706 Diesel engine system design
© Woodhead Publishing Limited, 2011
Surface topography plays a dominant role in mixed lubrication. Both
surface roughness pattern (oriented in the transverse, isotropic, or longitudinal
direction) and roughness magnitude have signicant inuence on piston
ring oil lm thickness, friction and wear. A rougher surface increases the
proportion of boundary friction, in the mixed lubrication. The piston ring
surface topography effect in the mixed lubrication was modeled by Sui and
Ariga (1993) and Michail and Barber (1995). Sui and Ariga (1993) concluded
that the friction of the oil ring and the second compression ring is the most
sensitive to surface roughness variations, while the top compression ring is
less affected by surface roughness. Tian et al. (1996b) studied the effect of
surface roughness on oil transport in the top liner region. Arcoumanis et al.
(1997) developed mixed lubrication models for Newtonian and non-Newtonian
shear thinning uids on rough surfaces. Gulwadi (2000) introduced a model
to calculate the ring–liner wear.
The forces and moments due to gas pressures, axial inertia, hydrodynamic
normal and shear forces and the reaction and friction forces at the ring–groove
pivot positions cause the ring to move axially and twist in the groove. Ruddy
et al. (1979), Keribar et al. (1991), Tian et al. (1996a, 1997, 1998), and
Gulwadi (2000) extended the axisymmetrical one-dimensional Reynolds
equation lubrication analysis by including the ring dynamics of the radial,
axial, and twist motions within the groove so that blow-by, oil consumption
and ring–groove wear can be analyzed in addition to a more accurate prediction
of ring friction. Tian et al. (1996a) also introduced a lubrication and asperity
contact model for the oil lm pressure between the ring and its groove.
Piston ring hydrodynamic lubrication and friction are signicantly affected
by the dynamic twist of the ring and the inter-ring gas pressure loading that
is inuenced by the ring axial motion. The inuences of particles on the
tribological performance of piston ring packs were numerically studied by
Meng et al. (2007b, 2010). Piston ring dynamics modeling can be conducted
with commercial software packages such as Ricardo’s RINGPAK (Keribar
et al., 1991; Gulwadi, 2000) and AVL’s EXCITE Piston&Rings and GLIDE
(Herbst and Priebsch, 2000).
The lateral dynamic friction force between the piston ring and the ring groove
is believed to be signicant, and this partly contributes to a circumferential
variation of the oil lm thickness of the ring. Non-axisymmetrical oil lm
distribution is also caused by other factors such as circumferentially non-
uniform ring elastic pressure (e.g., caused by improper design of the ring
free shape), bore distortion, the circumferential variation of the ring face
prole, dynamic ring twist, the non-uniform static twist caused by ring
groove deformation, circumferential ring gap position, and the different/
asymmetrical inter-ring gas pressures at the thrust and anti-thrust sides due
to the piston secondary motions. The modeling work by Das (1976) was
one of the earliest efforts to solve the two-dimensional Reynolds equation
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