Gokhan
Thread starter
Originally Posted by TurboLuver
Is this theory only or was it proven in practice as well?
Yes, definitely both in theory or practice. There are a lot of experimental and theoretical studies on this and it's a current research topic. Ring - cylinder liner lubrication is extremely complicated. Nevertheless, rings are incredibly robust in most cars and conditions despite their extremely challenging lubrication.
It's not to say that thicker oil will definitely be worse but certainly going arbitrarily thick can cause problems. You need to keep transport (flow) and cavitation in mind when you think of ring lubrication. Rings are not flooded with oil at all times as the bearings and (1) oil starvation frequently happens and (2) some cavitation always happens.
Another study was also saying that the worst conditions are cold starts and warm idles, where the oil is very thick and/or the cylinder bore is warped because of the temperature. The old saying "Do not race a cold engine" also applies to the rings and liners.
Blow-by and tribological performance of piston ring pack during cold start and warm idle operations
https://link.springer.com/article/10.1007/s11431-016-6021-6
Abstract:
To reduce the fuel consumption of internal combustion engines, more attention has been paid to the tribological performance of the piston ring pack during the cold start and idle operations. In this research, a numerical model considering the cylinder liner deformation and the piston ring conformability is developed to predict the blow-by, lubrication, friction and wear of the piston ring pack under different operating conditions. The gas flow rate, inter-ring gas pressures, minimum oil film thickness, frictional force and wear load during cold start are calculated and compared with those during warm idle operating conditions. The results show that cylinder liner deformation and piston ring conformability together obviously affect blow-by and other tribological performance. Meanwhile, it is found that friction loss is larger during cold start than during warm idle operating conditions. However, the wear process is more severe during warm idle operation than during cold start. From this research, the blow-by and tribological performance of the piston ring pack during cold start and warm idle operations are understood more deeply.
Conclusions:
A numerical model that considers cylinder liner deformation
and piston ring conformability was developed to
predict the blow-by and tribological performance of the
piston ring pack under cold start and warm idle conditions.
The following conclusions were drawn as follows.
During the transient period of cold start, a higher engine
speed is helpful to form the oil film.
Compared with Case 2 of the cold start in flooded lubrication,
the thinner oil film of the piston ring pack in Case 1
of the cold start in starved lubrication conditions leads to a
larger frictional force and FMEP. The wear load of the piston
ring pack in Case 1 is also more severe than that in Case 2.
In Cases 1 and 2, for ring 2 the minimum oil film thickness
at the downstroke is much smaller than that at the upstroke.
The frictional force and FMEP at the downstroke
is larger than that at the upstroke.
Compared with cold idle conditions, the larger liner
deformation in the vicinity of TDC leads to the higher gas
flow rate and inter-ring gas pressures during warm idle conditions.
The frictional force is larger during cold idle conditions,
whereas the wear process of the piston ring pack is more
severe during the warm idle conditions. Because of the
uneven liner deformation and ring profile, the wear of the
piston rings during the warm idle conditions is uneven in
both the circumferential and axial directions.
Through this research, the blow-by and tribological performance
of the piston ring pack during the cold start and
warm idle operating conditions are understood more deeply.
In future research, engine tests should be performed to validate
the numerical model.
Another study:
Analysis of the Piston Ring/Liner Oil Film Development During Warm-Up for an SI-Engine
https://www.researchgate.net/public...elopment_During_Warm-Up_for_an_SI-Engine
Abstract:
A one-dimensional ring-pack lubrication model developed at MIT is applied to simulate
the oil film behavior during the warm-up period of a Kohler spark ignition engine. This is
done by making assumptions for the evolution of the oil temperatures during warm-up
and that the oil control ring during downstrokes is fully flooded. The ring-pack lubrication
model includes features such as three different lubrication regimes, i.e., pure hydrodynamic
lubrication, boundary lubrication and pure asperity contact, nonsteady wetting
of both inlet and outlet of the piston ring, capability to use all ring face profiles that can
be approximated by piece-wise polynomials, and, finally, the ability to model the rheology
of multigrade oils. Not surprisingly, the simulations show that by far the most important
parameter is the temperature dependence of the oil viscosity.
Conclusions:
This is the first attempt to describe the oil film development
during the warm-up period for SI engines. A new temperature
profile of the cylinder liner has been proposed to make the calculations
possible. Interesting findings include:
Fig. 8 Average friction mean effective pressure for the ringpack
during the warm-up
Fig. 9 Cycle averaged ring-pack friction mean effective pressure
during the warm-up
• For the piston ring lubrication, only the high shear viscosity
is of importance. Multigrade oils effectively exhibit full shear
thinning behavior for this application.
• For all temperatures between 20°C and the warm condition,
all rings change lubrication regime near TDC to the mixed lubrication.
Thereby, asperity contact occurs during the whole
warm-up phase.
• The minimum oil film thickness between the oil control ring
and cylinder liner scales fairly well with the square root of the
viscosity. The thickness between other rings in the ring-pack does
not scale with viscosity in any simple manner. While the thickness
of the oil left on liner is coupled with the minimum oil film thickness
of the compression ring, no simple scaling is found.
• The cycle averaged ring-pack FMEP increases four to five
times at cold conditions ~20°C! compared to the warm condition
~100°C! for the baseline SAE 10W30 oil. By averaging the above-mentioned
FMEP over the whole warm-up phase, the average
warm-up FMEP ~20°C!100°C! is twice the warm FMEP ~100°!.
• The oil film left on the liner is important for modeling the
absorption/desorption mechanism of fuel hydrocarbons in the oil
film. The thickness of this oil layer is predicted to be smaller than
previously calculated and to be in the order of a half to two microns,
and vary dependent on the oil temperature. Furthermore,
the oil layer left on the liner is found to have a small, but still
significant, thickness in the region not overrun by the oil control
ring, i.e., the distance between TDC of the compression ring and
TDC of the oil control ring. The thickness is on the order of
0.2-0.5 mm, depending on the oil temperature. This finding is
important for a further study of the warm-up absorption/
desorption process to estimate the contribution from this source to
the engine-out unburned hydrocarbons.
The model of the oil film behavior during the warm-up has yet
to be verified by experiments, and will be the topic of subsequent
work.
Is this theory only or was it proven in practice as well?
Yes, definitely both in theory or practice. There are a lot of experimental and theoretical studies on this and it's a current research topic. Ring - cylinder liner lubrication is extremely complicated. Nevertheless, rings are incredibly robust in most cars and conditions despite their extremely challenging lubrication.
It's not to say that thicker oil will definitely be worse but certainly going arbitrarily thick can cause problems. You need to keep transport (flow) and cavitation in mind when you think of ring lubrication. Rings are not flooded with oil at all times as the bearings and (1) oil starvation frequently happens and (2) some cavitation always happens.
Another study was also saying that the worst conditions are cold starts and warm idles, where the oil is very thick and/or the cylinder bore is warped because of the temperature. The old saying "Do not race a cold engine" also applies to the rings and liners.
Blow-by and tribological performance of piston ring pack during cold start and warm idle operations
https://link.springer.com/article/10.1007/s11431-016-6021-6
Abstract:
To reduce the fuel consumption of internal combustion engines, more attention has been paid to the tribological performance of the piston ring pack during the cold start and idle operations. In this research, a numerical model considering the cylinder liner deformation and the piston ring conformability is developed to predict the blow-by, lubrication, friction and wear of the piston ring pack under different operating conditions. The gas flow rate, inter-ring gas pressures, minimum oil film thickness, frictional force and wear load during cold start are calculated and compared with those during warm idle operating conditions. The results show that cylinder liner deformation and piston ring conformability together obviously affect blow-by and other tribological performance. Meanwhile, it is found that friction loss is larger during cold start than during warm idle operating conditions. However, the wear process is more severe during warm idle operation than during cold start. From this research, the blow-by and tribological performance of the piston ring pack during cold start and warm idle operations are understood more deeply.
Conclusions:
A numerical model that considers cylinder liner deformation
and piston ring conformability was developed to
predict the blow-by and tribological performance of the
piston ring pack under cold start and warm idle conditions.
The following conclusions were drawn as follows.
During the transient period of cold start, a higher engine
speed is helpful to form the oil film.
Compared with Case 2 of the cold start in flooded lubrication,
the thinner oil film of the piston ring pack in Case 1
of the cold start in starved lubrication conditions leads to a
larger frictional force and FMEP. The wear load of the piston
ring pack in Case 1 is also more severe than that in Case 2.
In Cases 1 and 2, for ring 2 the minimum oil film thickness
at the downstroke is much smaller than that at the upstroke.
The frictional force and FMEP at the downstroke
is larger than that at the upstroke.
Compared with cold idle conditions, the larger liner
deformation in the vicinity of TDC leads to the higher gas
flow rate and inter-ring gas pressures during warm idle conditions.
The frictional force is larger during cold idle conditions,
whereas the wear process of the piston ring pack is more
severe during the warm idle conditions. Because of the
uneven liner deformation and ring profile, the wear of the
piston rings during the warm idle conditions is uneven in
both the circumferential and axial directions.
Through this research, the blow-by and tribological performance
of the piston ring pack during the cold start and
warm idle operating conditions are understood more deeply.
In future research, engine tests should be performed to validate
the numerical model.
Another study:
Analysis of the Piston Ring/Liner Oil Film Development During Warm-Up for an SI-Engine
https://www.researchgate.net/public...elopment_During_Warm-Up_for_an_SI-Engine
Abstract:
A one-dimensional ring-pack lubrication model developed at MIT is applied to simulate
the oil film behavior during the warm-up period of a Kohler spark ignition engine. This is
done by making assumptions for the evolution of the oil temperatures during warm-up
and that the oil control ring during downstrokes is fully flooded. The ring-pack lubrication
model includes features such as three different lubrication regimes, i.e., pure hydrodynamic
lubrication, boundary lubrication and pure asperity contact, nonsteady wetting
of both inlet and outlet of the piston ring, capability to use all ring face profiles that can
be approximated by piece-wise polynomials, and, finally, the ability to model the rheology
of multigrade oils. Not surprisingly, the simulations show that by far the most important
parameter is the temperature dependence of the oil viscosity.
Conclusions:
This is the first attempt to describe the oil film development
during the warm-up period for SI engines. A new temperature
profile of the cylinder liner has been proposed to make the calculations
possible. Interesting findings include:
Fig. 8 Average friction mean effective pressure for the ringpack
during the warm-up
Fig. 9 Cycle averaged ring-pack friction mean effective pressure
during the warm-up
• For the piston ring lubrication, only the high shear viscosity
is of importance. Multigrade oils effectively exhibit full shear
thinning behavior for this application.
• For all temperatures between 20°C and the warm condition,
all rings change lubrication regime near TDC to the mixed lubrication.
Thereby, asperity contact occurs during the whole
warm-up phase.
• The minimum oil film thickness between the oil control ring
and cylinder liner scales fairly well with the square root of the
viscosity. The thickness between other rings in the ring-pack does
not scale with viscosity in any simple manner. While the thickness
of the oil left on liner is coupled with the minimum oil film thickness
of the compression ring, no simple scaling is found.
• The cycle averaged ring-pack FMEP increases four to five
times at cold conditions ~20°C! compared to the warm condition
~100°C! for the baseline SAE 10W30 oil. By averaging the above-mentioned
FMEP over the whole warm-up phase, the average
warm-up FMEP ~20°C!100°C! is twice the warm FMEP ~100°!.
• The oil film left on the liner is important for modeling the
absorption/desorption mechanism of fuel hydrocarbons in the oil
film. The thickness of this oil layer is predicted to be smaller than
previously calculated and to be in the order of a half to two microns,
and vary dependent on the oil temperature. Furthermore,
the oil layer left on the liner is found to have a small, but still
significant, thickness in the region not overrun by the oil control
ring, i.e., the distance between TDC of the compression ring and
TDC of the oil control ring. The thickness is on the order of
0.2-0.5 mm, depending on the oil temperature. This finding is
important for a further study of the warm-up absorption/
desorption process to estimate the contribution from this source to
the engine-out unburned hydrocarbons.
The model of the oil film behavior during the warm-up has yet
to be verified by experiments, and will be the topic of subsequent
work.