Instantaneous speed signal included much working status information of diesel, and it has been used in severe misfire fault diagnosis successfully, but there is still lack of further research on early weak fault diagnosis. In this paper, several early faults are simulated in 6-135 type diesel through adjust cylinder fuel eject quantity, adjust exhaust valve clearance, simulate exhaust valve leak in differ degree. Experiment data is acquired at different running status, and instantaneous speed signal of diesel is calculated. Through analyzing the change rule of instantaneous speed signal in different status, it is found that the diesel instantaneous speed signal could be used in several early fault diagnosis, and the instantaneous speed unitary rise amplitude ratio P of every cylinder is a direct character parameter. At normal status, P value of every cylinder are in the range of 0.9~1.1. If the P value of certain cylinder descend over 10%, then it could be think as a fault cylinder. The method is sensitive to some fault which could influence gas press in cylinder such as fuel eject quantity not uniformity and valve leaking, and it is not sensitive to some fault which couldn’t influence gas press in cylinder such as cylinder liner wear.%瞬时转速蕴含了大量柴油机运行状态信息，已经成功用于失火严重故障的诊断，但对于早期弱故障的诊断尚缺乏进一步的研究。针对6-135型柴油机，通过在不同程度上调整气缸喷油量、调整气阀间隙、模拟气阀泄漏等方法模拟柴油机早期故障，在不同工况下进行实验数据采集，计算柴油机瞬时转速信号，并分析其不同状态下的变化规律。发现柴油机瞬时转速信号可以用于判断多种早期故障，各缸瞬时转速升程归一化比值P是较直接的特征参数。正常状态下，各缸瞬时转速P值在0.9～1.1以内，若某缸升程P值下降超过10%，可认为该气缸存在故障。该方法对于影响缸内压力的故障如各气缸喷油量不均匀、气阀泄漏等比较敏感，对于活塞-缸套间隙过大等对缸内压力影响程度小的故障不敏感。
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