While Automotive Service Excellence ( ASE) hasn't published test results yet, the question of engine oil life does tend to come up for those talking about CNG-powered vehicles. Some mechanics pull out the dipstick and take a look at the oil's color. Is the appearance of the dipstick a reliable indicator in CNG vehilces?
Dave Crowley, an instructor at Natural Gas Vehicle Institute, says that you can't rely on the visual appearance – it may look clean, but could be long overdue for a change. While the base oil doesn't wear out, it changes chemically over time, and critical additives are depleted. A few of the changes Crowley looks for:
- Viscosity: Using CNG eliminates fuel dilution of oil and it thickens over time, which can have a negative effect on a lubricant's ability to protect the CNG engine.
- Oxidation: CNG engines go to higher temperatures than gasoline and diesel engines in the combustion chamber and upper cylinder, increasing the rate of oxidation.
- pH Balance: Formation of acids is normal in ICEs, and some natural gas can bring additional acidity. Natural gas composition can vary by region and source, making it necessary to do oil analysis to get an accurate reading of the CNG-powered vehicle's oil quality.