The ablation surface of piston appears to be pear-shaped skin and dark grey, main reasons involve:

1. Abnormal combustion arising from the causes like improper adjustment between injection advance angle and knocking control system of engine may easily result in the dimpling of piston top or the ablation of ring bank.

2. Abnormal operation of injector gives rise to knocking of engine and easily results in piston top ablation or cracking, or more seriously breakthrough.

3. Abnormal operation of injector causes inferior fuel atomization, second injection or dropping and results in serious after-burning and overheats ablation of piston.

4. Bad heat dissipation or over high combustion temperature of engine results in failure emission of heat.

5. Inferior chemical composition and metallographic structure of piston materials.