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The piston-ring-liner system is an important source of oil consumption in modern diesel engines. One of the important factors affecting oil consumption from the piston-ring-liner system is the distortion of the cylinder bore. To the naked eye, a cylinder bore may look perfectly round but when examined more closely at a µm level, it can be highly distorted. Combustion pressure and temperature gradients during engine operation can lead to further distortions. In fact, bore distortion in internal combustion engines is normal and difficult to prevent.
Figure 1 illustrates the result of measurements of a distorted cylinder bore. The radial scale has been greatly distorted to illustrate the details.
Figure 1. Three-Dimensional View of Cylinder Bore Distortion
(Source: C-K Engineering)
Bore deformity affects the conformability or ability of piston rings to maintain contact with the cylinder liner. On a macro scale, bore deformity can be described by a Fourier series, Figure 2 :
(1)R(φ) = ∑ (Ai cos(φ) + Bi sin(φ))
R(φ) - radial coordinate,
φ - angular coordinate,
Ai, Bi - amplitude constants,
i - order, with the summation from i=0 to i=n,
n - highest order distortion to be considered.
Figure 2. Coordinate System and Fourier Series Bore Distortion Orders
Bore distortion is caused by a number of factors including machining tolerance (zero and 1st order distortions), forces generated from tightening head bolts (distortion order = number of head bolts), variations in liner cooling resulting in differences in thermal expansion, distortions caused by gas pressure—especially with thinner cylinder walls. Clamping of the liner between the cylinder head and the engine block can also cause outward deformation of the liner. The maximum distortion usually occurs near the upper edge of cylinder .
One way to reduce bore distortion is to carry out the final machining of the bore with a deck plate that can reproduce some of the distortions that occur from the cylinder head bolts. In one case, this was estimated to reduce in oil consumption by about 20% .
Other ways to reduce bore distortion include cylinder liner material selection and engine design details that reduce distorting forces. For example, in 2012, Federal Mogul announced a dual-material cylinder liner technology (Hybrid Liner) to reduce bore distortion in highly-loaded gasoline engines with an aluminum engine block. Conventionally, an aluminum engine block with a cast-in liner would not maintain the contact necessary to keep cylinder liner distortions to a minimum. However, the bonding between the sleeve and block surfaces can be improved by spray coating the outside of a cast iron sleeve with an AlSi12 alloy having a melting point below that of the aluminium engine block. This can not only reduce bore distortion and oil consumption but considerably improve heat transfer through the liner .