Tech Tips: Oil Carryover in CNG Fuel Stream – The Root of Many Problems

Do some of the compressed natural gas (CNG) powered vehicles in your fleet experience hard starting, rough idling, hesitation under acceleration, or does the engine stall when the driver attempts to accelerate away from a green signal light?  Often, when the driver reports the situation to the vehicle mechanic, the mechanic uses an engine diagnostic instrument to attempt to diagnose the problem.  The problem may not be detectable when using engine diagnostic tools.  The cause of the problem may be the presence of contaminates in the CNG fuel.

Because most CNG is compressed by using reciprocating compressors that are lubricated with oil, this oil finds its way into the CNG fuel stream during the compression cycle.  The solution to eliminating this oil in the CNG fuel stream (oil carryover) is through the use of coalescing filters that remove oil in an aerosol form.  The key to the effective use of coalescing filters is the proper sizing of the filter and the filter housing, proper number and location of the filters in the CNG fueling station piping, and frequent draining of the filter housing.

Virtually all compressed natural gas fueling stations contain some number of coalescing filters.  However, the location, size, number and frequency of draining the filter housings may not be adequate to prevent some amount of oil from being dispensed into the vehicle during fueling. The fuel dispensed into a CNG powered vehicle must be free of any liquid or solid contaminate.  To learn more about how coalescing filters can eliminate hard starting, rough idling, hesitation under acceleration, or engine stalls, please contact Leo Thomason, Training Director, AFV Institute, at leo@afvi.org.