1985/03/04 Caterpillar

All Natural Gas Engines

There have been some reports of natural gas engine failures that were caused by rapid deterioration of the lubricating oil. Most of these engines were used in gas compressor applications. The oil in these engines showed signs of heavy sludging and unacceptable viscosity increases (thickening of the oil). This was caused by excessive oxidation and nitration which can lead to plugged oil filters, deposits on the pistons, sticking piston rings, corrosion and bearing failures.

This article provides guidelines for oil selection and for the condemning limits on oil degradation. There are many conditions which can cause rapid deterioration of the lubricating oil. Some of these conditions are shown below.

... The limits on wear metal concentration remains the same as per the S.O.S program, except for copper. The General Notes, Section A, No. 9 of the Interpretation Guideline states, "Natural gas engine oil normally has a high copper concentration..." This implies that high copper concentration is acceptable. Recent information suggests that copper concentration above published guidelines is not normal and should be treated as a sign that the oil has degraded to an unacceptable point. The oil should be changed to prevent serious deposits and wear in the engine.

... To maintain a combustion chamber that is free of deposits, some of our Maintenance Guides state, "Use only natural gas engine oils. Natural gas engine oils meet or exceed the Supplement I diesel engine oil specification. These oils contain ashless dispersant additives and either ashless or organo-metallic oxidation inhibitors. Your oil supplier knows which oils meet those requirements". The EMA Book classifies natural gas engine oils as low ash (contains a maximum of 0.5% sulfated ash) and high ash (contains a minimum of 2.0% sulfated ash by weight). Caterpillar recommends that engines operating on processed natural gas use the low ash oils. In low ash oils, because of the low ash requirement, only a limited amount of detergent and alkaline additives can be used. The formulators depend upon a variety of ashless dispersants to obtain oil performance. Because of this, all oils meeting this definition may not have similar performance properties when compared to diesel engine oils which are rated as CC or CD. Remember, natural gas engine oils need only meet the requirements for CB (no engine performance test available) and therefore can be more prone to the formation of deposits.

... Oxidation and nitration are critical because they can cause the oil to thicken. They can also cause the formation of lacquer and maroon-colored deposits due to nitration. Both oxidation and nitration generate acids which cause corrosive wear and accelerate oil degradation.

The nitration rate of an oil is associated with the air to fuel ratio for the engine. There is evidence that suggests operating the engine with an air to fuel ratio between 10:1 and 11:1 promotes rapid nitration of the oil. This is the normal air to fuel ratio range for most Caterpillar natural gas engines, and permits the optimum fuel consumption at rated horsepower. In this range nitrous oxide (NO_x), as measured in the exhaust stream, is at its highest level. This range may cause the oil to degrade at an unacceptable rate. If nitration is determined to be the principle reason for oil degradation, it may be necessary to adjust the air to fuel ratio either higher or lower to minimize the nitration rate. If the air to fuel ratio is changed, it must be done with care because it may have a negative effect on the power of the engine or result in excessive exhaust temperature which could affect the service life of the engine.

Engine Data Sheet 195.0, Form LEHX1524, of the Caterpillar Technical Manual shows the effects of different air to fuel ratio settings on various engine functions. The graphs included in that publication are for 6 1/4" bore natural gas engines, but the general shapes of the curves are similar for all natural gas engines.

Remember, if the engine is operating with an oil sump temperature of less than 74°C (165°F), the oil sample may show a high nitration number. If the temperature of the oil in the sump is above 99°C (210°F), the oil sample may show a low nitration number and an elevated oxidation number.

... Low engine loads can cause low jacket water temperatures. This can result in excessive moisture and nitration products accumulating in the crankcase. These materials react with the oil and form a sticky, gummy product that can plug the oil filters. When this happens, unfiltered oil can circulate through the engine and cause deposits on the pistons and the piston rings to stick.

To minimize this problem, temperature regulators with an 88°C (190°F) initial opening temperature can be used. See the article, "Use Of A High Temperature Regulator Will Keep Condensation To A Minimum," on Page 4 of the November 7, 1983 Service Magazine.

The oil filters must not cause more than a 70 kPa (10 psi) decrease in the oil pressure and the oil must not exceed the condemning limits. This will make sure that the oil remains acceptable.

Condemning Limits

1. Viscosity of the used oil must not have increased by more than 3 cSt at 100°C (212°F) when compared to new oil viscosity.

2. The percent of solids or insolubles must not be more than 1% by ASTM D-893.

Differential infrared analysis of used oil must not exceed the following absorbance/CM-¹.

1. The maximum for oxidation is 20 Absorbance/CM-¹ at a wave number of 1710.

2. The maximum for nitration is 20 Absorbance/CM-¹ at a wave number of 1630.

If analysis of the used oil at the recommended oil change interval exceeds the condemning limits, the following courses of action can be taken.

1. Modify the jacket water temperature and/or adjust the air to fuel ratio to minimize the oil degradation rate.

2. Work with your oil supplier to arrive at an oil that will not exceed the limits.