Series 50G & 60G Troubleshooting

Section 4.12 Fuel Composition

Section 4.12
Fuel Composition

The quality of fuel used is a very important factor in obtaining satisfactory engine performance, long engine life, and acceptable exhaust emission levels. Listed in Table "Compressed Natural Gas Fuel Specifications" are the DDC fuel specifications for both CNG and LNG. Regardless of the type of fuel, fuel entering the engine must meet or exceed DDC's published CNG specifications. ‪



ASTM Test Method‪


Mole Percent


88% min.‪

D 1945‪


6% max.‪


1.7% max.‪

Other: C4 and Higher‪

0.3% max.‪

Other Gaseous Species

Mole Percent


0.1% max.‪

D 2650‪

Carbon dioxide + Nitrogen + Oxygen‪

4.5% max.‪

D 1945‪

Carbon Monoxide‪

0.1% max.‪

D 2650‪

Other Species


0% mass‪

No Test Method‪

Sulfur, Total‪

22 ppm/v‪

D 4468‪

Performance Related Properties

Motor Octane Number‪

115 min.‪

 D 2623 *‪

Wobbe Number‪

1290-1380 BTU/ft3

D 3588‪



Pressure Water Dew Point Temperature, Max.


D 1142‪

Pressure Hydrocarbon Dew Point Temperature, Max.

‡ Below which will form 1% condensate‪



Table 1. Compressed Natural Gas Fuel Specifications

* Test method D 2623 was obsoleted by ASTM in 1991. Wobbe Index (WI), also known as Wobbe Number, is a measure of fuel energy flow rate through a fixed orifice under given inlet conditions. Numerically, WI = (dry, higher heating value)/(specific gravity)
† The compressed natural gas shall not contain dust, sand, dirt, gums, oils, or other substances in an amount sufficient to be injurious to the fuel station equipment or the vehicle being fueled.
‡ The dew point at vehicle fuel storage container pressure shall be at least 5.6 °C (10 °F) below the 99.0% winter design temperature for Climatic Conditions for the United States in the American Society of Heating, Refrigerating and Air conditioning Engineer’s (ASHRAE) Handbook. Testing for water vapor shall be in accordance with ASTM D 1142, utilizing the Bureau of Mines apparatus.
¶ The natural gas at ambient conditions must have a distinctive odor potent enough for its presence to be detected down to a concentration in air of 1% by volume.

Check the Fuel Composition for problems that could cause Knock as follows:‪

  1. Ask for a gas analysis from your local gas supplier and confirm that the gas that you are using is within DDC's specifications listed in Table "Compressed Natural Gas Fuel Specifications" .
    1. If the fuel is within DDC's specification, then check the Internal Engine Components. Refer to "4.13 Internal Engine Components" .
    2. If the fuel is not within DDC's Specifications contact Technical Service.
  2. If the no longer knocks, troubleshooting is done.

Series 50G and 60G Troubleshooting Guide - 6SE482
Generated on 10-13-2008

Leave a Reply

Your email address will not be published. Required fields are marked *