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Tribology
Friction

Internal combustion engines generate a controllable torque within an engine speed range and deliver a brake horsepower of Pe=2πnM. The powertrain within the reciprocating engine (pistons, rings, cylinder barrels, connecting rods, crankshaft, bearings, oil) is subject to friction in operation which is a part of engine mechanical losses. The distribution of expended fuel energy over time between BHP and various engine power losses is presented in Fig. 20. The ring pack loss constitutes 20% of the engine friction loss (Fig. 21), with the oil ring accounting for about 60% of this figure.

Friction is observable as a resistance against the relative movement of contacting surfaces. Contact occurs directly (contact between solids), indirectly (lubrication) or by a combination of both in the case of mixed friction. Frictional forces are a function of material properties and physical and chemical system characteristics. Other contributory effects are the surface structures, manifested by the ring running face contour, the roughness, and the deviation from the ideal cylinder shape. These structures do not have full-face contact, so that local contact pressures diverge significantly from the specific contact pressure. In the presence of contact and mixed friction, wear mechanisms occur which are accompanied by loss of energy. Frictional heat can reach very significant levels locally and may trigger material damage which can result in the failure of tribological systems (scuffing, seizure).

The Stribeck Curve (Fig. 22, schematic) shows the coefficient of friction f and oil film thickness h as a function of the velocity, the oil viscosity and the specific contact pressure at the surfaces in relative motion.

The piston ring operating point on the Stribeck Curve is dependent on the crank angle and even at constant engine speed shifts back and forth between the functional values for zero and maximum velocity, while in the region of the dead centres it is in the mixed friction zone. Owing to the squeeze effect, remains of oil film can be retained at the dead centres at high revolutions. Maximum relative velocity is determined by the engine speed. With its strong correlation to temperature, the dynamic viscosity is dependent on the engine operating point.
Modern computer programs are capable of numerically calculating oil film thicknesses and ring frictional forces while considering surface roughness and orientation as well as partial oil fill states of the gap between the components [34, 35, 36, 37, 38]. To determine ring friction loss, the product of ring frictional force and velocity is then integrated for whole working cycles.
In the region of the dead centres the rings operate in the mixed friction zone; the frictional forces within the working cycle are at their greatest here but contribute only negligibly to the friction loss as this is where relative velocity approaches zero. Generally speaking, the friction loss of ring packs may be assumed to be roughly proportional to the engine speed. Its correlation to load is not significant. For piston rings the friction mean effective pressure is defined as pmr,ring=PR,ring/(i n Vh).

Friction reduction is concerned with optimizing the functional behaviour of ring packs in order to improve engine fuel economy. It often conflicts, however, with the optimization of oil consumption and blowby.

The main actions to reduce friction are:


Fig. 20: Percentage Distribution of the Fuel Energy (mB Hu,mixture[kW]) between BHP and Engine Power Losses, as per [32]


Fig. 21: Percentage Distribution of Power Losses between Different Components and Accessories, as per [32]


Fig. 22: Stribeck Curve [33]

 

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