Diesel Engine Performance With Medium Viscosity Index Lube Oil

By James L. Wilkinson

In the 1930's, the medium-speed diesel engine came into its own as a prime mover in rail, marine, and stationary applications.

It was soon realized by engine builders and o p e r a t o r s that Medium Viscosity Index (MVI) lube oils had distinct advantages in performance over High V i s c o s i t y Index (HVI) oils in these diesel engines.

Historically, the preference for MVI lube oils is based primarily upon experience in 2-stroke, medium-speed diesel engines. This preference has carried over to 4-stroke designs also, although the benefits, while evident, are not as pronounced as in the 2-stroke engines.

The main feature of the MVI lube oil is in the nature of the residual deposits formed in the engine as contrasted with the deposits formed by the HVI lubricants. Additives aside, the difference in the deposit formation by the two types of oil points up the excellent performance of MVI lube oil in service.

In the infancy of the medium-speed diesel engine, as we now know it, oils were not compounded but were straight blends of refined lubricating oils. Performance of the lubricant was directly related to the base oil.

It was soon found that MVI oils formed softer, less dense carbon deposits than HVI oils. The deposits formed by the HVI oils were harder, more adhesive, and tended to build up to high levels in deposit-prone areas.

Deposits from the MVI oils, on the other hand, in addition to being softer tended to slough off and not build up beyond a certain low level as a result of normal engine operation.

Even in today's modern oils, the carbon deposits reflect the nature of the base oils, MVI or HVI, regardless of the benefits of additives. Thus the benefits of MVI oil still apply, namely softer carbon deposits and less of them. Additive technology has improved the performance characteristics of both oils about equally, and the performance gap of the 1930's still exists.

The effect of the carbon deposits is most noticeable in scavenging air and exhaust ports in 2-stroke engines and on the top lands and in ring grooves of the pistons in all engines.

In 2-stroke medium-speed diesels, port blocking is an important factor in performance because of its effect on engine power.

It is also an economic factor in the downtime and labor expense of port-cleaning operations.

Deposits formed by MVI lube oils tend to be crumbly, and in the port area will build up to moderate and usually acceptable levels. However, once they attain these levels they are broken off by the normal aspiration of the engine and do not build up further.

In some instances, the use of MVI oils will eliminate the need for any port cleaning between scheduled overhauls.

HVI oils form more adhesive and dense carbon deposits in the port areas. These deposits build steadily, and engine aspiration during operation is usually insufficient to maintain them at a low level. These hard, dense deposits are difficult to remove and can require shutdown for laborious hand scraping.

In one reported instance, Fairbanks Morse Model 38 opposed-piston engines required port cleaning after 1,500 hours operation with HVI oil. Downtime was lengthy and labor costs high. After switching to an MVI oil, the engines, when inspected, had operated in excess of 5,000 hours without portcleaning.

Intake ports were 100 percent open, exhaust ports 90 percent open.

Top land and ring groove deposits are the other most prominent points of carbon deposition.

Here also, the softer, less adhesive deposits of the MVI oil are the least troublesome.

The deposits are more easily removed by normal engine operation and do not build up to excessive levels on the top lands or in the ring grooves.

The hard and adhesive HVI oil carbon deposits can, and often do, build up to excessive levels. This causes ring "proudness"; in effect, the deposit prevents the ring from recessing completely into the groove. And, the ring groove fill reduces ring side clearance, which can affect power output, oil consumption, and hydrocarbon emissions adversely.

This can lead to ring sticking and ring breakage. In extreme cases in 2-stroke diesels, it also may cause port scalloping.

Excessive carbon buildup on the top land of the piston caused by HVI lube oil can reduce clearances sufficiently to prevent combustion pressures from pushing rings against the cylinder liner normally for sealing. Again, the result is poor performance — power loss and increased oil consumption. In extreme cases, these land deposits also can cause excessive bore wear or "bore polishing." In this instance, in either 2-stroke or 4-stroke diesels, a power pack replacement can become necessary.

The modern MVI lube oil is a far cry from the simple non-compounded oils of the 1930's.

Additive technology has produced long-life oils with dispersant action to keep engines clean by keeping contaminants in suspension in the oil rather than depositing out on engine surfaces. High alkalinity (TBN-E) and excellent alkalinity retention help neutralize corrosive combustion products to reduce corrosive wear. Oxidation inhibitors and corrosion inhibitors protect both oil and engine.

Filters last longer.

Today's MVI lube oil provides the same advantages as its predecessor MVI oils in forming carbon deposits that are soft and friable; a benefit of an all-neutral oil. And, because of additives, many MVI oils can be used without change in 2-stroke diesels if their condition is closely monitored by a used-oil analysis program. Changing oil based on analysis will maximize oil life in all engine types.

Projected availability of napthenic MVI lube stocks is shown in Figure 1, along with projected demand. About 1984/1985, total MVI lube stocks will be unable to fill the demand for conventional MVI oil applications.

The switch from MVI to HVI base oils is already in progress. Overall, MVI lube stock availability is expected to decline by about 50 percent by 1990.

Shell Oil Company presently manufactures MVI oil at its Martinez, Calif., refinery.

This is the only Shell refinery currently producing MVI lube oils. However, in Texas, Shell has another source of MVI lube crude.

To make this crude available as MVI lube oil, Shell is building a new addition to its Deer Park, Texas, plant. This expansion project is scheduled for completion by the end of 1980 and will more than double the company's supply of MVI lube oil. A further expansion of the Deer Park plant is already scheduled for 1984/1985, which will provide an additional 30 percent capacity.

In addition to increased MVI base oil supply, distribution East of the Rockies will be facilitated by the plant expansions.

Shell believes that the majority of engine builders and operators will prefer to operate engines with MVI lube oils for as long as possible. The successful use of these oils in medium-speed diesels is documented by a long history of successful performance. It behooves the operator to conserve present supplies as much as possible to help the future supply position.

With modern high quality MVI lube oils having the capability of extremely long oil life, with good engine protection, the implementation of a used-oil analysis program can be helpful in determining when (or even if ever) oil needs changing. In addition, such an analysis program is a useful maintenance tool when trace metals analysis is included.

With such a program, oil is changed only when necessary, if at all. This saves valuable MVI lube crude reserves, and can save money. It also can help detect engine problems and avert untimely breakdowns that can be costly.

With the additional MVI lube supply being placed in the market by Shell Oil Company's expansions, and operator conservation (such as that outlined above), the crossover point on Figure 1, demand exceeding supply, can possibly be extended

Other stories from July 1980 issue

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