زيوت الموتور والديزل (كيف تحدد مواصفاتها)

Oil companies provide data on their oils most often referred to as "typical inspection data".

This is an average of the actual physical and a few common chemical properties of their oils.

This information is available to the public through their distributors or by writing or calling the company directly.

I have compiled a list of the most popular, premium oils so that a ready comparison can be made.

If your favorite oil is not on the list get the data from the distributor and use what I have as a data base.

This article is going to look at six of the most important properties of a motor oil readily available to the public: viscosity, viscosity index (VI), flash point, pour point, % sulfated ash, and % zinc.

Viscosity is the measure of how thick an oil is. This is the most important property for an engine.

An oil with too low a viscosity can shear and loose film strength at high temperatures.

An oil with too high a viscosity may not pump to the proper parts at low temperatures and the film may tear at high rpm.

The weights given on oils are arbitrary numbers assigned by the S.A.E. (Society of Automotive Engineers).

These numbers correspond to "real" viscosity, as measured by several accepted techniques.

These measurements are taken at specific temperatures. Oils that fall into a certain range are designated 5, 10, 20, 30, 40, 50 by the S.A.E.

The W means the oil meets specifications for viscosity at 0 F and is therefore suitable for Winter use.

The following chart shows the relationship of "real" viscosity to their S.A.E. assigned numbers. The relationship of gear oils to engine oils is also shown.

_______________________________________________________________

| |

| SAE Gear Viscosity Number |

| ________________________________________________________ |

| |75W |80W |85W| 90 | 140 | |

| |____|_____|___|______________|________________________| |

| |

| SAE Crank Case Viscosity Number |

| ____________________________ |

| |10| 20 | 30 | 40 | 50 | |

| |__|_____|____|_____|______| |

______________________________________________________________

2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42

viscosity cSt @ 100 degrees C

Multi-viscosity oils work like this: Polymers are added to a light base(5W, 10W, 20W), which prevent the oil from thinning as much as it warms up. At cold temperatures the polymers are coiled up and allow the oil to flow as their low numbers indicate.

As the oil warms up the polymers begin to unwind into long chains that prevent the oil from thinning as much as it normally would.

The result is that at 100 degrees C the oil has thinned only as much as the higher viscosity number indicates. Another way of looking at multi-viscosity oils is to think of a 20W-50 as a 20 weight oil that will not thin more than a 50 weight would when hot.

Multi-viscosity oils are one of the great improvements in oils, but they should be chosen wisely.

Always use a multi-grade with the narrowest span of viscosity that is appropriate for the temperatures you are going to encounter.

In the winter base your decision on the lowest temperature you will encounter, in the summer, the highest temperature you expect.

The polymers can shear and burn forming deposits that can cause ring sticking and other problems. 10W-40 and 5W-30 require a lot of polymers (synthetics excluded) to achieve that range.

This has caused problems in diesel engines, but fewer polymers are better for all engines. The wide viscosity range oils, in general, are more prone to viscosity and thermal breakdown due to the high polymer content.

It is the oil that lubricates, not the additives. Oils that can do their job with the fewest additives are the best.

Very few manufactures recommend 10W-40 any more, and some threaten to void warranties if it is used. It was not included in this article for that reason.

20W-50 is the same 30 point spread, but because it starts with a heavier base it requires less viscosity index improvers (polymers) to do the job.

Follow your manufacturer's recommendations as to which weights are appropriate for your vehicle.

Viscosity Index (VI) is an empirical number indicating the rate of change in viscosity of an oil within a given temperature range.

Higher numbers indicate a low change, lower numbers indicate a relatively large change.

The higher the number, the better. This is one major property of an oil that keeps your bearings happy.

These numbers can only be compared within a viscosity range. It is not an indication of how well the oil resists thermal breakdown.

Flash point is the temperature at which an oil gives off vapors that can be ignited with a flame held over the oil. The lower the flash point the greater tendency for the oil to suffer vaporization loss at high temperatures and to burn off on hot cylinder walls and pistons.

The flash point can be an indicator of the quality of the base stock used. The higher the flash point the better. 400 F is the minimum to prevent possible high consumption. Flash point is in degrees F. Pour point is 5 degrees F above the point at which a chilled oil shows no movement at the surface for 5 seconds when inclined.

This measurement is especially important for oils used in the winter. A borderline pumping temperature is given by some manufacturers. This is the temperature at which the oil will pump and maintain adequate oil pressure.

This was not given by a lot of the manufacturers, but seems to be about 20 degrees F above the pour point. The lower the pour point the better. Pour point is in degrees F.

% sulfated ash is how much solid material is left when the oil burns. A high ash content will tend to form more sludge and deposits in the engine.

Low ash content also seems to promote long valve life. Look for oils with a low ash content.

% zinc is the amount of zinc used as an extreme pressure, anti- wear additive.

The zinc is only used when there is actual metal to metal contact in the engine.

Hopefully the oil will do its job and this will rarely occur, but if it does, the zinc compounds react with the metal to prevent scuffing and wear.

A level of .11% is enough to protect an automobile engine for the extended oil drain interval, under normal use.

Those of you with high revving, air cooled motorcycles or turbo charged cars or bikes might want to look at the oils with the higher zinc content. More doesn't give you better protection, it gives you longer protection if the rate of metal to metal contact is abnormally high. High zinc content can lead to deposit formation and plug fouling.

The Data listed alphabetically by oil weight

All of the oils above meet current SG/CD ratings and all vehicle manufacture's warranty requirements in the proper viscosity.

All are "good enough", but those with the better numbers are icing on the cake.

The more expensive synthetics the only truly significant differences, due to their superior high temperature oxidation resistance, high film strength, very low tendency to form deposits, stable viscosity base, and low temperature flow characteristics.

Synthetics are superior lubricants compared to traditional petroleum oils. You will have to decide if their high cost is justified in your application.

The extended oil drain intervals given by the vehicle manufacturers (typically 7500 miles) and synthetic oil companies (up to 25,000 miles) are for what is called normal service. Normal service is defined as the engine at normal operating temperature, at highway speeds, and in a dust free environment. Stop and go, city driving, trips of less than 10 miles, or extreme heat or cold puts the oil change interval into the severe service category, which is 3000 miles for most vehicles. Synthetics can be run two to three times the mileage of petroleum oils with no problems.

They do not react to combustion and combustion by-products to the extent that the dead dinosaur juice does. The longer drain intervals possible help take the bite out of the higher cost of the synthetics. If your car or bike is still under warranty you will have to stick to the recommended drain intervals. These are set for petroleum oils and the manufacturers make no official allowance for the use of synthetics.

Oil additives should not be used. The oil companies have gone to great lengths to develop an additive package that meets the vehicle's requirements. Some of these additives are synergistic, that is the effect of two additives together is greater than the effect of each acting separately. If you add anything to the oil you may upset this balance and prevent the oil from performing to specification.

The numbers above are not, by any means, all there is to determining what makes a top quality oil. The exact base stock used, the type, quality, and quantity of additives used are very important. The given data combined with the manufacturer's claims, your personal experience, and the reputation of the oil among others who use it should help you make an informed choice.

Oil performs several functions:

Lubrication

Corrosion protection

Cooling

Contaminant transfer

What I learned is that there are numerous tradeoffs made in all these areas and it is useful to look at some of them.

Some of these areas are in conflict. You can count on regular, automotive oils to be a good compromise for use in modern automotive engines. If your use is otherwise, it helps to understand what they did.

DETERGENT/NONDETERGENT-

Detergent additives are just surfactants which lower the surface tension and allow small particles to remain in suspension more easily.

This is to transfer contaminates to the filter so they can be removed. You do not want deposits to form throughout the engine because that makes them hard to remove and insulates the passages so that the oil can't remove and equalize the heat.

The base viscosity is increased somewhat by other additives to compenste for lower surface tension. Back when overhaul intervals were shorter, The deposits would get cleaned up periodicaly before they got too bad. I wouldn't use nondetergent in an engine with modern parts.

There is little problem with detergent oils unless they are subjected to very high temps or pressures or used in hydraulic systems. High temp and pressure can convert some additives to shellac like compounds that are really hard to remove.

I think the diesel rated versions are less prone to this but it's not a big deal in most applications. Also, there may be extra foaming if air can get introduced somewhere. I had trouble with my JD M with detergent oil in the lift system.

One problem you can have as mentioned by others is adding detergent oil to an engine that has a huge amount of deposits in it. As the detergent softens these deposits, there is a risk of a chunk coming loose and blocking something.

The risk is real but then if you have this much stuff in there, you have a time bomb waiting to go off anyway because a big temperature swing can trigger the same thing.

The right answer is to tear it down and clean it up. My answer is to run detergent oil, at moderate load, and change oil and filter frequently for 3 or 4 changes and hope for the best.

OIL GRADE-

The numbers, 30 weight, 15-30 weight, etc. relate to the viscosity of the oil. Viscosity is a measure of the "shear strength" of a thin layer of oil. This is important because of the way a plain bearing works.

It is not the oil pressure generated by the oil pump that keeps the crank separated from the crank bearings (for example). Rather, it is a local area of higher pressure generated by the shearing action of the parts moving relative to each other.

This "hydro-dynamic bearing" layer is what resists the forces of pistons and so on. If the oil is too thick, this layer will not form fully or oil may not even get into the space to start with. Too thin and the layer will not be thick enough to prevent contact at the high spots.

For an engine used fairly little, when oil changes are infrequent, multi-viscosity oils are important so that lubrication is adequate in either winter or summer. However time, temp, and chemicals, break down the additives that control the viscosity in these oils so that it changes over time.

One reason they recommend at least annual changes. If the tractor is only used in a narrow temperature range, then a single weight oil would last longer without change since it theoretically contains less additives. However, oil companies may use the same base stock and add stuff to get various single grades, so I wouldn't count on more stability over time.

CORROSION-

An important function of modern oils is corrosion inhibiting. That is because there are so many different metals in a modern engine. If the Ph is too far out of whack, galvanic corrosion can eat things up.

There is a microscopic variation of this called "fretting corrosion" that can get to parts that are in contact but move very little under quite a bit of pressure. That is what gives the odd stain patterns on shafts and surfaces that are together a long time.

The Ph of the oil is really affected by the water it picks up because the water will react with gasses to form acids.

That is why lots of short trips are bad, lots of blowby and gasses and lots of moisture that does not have time to be driven out.

Oil starts out a bit on the basic side and gets more acid over time. I think they assume that most cars get a fair number of short trips.

This is a bit of a problem for a tractor that does not get used too much since a basic Ph will corrode aluminum and an acid will corrode zinc and iron. Ideally you could keep "half worn" oil all the time but you can't. I use this as a justification for not changing oil too frequently.

FILTRATION-

This a place where a lot of compromises are made and specs are pretty vague. In general, filters are rated by the pore size in microns. However, this is not a absolute limit due to the construction of the filter.

The rating indicates that the filter will trap 90% of particles above that size. What they don't tell you is what is the biggest particle size it will pass. Good quality (expensive) media with a 20 micron rating will pass some particles up to 45 microns.

Fuel filters have about an 8 micron rating with a 25 micron max. The tradeoff here is flow rate. The higher the filter efficiency, the lower the flow.

A good "trick" some companies use for marketing is to go to a smaller rating paper that has bigger max pore size to get back some flow rate. They advertize "We have 16 microns and they have only

20" but they don't tell you they will pass a bunch of 60 micron stuff that "they" would trap. Interestingly, there is a standard "test dirt" you can buy for these tests that has a known distribution of particle sizes.

Engines are pretty tolerant of particles up to about 30-35 microns so the small end of the scale is really not what you worry about.

Most filters actually improve in efficiency with use. That is because the big holes get bridged over and the surface becomes more uniform. The best quality filters have a micron rating that is fairly high, say around 28 but is very uniform pore size.

This paper improves pretty fast to around an 18 actual perfomance and stays that way a long time. A bad filter on the other hand has small pores so it tests well when new but a wide variation in sizes.

The small pores tend to plug over in use and the big holes open up with the added flow through them. This results in a worsening efficiency condition. A filter that plugs too fast can also rupture or cause the unit to "bypass" the filter so that there is no filtering at all.

Unfortunately, there is little you or I can tell about filters until it is too late. If you could get them, you could compare the manufacturers graph of efficiency, flow loss and max particle size over time (actually volume filtered).

The best have a broad time in the middle range with high efficiency and low flow loss. The manufacturers have these, but I have never seen such graphs available at the retailer. The best bet is to buy good quality brand name filters.

The highest price ones are probably no better and the lowest price ones may or not be the same. It isn't worth the $3 difference to find out. Never use fuel filters for lube oil, by the way. They will plug too fast and it is overkill.

If a tractor is not used very much, say just for shows, I would consider changing oil each year and filter every other time to limit the amount of big particles getting through. This would not be true if you are "cleaning up" a dirty engine with detergent oil.

OIL TESTING -

I should mention that a lot of truckers etc use a testing service to keep track of their engines. It is a little expensive for us commoners but might be useful in some cases. The oil is sampled and sent to a lab.

They can test it for metal content and for Ph and lubricity. More money buys more detail. Some people have used the technique to determine change interval though I doubt it is worth it for th at unless you are talking a lot of oil (locomotives hold ~250 gallons in the pan).

A better use would be to assess the state of an engine that is in question. High readings of iron indicate a wearing bore or cam/liter, aluminum is piston maybe bearing, lead is bearings, etc.

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