Oil Specifications, Diesel Engines:
What's Current? What's Coming?
The current commercial vehicle diesel fueled motor oil specifications are API CG-4, CF and CF-2. Oils meeting the next four-cycle on-road specification should be on the market by 1999.
The SAE, ASTM, API, Engine Manufacturers Association (EMA) and the Chemical Manufacturers Association (CMA) write commercial vehicle oil specifications in North America. Commercial vehicle oil specifications are written for oils lubricating two-cycle and four-cycle diesel engines and for "universal" oils, which may be used to lubricate gasoline and diesel engines. The EMA sends requirements for a new oil to the SAE technical committee, TC-2. TC-2 also gathers the concerns of other interested parties, such as fleet operators. TC-2 passes its recommendation to create a new specification to the ASTM Heavy-Duty Oil Classification Panel (HDOCP). HDOCP members include representatives of oil companies, additive suppliers and engine makers. HDOCP defines the tests and limits of the new specification. Four ASTM committees vote on the tests and limits proposed for the new spec before it is passed. Once the spec passes, oil company formulations are tested under the auspices of the CMA.
Current Specifications
Current commercial vehicle specifications include CG-4, CF-4 and CF for four-cycle diesels and CF-2 for two-cycle diesels. Universal oils bear CG-4/SJ or CG-4/SH designations. (SH is still current as a commercial oil specification when used with a "C" category that appears first in the designation.) CG-4 was set to be introduced in 1994 to meet the needs of the low emission engines introduced in 1994 and to meet the needs of engines burning fuel with 0.05 percent weight or less sulfur, primarily on-road trucks. CG-4 oils are multigrade only. The specification was introduced in 1995 and replaced CF-4 in North America. CG-4 has not been well-accepted outside North America. In other countries, especially those where diesel fuel with greater than 0.05 percent-weight sulfur is common, API CF-4 is the preferred four-cycle on-road engine oil.

CF oils are formulated for use in four-cycle off-road engines, which may burn diesel fuel with greater than 0.05 percent-weight sulfur. CF oils are also specified 		for use in some turbocharged on- or off-road diesel engines. CF oils may be monograde or multigrade oils. The specification was introduced in 1994 and replaced the API CD category.

CF-2 oils are formulated for use in severe service two-cycle engines. Many CF-2 oils are monogrades; however, the specification does not exclude multigrade oils. The specification was introduced in 1994 and replaced the CD-II specification.
Future Specifications
API PC-7 ("Proposed Category 7") was originally scheduled for release January 1, 1998, to coincide with the introduction of 1998 low emission four-cycle on-road engines. Testing and funding problems have delayed the introduction of the new specification until sometime in 1999.

The 1998 engines will be significantly different from earlier engines. They are expected to feature greater injection retarding for control of nitrogen of oxide emissions; articulated pistons with iron crowns and steel skirts, which allow the piston to go higher in the cylinder and create more pressure for more efficient combustion and more power; higher injection pressures, 		which must accompany higher in-cylinder pressures; and hydraulically-activated electronically-controlled unit injectors (HEUI) systems, which also increase combustion efficiency. HEUI systems have already been introduced; they will be more widespread in 1998 engines than they were in previous models.

PC-7 oils must offer excellent dispersancy to handle the increased soot that will accompany greater injection retarding; excellent detergency to handle the increased deposits that accompany the high power density associated with articulated pistons; excellent thermal stability to handle the heat generated by higher pressures; and excellent shear stability and foam control to resist the physical oil damage and foaming caused by HEUI systems.

In addition, the EMA wants PC-7 oils to offer twice the service life offered by CG-4 oils. Large fleets are demanding extended oil drain intervals for the cost savings associated with them. 		The superior thermal and oxidative stability of synthetic oils will be an advantage in the higher-pressure, higher-temperature environment of the 1998 engines, especially in light of the desire for extended oil service life.

Additionally, the demand for increased fuel economy is creating a desire for lower viscosity diesel engine oils, especially 10W-40 grades. Due to their superior film strength and lower volatility, synthetic and semi-synthetic oils may be required in the formulation of these low viscosity grade diesel oils.