1979/03/07 Caterpillar

All Caterpillar Engines

Basically, there are two possible causes of severe wear and internal damage during operation of a diesel engine: Abrasive wear and corrosive wear. Abrasive wear is caused by hard, sharp particles of dirt which scratch metal surfaces and rapidly cause severe damage if allowed to enter an engine. Most dealer and customer personnel understand abrasive wear, and know the importance of proper maintenance of air filters, oil filters, breathers, etc. to keep dirt out of the engine.

Corrosive wear is not so commonly understood. It has not been a problem in areas where the sulfur content of diesel fuel is controlled at low levels. However, there are some areas where fuel sulfur content is not controlled, and some fuels in these areas have a very high sulfur content. The use of these fuels can shorten engine service life to as little as one-fifth of normal. For this reason, it is very important to understand the problem of corrosive wear and the proper maintenance necessary to protect the engine from damage.

In comparison with abrasive wear, which causes direct mechanical damage, corrosive wear is the result of chemical action. During the combustion process, the sulfur in the fuel forms sulfur oxides which combine with moisture in the air to produce sulfuric and sulfurous acids. These acids chemically attack cylinder liner walls and piston rings, and can damage exhaust valve guides and engine bearings. The amount of acid produced is in direct relation to the amount of sulfur and water present during combustion. The amount of sulfur depends on the fuel sulfur content and the rate of fuel consumption, or engine load factor. The amount of water depends on the humidity, or moisture in the air, and the rate of condensation, which is highest at low engine temperatures.

The corrosive damage from these acids is more deceptive than abrasive wear. It gradually accumulates with no warning until the damage is done. The engine will run normally with no change in power or starting characteristics until the piston rings stick or the rings and liners are worn enough to cause excessive oil consumption and/or leakage of combustion gases. The corrosion quietly destroys the engine as the acid products accumulate in the oil.

The rate and extent of corrosive wear and damage depend on these factors:

1. Percent of sulfur in the fuel.

2. Load factor, or amount of fuel burned in the engine.

3. Engine temperature.

4. Type of engine oil used.

5. Oil change intervals.

6. Humidity in the atmosphere.

To keep the effect of these factors at a minimum, follow recommended procedures in operation and maintenance. The recommendations are:

1. Use the best fuel available, and know the percent of fuel sulfur content. In areas where the fuel sulfur specification is not available, it will be necessary to have the fuel tested for sulfur content. Refineries in many countries do not have equipment to reduce fuel sulfur content to a low level (less than 0.4%), and the sulfur content of the fuel is frequently as high as 0.8% to 1.8%. A Special Instruction, Form SEHS7067, is a good source of information about fuels, but it will be of little use in areas where no fuel specifications are available.

2. The application of the engine has an effect on corrosive wear by the amount of fuel burned. For example, if an engine uses 100 gallons (380 liters) of fuel per day, and the fuel has a sulfur content of 1.0%, 7.1 lbs (3.22 kg) sulfur will enter the engine each day. When the rate of fuel consumption is higher than normal, the oil must do more work to keep the engine clean. When the rate of fuel consumption is lower than normal, the engine may run below its normal operating temperature. Low engine temperature increases the amount of water produced in the cylinders, which also causes the oil to deteriorate. In addition, it increases acid accumulation and can cause exhaust valve problems.

3. Correct engine temperature is critical. High engine temperature increases the possibility of piston rings sticking. Low engine temperature increases moisture condensation, so more water is produced in the cylinders. This depletes the oil additives, and combines with the sulfur products to make more acid. The worst possible condition is operation in a heavy load application (high rate of fuel consumption) when the engine is overcooled (below normal temperature). To keep these problems at a minimum, be sure the engine cooling system operates in the normal temperature range. The temperature regulator must operate correctly to keep the engine temperature above 165°F (74°C). The maximum temperature is shown by the red area on the engine temperature gauge. At or near sea level, the maximum temperature is approximately 225°F (107°C) in a pressurized system, or 212°F (100°C) in a system which is not pressurized.

4. Always use an oil that has an API (American Petroleum Institute) classification of CD. Oils with a CD classification are available worldwide. A list of these oils is in Form GEG05044-02, "Engine Manufacturers Association Lubricating Oils Data Book".

5. The problem of increased wear with high sulfur fuel is not new. For many years, every Lubrication and Maintenance Guide has had the following footnote to the normal oil change interval: "Normal oil change interval when fuel sulfur content is less than 0.4%. When fuel sulfur content is 0.4% to 1.0%, reduce oil change interval one-half. When fuel sulfur content is above 1.0%, reduce oil change to one-fourth the normal interval."

6. The effects of high fuel sulfur are made worse by conditions of high humidity. This cannot be controlled, but remember that engine wear and other possible damage will be less, for example, in a hot desert area than in a coastal area.

In summary, do not wait until the engine shows signs of wear to take precautions against fuel sulfur damage. The use of a better oil and/or shorter oil change intervals after the engine has developed excessive blowby (leakage of combustion gases) or high oil consumption will not correct the damage. Take these steps when the engine is new:

1. Know the sulfur content of your engine fuel.

2. Use an oil having an API classification of CD.

3. Shorten the oil change intervals if necessary, according to fuel sulfur content.

4. Change oil filters at the correct change interval, as shown in the Lubrication and Maintenance Guide. It is not necessary to shorten filter change intervals when oil change intervals are reduced.

5. Be sure the engine temperature is above 165°F (74°C) during operation. Always keep the engine temperature in the normal operating range.