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Compressed natural gas (CNG) has been used as a fuel for internal combustion engines for decades. CNG engines, in most cases, have been used in stationary applications for power generation, and pipeline gas transportation. More recently, CNG has found its way into mobile applications. Locally operating fleets, such as city bus fleets, have shown a lot of interest in CNG. Bus fleets operate most of the time in urban areas, where lower emission levels are mandated, and the operation allows the busses to return to a central refueling station on a daily basis. CNG engines are different from diesel engines in terms of lubricant requirements. Higher operating temperatures may require better oxidation and nitration control, and high ash levels can be a detriment. With the increased use of CNG as a fuel for commercial vehicle engines, it becomes more and more important to have a procedure to evaluate lubricants for this application.

During the Coordinating Research Council's (CRC) program to select an engine to replace the BMW 31 8i as the industry standard intake valve deposit test engine, an opportunity was identified to compare the driveability performance of the engines evaluated with and without deposits. This Coordinating Research Council project confirmed that intake valve deposits increase driveability demerits as measured by the BMW Driveability Test Procedure. This was true for the BMW 31 8i reference vehicles as well as the three candidate engines. The driveability demerits for the reference engine as well as the three candidate engines increased with decreasing fuel volatility of the engines considered. The BMW 31 8i was by far the most sensitive to intake valve deposits. A modification of the CRC Driveability Test Procedure did not produce the clear separation found with the BMW Driveability Test.