Steel rings with Physical Vapor Deposition (PVD), Titanium Nitride (TiN), or ceramic coatings are being used successfully in other forms of motorsports. These rings and coatings are hypersensitive to the type of bore material and finish required. In some instances, these coatings may offer some advantages with a Nikasil-coated bore. Such combinations may yield the lowest friction yet but can be extremely difficult to achieve ring seal.
Measuring Piston Ring Tension The traditional way to measure ring tension is to load (install) the ring into the bore and use a spring scale to determine the force required to pull the ring. This is a crude but effective way of measuring ring tension, more commonly called ring "drag." Properly performed, this method requires a great deal of engine builder experience to be truly effective.
OEMs measure ring tension with a machine that loads the ring tangentially. This type of machine is costly but is extremely accurate and repeatable. A tangential ring tension gauge usually loads the ring into a dummy ring grove while a steel band is wrapped around it. Bearings allow the band to apply a true tangential load to the ring. One end of the band is attached to a load cell to measure force. The other is pulled. Load is applied until the proper ring gap is found and the force is read. Eventually, as the practice of low-tension rings continues, more engine builders will use this type of gauge. It will be the only way to effectively measure the ring tension, with ultra low-tension rings.
Cylinder Pressure Vs. Crank Angle Where The Ring Needs To Seal
Study the first illustration showing cylinder pressure history as a function of crankshaft angle. Note that peak cylinder pressure during "normal" combustion occurs just past TDC. While piston ring seal is important during the compression stroke and prior to ignition, it is especially critical to create and maintain pressure seal in upper portions of the cylinder in this post-TDC area. The higher the rpm, the longer the delay before peak pressure is developed.
The measurement of engine output often includes a comparison of negative and positive work performed on a piston. Simply stated, pre-combustion pressure ("work") is applied to a piston on its upward stroke during compression. Depending upon the crankshaft angle at which ignition occurs (prior to TDC), this amount of "negative" work will vary. In other words, the earlier the spark, the greater the negative work applied, all else being equal. In fact, again noting the illustration, the nearer the point of ignition (A) and the earlier the point of peak cylinder pressure (P1 or P2), the greater the amount of position work applied to the piston. Once a piston has passed TDC on its power stroke, positive work is applied.
At the risk of oversimplification, the difference between positive and negative work is a measure of IMEP (indicated mean effective pressure). As an aside, the faster the combustion rate (burn), the later can be the initiation of spark ... resulting in decreased negative work and increased positive work on the piston. Technically, indicated work is the difference between work done on a piston during its compression stroke and work done on it during expansion. Graphically, this relationship is often expressed in the form of P-V (pressure-volume) diagrams or so-called pressure "loops."
Based on decreasing combustion space and rising combustion temperature, controlled cylinder pressure (absent any pre-ignition or detonation) will peak past TDC, owning in part to delays from gas dynamics. Ordinarily, it is within the first 30 crankshaft degrees past TDC where ring seal is particularly critical. This leads to the necessity (particularly at this point) for pistons being square in their bores, rings that are in proper contact with cylinder walls, bore concentricity at temperature, and skirt drag minimized for friction horsepower loss reduction.
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