Module 2:Genesis and Mechanism of Formation of Engine Emissions
  Lecture 7:Mechanisms of CO and HC Formation  in SI Engines
 

 

FORMATION OF UNBURNED HYDROCARBONS

  • The unburned hydrocarbons are also called volatile organic compounds (VOCs).
  • Unburned hydrocarbon emissions result as part of the fuel inducted into the engine escapes combustion. Most of hydrocarbons in exhaust HC are the same compounds as in the fuel. A typical petroleum fuel contains 100 to 200 different hydrocarbons (10 to 20 are the major constituents) and other organic compounds. Thus, if the fuels are rich in aromatics and olefins the exhaust hydrocarbons also consist of a high fraction of aromatics and olefins, which are photo-chemically more reactive.
  • Almost 400 hundred different organic compounds are present in the engine exhaust. A number of these compounds are formed during combustion process in the engine cylinder.
  • Nearly 50% mass of organic compounds emitted in the exhaust is similar to fuel in composition. The balance 50% is composed of the chemical species which are produced by thermal cracking, pyrolysis, chemical synthesis and partial oxidation of the fuel molecules during combustion.
  • Methane is also present in significant amounts in the exhaust of gasoline and diesel engines. As methane is not photo-chemically reactive, hydrocarbon emissions now, are also measured neglecting methane emissions and  these are termed as non-methane hydrocarbons or non-methane organic gases (NMHC/NMOG).
  • Hydrocarbon concentration in the exhaust is measured by flame ionization analyzer (FIA), which is basically a carbon atom counter. The total hydrocarbon concentration measured by this method is specified   in parts per million as methane or C1 (ppmC1 or simply ppmC). It means that if the FIA is calibrated with propane (C3H8), the HC measurement reading is to be multiplied by a factor of 3 to obtain HC concentration in ppmC.

Several engine processes contribute to the unburned fuel emissions. The sources of unburned hydrocarbon emissions also vary with the engine design; whether it is a homogeneous SI or a heterogeneous CI ignition engine, whether 4-stroke or crankcase scavenged 2-stroke engine. In the crankcase scavenged, small two stroke SI engines, fuel-air mixture bypasses combustion and is directly ‘short-circuited’ to the exhaust port during scavenging period and the mixture short-circuiting is the main source of hydrocarbon emissions in these engines.