, but the head parts did show increase in piston diameter after the experiment, in particular in case of EGR application. It should also be considered that the reason that the wear rate of skirt part at a front-rear spot was identical regardless of EGR may have been due to the adhesion of soot and the erosion by H2SO4 on a piston surface as discussed previously.
Figure 9 presents the mean wear rates and piston outer diameters of Figs. 7 and 8. The skirt parts showed a slightly higher wear rate in case of EGR application, while the head parts showed a remarkably higher piston diameter increase with EGR application. According to previous studies(13,14), it has been found that great amounts of soot particulate were deposited on the head part close to the piston crown, and the piston crown part neighboring intake and exhaust valves. The results in the present study may be similar to those of previous works. That is, it can be considered from the results shown in this study that the head parts in case of EGR application made a relatively frequent contact with combustion gases, resulting in great amounts of soot deposit. And the erosion wear increased because H2SO4formed may lead to precipitate formed around soot particles as mentioned above. It is also noted that the abrasion wear may be produced by abrasive powder on the roughened eroded surfaces.
Fig. 10 Mean wear rate of piston ring thickness for 3 spots with intervals of 120｡?egin-ning from the ring and gap
3.3 Wear Rate of Piston Rings
The thickness of piston rings before the experiment was; 2.5 mm for the compression tings of No. 1 to No. 3; and 4.0 mm for the oil tings of No. 1 and No. 2. The width of all tings was the same, 4.15 mm. The thickness and width of each piston ring after the experiment were obtained by measuring at three measurement positions with angle intervals of 120° from the ring end gap and the average values were calculated. To compare with the measured values before and after the experiment, the obtained wear amount of the top compression ring without EGR was defined as a wear rate of 100% to convert the other measurements into relative wear percentages. Figs. 10 and 11 show thickness and width mean wear rates respectively.
As can be found in the figures, in case of 20% EGR rate, the thickness mean wear rates of top and No. 2 compression rings were much higher than the reference value of top ring without EGR, while No. 3 compression ring and oil rings of Nos. 1 and 2 showed unusually lower mean wear rates with EGR application than without EGR, although all the mean wear rates were greater than the reference value. The mean wear rates in ring width were remarkably higher in top compression ring than in any other ring, with even greater wear rate in case of EGR application. However, the compression tings of Nos. 2 and No. 3, and the oil rings of Nos.1 and No. 2 showed lower mean wear rates than the reference value regardless of EGR application. Nos. 2 and 3 compression tings, on the other hand, showed slightly decreased wear rate with EGR than that without EGR, and Nos. 1 and 2 oil rings showed higher wear rate with EGR, as compared to the oil rings thickness wear rate.
