Fuel Quality Calculator

Cummins Westport natural gas engines can operate on CNG, LNG or biomethane that meets Cummins published fuel specifications.

Customers should note that pipeline natural gas fuel quality can vary on a regional and seasonal basis. LNG fuel quality may change if it is held in storage for extended periods of time. Consult your fuel provider on a regular basis to ensure that natural gas fuel quality, in particular, the methane number, meets engine requirements.

The minimum methane number requirement for the C, B and L Gas Plus engines is 65 or greater. The ISL G and the ISX12 G require a minimum methane number of 75 or greater.

To ensure the fuel you are using meets the required specifications, we've provided an online calculator for your reference. This specification covers the natural gas fuel requirements for the following Cummins Westport automotive spark-ignited natural gas engine models:

ISX12 G  • ISL G  •  L Gas Plus  •  C Gas Plus  •  B Gas Plus

Cummins Westport’s (CWI) website fuel quality calculator was using an SAE based Methane Number (MN) calculation (SAE 922359 Eqn 4).  As of Nov 9, 2015, CWI's fuel quality calculator switched to a Cummins Proprietary methane number calculation. The Cummins Proprietary calculation provides a more accurate representation of the true Methane Number of the fuel.  CWI recognizes these two methods will result in different calculated methane numbers for a given fuel composition, but this difference is expected to be small and not likely change CWI's acceptability of most automotive natural gas fuel compositions.

If the calculated methane number of a specific fuel composition is just above or below the acceptable limit, then CWI should be contacted to discuss acceptability as hourly and daily variation in fuel composition will likely have a greater impact on the resulting methane number than the calculation method used.  The Cummins proprietary methane number calculation is a numerical, regression-analysis based method utilizing an extensive set of experimental test data. While neither this data nor the resulting calculation can be shared, this fuel quality calculator can be used to determine methane number for a given fuel composition using this method.

Using the calculator
Fill in the Fuel Composition values so that the total equals 100%. The Results will be calculated automatically. Methane Number, Lower Heating Value, and Sulfur Content criteria must be met to pass a given fuel for a specific engine.

Sample Name:
All values should be entered as mole %
Methane, CH4 %
Ethane,C2H6 %
Propane, C3H8 %
i-Butane, C4H10 %
n-Butane, C4H10 %
i-Pentane, C5H12 %
n-Pentane, C5H12 %
n-Hexane, C6H14 %
n-Heptane, C7H16 %
n-Octane, C8H18 %
n-Nonane, C9H20 %
n-Decane, C10H22 %
Hydrogen, H2 %
H2S ppmv
Carbon Monoxide, CO %
Carbon Dioxide, CO2 %
Nitrogen, N2 %
Oxygen, O2 %
Sample Percentage
MN Rich Burn


  1. The minimum methane number requirement for the C, B and L Gas Plus engines is 65 or greater.
  2. The ISX12 G, ISL G, and ISB6.7 G require a minimum methane number of 75 or greater.
  3. The minimum Lower Heating value (LHV) for all CWI engines is 16,100 btu/lbm.
  4. Fuel as delivered to engine regardless if liquid or gaseous.
  5. Gas with chlorine additives are not permitted.
  6. The fuel shall not contain water, dust, sand, dirt, oils, or any other substance that can harm engine operation.

Green   Pass:  Fuel composition acceptable.

Amber   Pass:  Fuel composition acceptable, but methane number is close to minimum specification. Be aware of potential variations in fuel composition as described above.

Red   Fail:  Fuel composition does not meet Cummins Westport requirements.