Dr. Lee:
I was wondering if you could tell us why the sulphur (I think - whatever it is that makes our oil get black) is
1) bad for motor oil &
2) good for the injection pump.
Thanks, Brian
* * * * * * * *
Hi Brian,
Like most good questions, the answer is not easy.
First, sulfur can appear in many different forms. As hydrogen sulfide (found in various amounts in crude oil) it is very corrosive, and also poisonous. During refining this and other sulfur compounds are mostly removed or else converted to less harmful compounds of sulfur, carbon, hydrogen, and a few other elements.
Refined oils then get separated, hydrocracked, catalytically reformed, purified, etc. and we get them as diesel fuel, and as lube oil.
Taking the sulfur out of fuel is expensive. But the sulfur in the fuel will create sulfuric acid when it burns, and some of that gets into our lube oil via blowby, and tries to dissolve bearings, gears, crankshafts, etc. Sulfuric acid also poisons catalytic converters, which are on some diesels, and coming on future diesel engines. So the guv'mint has said "Take the sulfur out of the fuel". In taking out the sulfur, other oil components are either removed or broken down, and those other non-sulfur oil components were very good at lubricating injection pumps and injectors. Without those other compounds that were also inadvertently removed, wear and degradation of fuel system components increases.
So we are encouraged to use fuel system additives to restore the lubricity that was lost as a BYPRODUCT of lowering the sulfur in the fuel. Over the years, some have confused the correlation of low sulfur and low lubricity with a cause-and-effect relationship. Nope, the lack of sulfur does not CAUSE low lubricity, it just occurs at the same time. We can restore lubricity without adding sulfur. If the refiners would blend in the correct additives, we would not have to be buying Stanadyne Blue, Power Service, Plus8 and similar additives.
In lube oil, things are somewhat different. About 50 years ago lubrication chemists and lubrication engineers (they are not the same thing, and they will let you know it) discovered zinc dithiophosphate. The "thio" part means it contains sulfur. ZDP for short (unless you work at Lubrizol, then it is ZnDP) is sort of the miracle drug for motor oil. It is a very effective anti-oxidant, so it keeps motor oil from breaking down and varnishing up the engine at high operating temperatures. It is also a very excellent anti-wear agent, particularly in rubbing friction like camshafts, tappets, and piston rings and valve stems. Many gasoline engines (Chevy 350, Ford 4 cylinder) just will wear out in months without some ZDP in the lube oil.
But ZDP in the lube oil ends up in the combustion chamber, burns, and puts both sulfur and phosphorus into the exhaust. Both sulfur and phosphorus are poisons to the catalytic converters on gas engines, so the guv'mint is pressuring the lube oil industry to cut back on ZDP in lube oil, which they have.
The oils we usually use, CG-4, CH-4, and CI-4 have less ZDP with each generation of specifications. That is because these are "universal" oils - useable in gasoline engines as well as our diesels, and to keep the SJ, SL etc. ratings, they have to cut way back on sulfur and phosphorus.
So you see Brian, sulfur in the form of ZDP is good in lube oil, but sulfur in fuel forming sulfuric acid is bad for everything.
You can restore some of the ZDP to your oil by adding genuine STP to your lube oil. STP is a mix of viscosity index improvers (VI or thickeners) and ZDP. We don't need or want the viscosity index improvers in our oil (they mess up pistons and rings) so I use either STP new car formula, or STP 4-cylinder formula. These have all the ZDP, but less VI so they are good for diesel oil.
There will be a quiz tomorrow.
Dr. Lee :cool:
[ 07-01-2004, 04:26 AM: Message edited by: CleviteKid ]
