Cylinder / Piston Wear Measurement
There are various wear measurement methods available for assessing changes in the tribological system in terms of wear behaviour. Depending on the method used data can be obtained on mass wear, volumetric, localized or integral wear behaviour, and wear forms. All these methods are legitimate for particular problems but differ greatly in complexity.
The most common procedures are described briefly below.
Integral Wear Measurement
With this very simple and quick method the mean radial wear on the ring running face is determined based on the change in circumferential length as a result of engine running (Fig. 33). In this wear measurement procedure the closed gap is measured before and after engine running in a bore gauge having the nominal dimension of the engine. The mean radial wear is then calculated from the measured closed gap increase. If the closed gap was not measured before the engine test it is usual to calculate the wear based on the closed gap tolerances stated in the drawing.
This method does not allow a determination of the cylinder wear.
Localized (Geometrical) Wear Measurement
In this complicated wear measurement procedure a profile trace (preferred magnification 1000/40x) of the ring running face is produced at a specific point on the ring both before and after engine running (Fig. 34). The wear is determined by comparing the profiles from both measurements. Wear is normally defined as radial abrasion of the profile. Usually 5 measurement points are selected on the ring (10° and 90° to the right and left of the gap, and opposite the gap), though a different number may be chosen depending on the nominal diameter and/or purpose of the investigation. With this method localized radial wear and wear forms as well as mean wear is determined .
Cylinder wear measurements at the top ring reversal point are performed using a similar procedure except that in this case the bore surface is only measured after stressing (Fig. 35, Fig. 36, Fig. 37). The original condition of the bore surface is inferred either from mathematical functions or manual evaluations, according to the measurement system used. The previous normal scope of measurement comprising 8 measurement points evenly distributed around the circumference has been increased for modern engines to as many as 24 measurement points per cylinder. This is necessary so that wear phenomena triggered by multi-valve technology and direct injection diesel fuel injectors can be reliably identified. The measured values are evaluated as localized wear and/or mean wear.
Wear Measurement using the Radiotracer Technique
In this procedure the components of interest are differently tagged with a radioactive tracer. During engine running the radioactively tagged material is abraded from the surface of the components. The debris enters the oil circuit where it is registered by special measuring devices. Ring and cylinder wear can be individually distinguished due to the different activation. The wear behaviour at different engine operating points can be investigated directly. However, this procedure only enables the mean mass wear of the components to be determined.
Fig. 33: Integral Wear Measurement
Fig. 34: Piston Ring Running Face Wear
Fig. 35: An Individual Cylinder Wear Measurement
Fig. 36: Cylinder Wear Measurement every 15°
Fig. 37: Topographical View of Cylinder Wear