Viscosity Affects Piston Rings

[Jimbo]

quote:

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Originally posted by Terry:
Vis in a engine is like shoes on the feet, too tight and it rubs you sore, too loose and you slop around all over the place.

Both hurt a racing engine really bad...

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Exactly. My motorcyle racing buddy has found maximum power in his Honda engines running SAE 40. That goes against the "thinner oil = less drag = more hp" mantra. His theory is that in Honda motorcyle engines, thick oil is necessary for maximum ring seal to create maximum compression and therefore maximum power.

My own experiences tearing down gasoline engines is that stuck rings due to coking, not wear, often led to loss of compression. This was mostly back in the 1970's, when oxidation of multigrade oils was a severe problem.

 

[1sttruck]

http://www.vtt.fi/inf/pdf/tiedotteet/2002/T2178.pdf

Piston ring tribology

A literature survey

Works by several authors, using test rigs and motored engines, show that a thicker oil film can be reached by increasing the engine speed or the oil viscosity, or by the decrease of the load (cylinder pressure) or temperature. However, results that do not completely follow the trends expected from the theory have been published. The degree of the influence of these factors is different and their interaction in a firing engine has an important impact on the lubricating oil film thickness. (Richardson and Borman, 1992, Dearlove and Cheng, 1995, Shenghua et al., 1996, Harigaya et al., 2000).

...In the conclusion of the study the author states among other things that:
• The oil film thickness increases almost linearly with speed during the compression stroke.
• The idle running conditions increase the oil film thickness for the top ring during the expansion stroke.
• The trends of a computer modelled top ring oil film thickness and the measurements showed good agreement when the speed and load changed, but the calculated values were about 3 to 6 times thicker than the measured values over the speed and load

 

[1sttruck]

http://www.erc.wisc.edu/publications/thesis/thesis/thesis_Dembroski.html

Dembroski, Terry Joseph - MS
Piston Heat Transfer in An air-cooled Engine

Thermal energy flow within the piston of an air-cooled engine was investigated by measuring the heat flux in the engine cylinder liner in the region of travel of the piston rings. Engine sump oil temperature and oil viscosity were varied to change the piston ring heat flow by altering the oil film between the cylinder wall and piston ring. Engine load was varied to change heat flow into the piston. Also, the crankcase oil splash cooling of the piston was varied to change heat flow leaving the piston.

When sump oil temperature was reduced, more heat flowed to the cooler sump. Change in oil viscosity had little effect on the cylinder wall heat flow but increased cylinder wall temperature due to an increase in frictional heating with lower oil viscosity. It was thus determined that this change in heat flow was due to the change in sump oil temperature and was not due to a change in oil film characteristics.

A decrease in engine load decreased the average temperature and heat flux in the cylinder wall. The largest decrease in heat flux was on the exhaust side of the engine where the largest fraction of cylinder heat is convected to the external airflow. It was found that the change in heat flux caused by the decrease in engine load was likely due to a change in the piston ring pack heat flow.

For changes in the crankcase oil splash cooling of the piston, there were only small changes in average temperature and no changes in the heat flux. Although the piston was still believed to lose heat to the crankcase oil splash, its percentage of piston heat loss was too small to affect piston ring pack heat flow.

Superposition was evaluated for sump oil temperature and engine load as well as sump oil temperature and oil viscosity. In both cases, significant error was found with the linear superposition assumption. Linear superposition was assumed not to hold for these complex heat flows.

 

[1sttruck]

An old NASA article, supports 'thinner = cooler':

http://naca.larc.nasa.gov/digidoc/report/tr/98/NACA-TR-698.PDF

OIL VISCOSITY AND TEMPERATURE
The effect of oil viscosity on piston temperatures is obtained from a comparison of figures 11 and 9 (u), Curve 1 of figure 9 (a), corrected to the cylinder-head temperatures of figure 11, is included in figure 11 for convenience. The results shown in figure 11 were obtained under the same test conditions as those of figure 9 (a) except that a lighter lubricating oil was used, The change from S. A. E. 60 oil, which was used for all of the other tests, to S. A. E. 30 oil (fig. 11) resulted in a decrease in piston temperature of about 20° F, correction being made for the higher average cylinder-head temperature of figure 11. This decrease in piston temperatures with decreased oil viscosity may be due to one or more of the following factors: (1) Improvement in the heat transfer between the piston and the cylinder walls; (2) decrease in friction heating; (3) increased oil flow and consequently greater oil cooling of the piston; and (4) unobserved changes in engine conditions. The variation of piston temperatures with oil (out) temperature for constant engine conditions and cylinder temperatures is shown in figure 12. The decrease in piston temperatures with initial increase in oil temperature is probably due to the viscosity effects previously indicated. The leveling off and the subsequent increase of piston temperatures with further increase in oil temperature may be the result of the counteraction of decreased oil cooling of the piston with increased oil temperature.

 

[1sttruck]

Considering the NASA article, using thinner oil to get cooler oil temps is obviously not the sole criteria for choosing an oil. The article appears to be pretty old, straight 60W seemed to be the standard oil for piston engines in aircraft, and all these years later, after trying and largely rejecting synthetics and now having multi-viscosity oils available, single grade 50W and 60W oils seem to be enjoying a comeback.

One article on oil consumption suggested that if your Lycoming/Continental (?) engine was using only a quart every 30 to 40 hrs that it was on the verge of not using enough oil. The thought seemed to be that the rings and cylinders depend on oil film, so oil consumption is good for the engine at the right levels.

Maybe two rules of thumb for choosing an oil are that it needs to be thick enough to provide enough oil pressure for the temperature and driving conditions, and also needs to be thin enough to keep the rings and cylinders lubed, indicated by some reasonable level of oil consumption.

 

[MolaKule]

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Change in oil viscosity had little effect on the cylinder wall heat flow but increased cylinder wall temperature due to an increase in frictional heating with lower oil viscosity. It was thus determined that this change in heat flow was due to the change in sump oil temperature and was not due to a change in oil film characteristics.

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Huh? Compare last sentence (conclusion) with previous sentence (declaration or hypothesis).

 

[1sttruck]

I think that they're making distinctions between heat and heat flow.

 

[MolaKule]

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I think that they're making distinctions between heat and heat flow.

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You can't separate the two.

It is obvious that a change in sump temp changes the rate of heat flow.

The part that doesn't make sense is this:

quote:

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but increased cylinder wall temperature due to an increase in frictional heating with lower oil viscosity....and was not due to a change in oil film characteristics.

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An Oil film is going to reduce (thin) with increased temperature unless you have one heck of a Non-Newtonian fluid; normally lower oil viscosity results in lower cylinder wall temps unless the oil film has been heated externally and thinned to the point that it's viscosity is allowing asperity contact as in boundary verses hydrodynamic lubrication.

In the least, this sentence needs clarification.

[ May 23, 2005, 04:48 PM: Message edited by: MolaKule ]