TBN in gasoline engines?

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Jun 3, 2003
BC, Canada
What function do high TBN levels in an engine oil play in gasoline fueled engines? Many posters quote using a HDMO in their cars because of the high TBN levels. I always thought that a high TBN was necessary to combat acids formed from the use of high sulphur fuels, and not so much a concern in other applications. Are people buying oils based (no pun) on the TBN and zink levels and do oil companies ut excessive amounts of both in an oil to sell the product? What roles do boundry lubricant additives play in a gasoline engine that has a low drag rollerized valve train and a 2:1 con-rod ratio that eliminates piston side loading?
Proper TBN level is needed to neutralize some products of oxidation...one of the chemists here will fill this out and correct it. Most folks know nothing about TBN and don't shop for it. What's important for folks extending drain intervals is how well TBN holds up, not necessarily how high it is to start with, and the correct combination of antioxidants contributes to this. Zinc is combined with phosphorus in zinc dialkyldithiophosphate or a related compound serving as an anti-wear agent. Phosphorus is limited to 1000 ppm in order for the oil to qualify as API-SL, so zinc is limited by fact, not rule. Boundary lubrication is necessary for start up, and in all places in the engine where a hydrodynamic film of oil is not present...wrist pins, piston skirts (2:1 rod ratio just minimizes side loading--it's still there), cam lobes, thrust faces, etc. Ken
Ken2; Good proof reading. Yes "eliminates piston side loading" is wrong. Long con-rods reduce side loading. I built a Chev 327 with 6.25" rods to reduce piston side loading and the increased wear it creates. Ken2, You mentioned the correct combination of antioxidants......is that combination the same or different for gasoline and Diesel engines. Is the use of HDMOs the best practice for gasoline fueled automobiles? Is there no harm in using HDMOs in everything? Do the HDMOs have more of the same additives as PCMOs that may or may not be used up as the miles roll by? Are additives Diesel or gasoline specific or are they generic in nature?
PCMO for gasoline engines and HDMO for heavy duty diesel engines have different additive packages. The diesel lube has a quite different job to do--handling soot is just one item. It's always a debate whether PCMO or a dual-rated 15W-40 diesel/gasoline engine lube oil is the best way to go. If I was working an gasoline engine very hard in a hot climate, I'd tend toward the 15W-40. In Kamloops...lighter stuff. Many of the additives that make up the additive package in oils are the same for diesel and gasoline engines, but may be blended in different amounts and different proportions for the different jobs. Ken
K2; Today in Kamloops (no its not a breakfast cereal), it is 34C at the airport or about 94F. The hottest day I remember was 43C or 108F, and that was also at the airport where it is nice and cool compared to in town or on the highway.
TBN depletion is still one of the primary degradation mechanisms in gas engines. Both oxidation and nitration (from NOx) lead to the formation of various types of organic acids. These acids deplete the alkaline detergent additives - calcium and magnesium - and the oil eventually has to be changed. HD diesel oils are primarily formulated to neutralize the sulphuric acid and suspend the soot that comes from burning diesel fuel.... TooSlick
So a high TBN cannot be achieved by adding tums to the oil? How well the TBN levels hold up, and not the new oil TBN value is important according to K2. This must mean that the additive levels published by oil companies are really just a guide-line and should only be used for a comparitive value only. I suspect there are expensive and/or cost effective additive packages for engine oils that differ for each application intended use and base-oil. Would a high anti-wear additive package be a band-aid to cover up a poor base oil quality? Can an high quality base oil perform as well with a lower additive level than a low quality base oil?
Actually, the form of calcium added to overbase detergents in some formulations is the same as tums (calcium carbonate/hydroxide). If you did add tums to oil it would raise the TBN. But please don't try this. [SPAZ!]
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