Fig. 5 Wear rate and inner diameter of cylinder liner for a front-rear spot at the respective positions measured from the top
Fig. 5 shows the measured values at a front-rear spot and the wear rate of a relative percentage in the same way mentioned above. This figure indicates that the wear rate with EGR was lower than that without EGR up to the spot 90 mm away from the top, and that the wear rate due to EGR application grew higher at closer distances to the second half part.
Fig. 6 shows the mean wear rate for right-left and front-rear spots. As can be seen from the figure, the wear amount without EGR was greater than that with EGR up to the spot 60 mm away from the top. Longer distanced spots than 60 mm, however, produced higher wear amount for applying EGR. The relative wear rate for the spot measurement is also shown in Fig. 6, where the first half beginning from the cylinder top showed higher wear rate in no EGR than that with EGR application, and the wear rate with EGR was greater in the second half. In particular, No. 1 cylinder without EGR showed a similar wear rate regardless of the measuring positions, though showing slight variations. No. 2 cylinder with EGR showed a significantly higher wear rate in the second half than in the first half.
Shiozaki et al.(6) have conducted a special simulation mode operation applying an EGR rate of 20% to investigate the wear rate of cylinder liner and piston rings on an in-line, six-cylinder (13.3 l), direct injection, heavy-duty diesel engine with JIS No. 2 light oil. They have shown that approximately four times higher wear rate was observed in case of EGR. They attributed this result to erosion due to H2SO4 and abrasion by particulate such as abrasive powder and soot.
3.2 Wear Rate of Piston
The wear amounts of piston at the right-left spots of the head part 10 mm away from the piston crown and the skirt part 100 mm away from the piston crown were measured before and after the experiment. And all the measured wear amounts were converted into relative percentages by defining the piston skirt spot with-out EGR as a wear rate of 100%.
Figure 7 shows the measured values of piston outer diameters and the wear rate. As can be seen from the figure, the skirt part at a right-left spot produced somewhat higher wear rate in case of EGR application, but the head part showed increase in piston diameter regardless of EGR application, especially higher in case of EGR application. This peculiar result suggests the possibility that though the piston head suffered significant wear, the roughened piston due to soot and other materials adhered to a piston surface or erosion by H2SO4 may have contributed to apparent piston diameter increase. In this experiment, a part of the roughened piston surface was wiped off by a scraper, but complete wiping could have caused a severe damage to the piston, making exact measurements impossible. As a solution to this
