Specific Emission and Emissions Index

   2.13   Specific Emission and Emissions Index

Levels of emissions of oxides of nitrogen ( nitric oxide, NO, and nitrogen dioxide, NO₂, usually grouped together as NO_x ), carbon monoxide (CO), unburned hydrocarbons (Hc) and particulates are important engine operating characteristics.

The concentrations of gaseous emissions in the engine
exhaust gases are usually measured in parts per million
or percent by volume ( which corresponds to the mole
fraction multiplied by 106 or by 10², respectively).

Normalized indicators of emissions levels arc more
useful, however, and two of these are in common use.

Specific emissions are the mass flow rate of pollutant
per unit power output:

      sNO_x= m^. _NOx / P                  Eq (a)

      sCO = m^. _CO / P                     Eq (b)


      sHC = m^. _HC / P                  Eq (c)

      sPart = m^. _Part / P                  Eq (d)

Indicated and brake specific emissions can be defined. Units in common use are:

1- mg/J               2- g/kW.h             3- g/hp.h

Alternatively, emission rules can be normalized by the fuel flow rate. An emission index (EI) is commonly used :

                                                            m^. _NOx (g/s)
                                                                               EI _NOx = ----------------------------
                                                                                                                         m^._f (kg/s)

with similar expressions for CO, HC. and particulates.