NeqSim’s Standard_ISO15403 class calculates a motor octane number (MON) and a methane-number result for compressed-natural-gas compositions. The class is a calculation helper; it does not perform a complete conformity assessment against ISO 15403-1:2006.

Implemented correlation

The current implementation evaluates

\[MON=137.78x_{CH_4}+29.948x_{C_2H_6}-18.193x_{C_3H_8}-167.062(x_{nC_4}+x_{iC_4})+181.233x_{CO_2}+26.944x_{N_2}\]

and then

\[NM=1.445MON-103.42\]

where each $x_i$ is the overall mole fraction stored by the thermodynamic system. Call calculate() before reading a result. The supported result keys are "MON" and "NM"; the usual methane-number abbreviation is not an accepted getter key.

The six terms above are the complete component coverage of the current class. Hydrogen, C5+ hydrocarbons, and other unlisted components contribute zero to the implemented sum, so mixtures containing material amounts of those components require a method whose validity range covers them.

Pure methane is a useful implementation anchor, not a definition of the methane-number scale: this correlation returns MON = 137.78 and NM = 95.6721. A hydrogen-only system would produce NM = -103.42 because hydrogen has no term; that extrapolation is outside the implemented component coverage and must not be interpreted as a hydrogen-fuel rating.

Complete Java example

This assertion-enabled Java 8 program uses the same one-mole composition basis for every case at 200 bara absolute. The sensitivity cases replace two mole percentage points of methane with either carbon dioxide or nitrogen instead of adding material to an existing system. Run it with assertions enabled (java -ea).

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import neqsim.standards.gasquality.Standard_ISO15403;
import neqsim.thermo.system.SystemInterface;
import neqsim.thermo.system.SystemSrkEos;

public final class Iso15403Example {
  private static final Logger logger = LogManager.getLogger(Iso15403Example.class);
  private static final double PRESSURE_BARA_ABSOLUTE = 200.0;

  private Iso15403Example() {}

  public static void main(String[] args) {
    Standard_ISO15403 base =
        new Standard_ISO15403(createCng(0.92, 0.01, 0.01));
    base.calculate();
    double baseMon = base.getValue("MON");
    double baseNm = base.getValue("NM");

    Standard_ISO15403 carbonDioxideCase =
        new Standard_ISO15403(createCng(0.90, 0.03, 0.01));
    carbonDioxideCase.calculate();
    double carbonDioxideNm = carbonDioxideCase.getValue("NM");

    Standard_ISO15403 nitrogenCase =
        new Standard_ISO15403(createCng(0.90, 0.01, 0.03));
    nitrogenCase.calculate();
    double nitrogenNm = nitrogenCase.getValue("NM");

    assert Double.isFinite(baseMon);
    assert Double.isFinite(baseNm);
    assert Math.abs(baseMon - 128.18474) < 1.0e-8;
    assert Math.abs(baseNm - 81.8069493) < 1.0e-8;
    assert Math.abs(carbonDioxideNm - 83.0627410) < 1.0e-8;
    assert Math.abs(nitrogenNm - 78.6037889) < 1.0e-8;
    assert carbonDioxideNm > baseNm;
    assert nitrogenNm < baseNm;
    assert "".equals(base.getUnit("MON"));
    assert "".equals(base.getUnit("NM"));
    assert base.isOnSpec();

    logger.info("Base MON={}, base NM={} (dimensionless)", baseMon, baseNm);
    logger.info("NM after replacing methane with CO2={}", carbonDioxideNm);
    logger.info("NM after replacing methane with N2={}", nitrogenNm);
  }

  private static SystemInterface createCng(
      double methane, double carbonDioxide, double nitrogen) {
    double totalMoles = methane + 0.04 + 0.01 + 0.005 + 0.005 + carbonDioxide + nitrogen;
    assert Math.abs(totalMoles - 1.0) < 1.0e-12 : "Composition must total one mole";

    SystemInterface gas = new SystemSrkEos(288.15, PRESSURE_BARA_ABSOLUTE);
    assert Math.abs(gas.getPressure("bara") - PRESSURE_BARA_ABSOLUTE) < 1.0e-12;
    gas.addComponent("methane", methane);
    gas.addComponent("ethane", 0.04);
    gas.addComponent("propane", 0.01);
    gas.addComponent("n-butane", 0.005);
    gas.addComponent("i-butane", 0.005);
    gas.addComponent("CO2", carbonDioxide);
    gas.addComponent("nitrogen", nitrogen);
    gas.init(0);
    return gas;
  }
}

The assertions are executable documentation of the current implementation, not ISO acceptance limits. The repository regression compiles this exact fence for Java 8 and invokes it with assertions enabled. For these three normalized compositions, the current source correlation gives:

Case MON NM Engineering interpretation
Base composition 128.18474 81.8069493 Reference case
Replace 2 mol% methane with CO2 129.05380 83.0627410 Increases this correlation result
Replace 2 mol% methane with N2 125.96802 78.6037889 Decreases this correlation result

These trends are properties of the implemented coefficients and the stated replacement experiment. They are not universal claims about engine knock or arbitrary dilution paths.

API and engineering boundaries

API Current behavior
new Standard_ISO15403(system) Uses the system’s overall composition
calculate() Updates the stored MON and NM results
getValue("MON") Returns the dimensionless motor octane number
getValue("NM") Returns the dimensionless methane-number result
getUnit(...) Returns an empty string
isOnSpec() Always returns true; no acceptance limits are evaluated

Do not use isOnSpec() as evidence that a fuel complies with ISO 15403, a national fuel specification, or an engine manufacturer’s limits. A conformity assessment also needs the applicable standard edition, sampling and analysis requirements, validated composition range, contractual limits, and any other required fuel properties.