Air –Fuel Ratio

The effect of air-fuel ratio on engine emissions has already been discussed in Lectures 3 , 5 and 7.

• Carbon monoxide results due to deficiency of oxygen during combustion and is reduced as the mixture is leaned. CO emissions are reduced to very low values as the mixture is leaned to = 0.90 – 0.95 i.e. air-fuel ratio is increased above the stoichiometric value by 5 to 10%. Further leaning of mixture shows very little additional reduction in the CO emissions.
• With increase in air fuel ratio, the initial concentration of hydrocarbons in the mixture is reduced and more oxygen is available for oxidation. Hydrocarbon emissions therefore, decrease with increase in air-fuel ratio until mixture becomes too lean when partial or complete engine misfire results which cause a sharp increase in HC emissions For < 0.8 engine may misfire more frequently thereby increasing HC emissions sharply.
• The highest burned gas temperatures are obtained for mixtures that are slightly (5 to 10 percent) richer than stoichiometric. On the other hand, there is little excess oxygen available under rich mixture conditions. As the mixture becomes lean, concentration of free oxygen increases but combustion temperature start decreasing. The interaction between these two parameters results in peak NO being obtained at about = 0.9 –0.95.

Residual Gas and EGR

Burned residual gases left from the previous cycle or part of the exhaust gas recirculated back to engine act as charge diluents. The charge dilution by recirculation of part of the exhaust gas back to the engine is called  exhaust gas recirculation (EGR). The combustion temperatures decrease due to charge dilution caused by the residual burned gases or EGR , the decrease in combution temperatures is nearly proportional to the heat capacity of the diluents as discusses earlier. The lower combustion temperatures resulting from the residual gas dilution/EGR reduces NO formation and emissions as shown on Fig 3.3.

As the EGR is increased the combustion rates become more and more slow, and combustion becomes unstable. With increase in EGR  cycle-to-cycle combustion variations increase and, more and more engine cycles having only partial combustion are observed . The frequency of partial burn cycles increases and these turn into misfired cycles at EGR rates of about more than 20%.  . In the partial burn and misfire cycles, combustion remains incomplete and results in high HC emissions. Moreover with EGR the burned gas temperatures are reduced and post-flame oxidation also reduces. Increase in HC becomes sharp as EGR increases beyond about 20 percent for a normal combustion engine.  With EGR rates of 20 percent or higher Fast burn engines due to higher flame speeds have higher burned gas temperatures and tolerate higher EGR rates before the combustion
becomes very unstable and loss in fuel efficiency becomes unacceptably high. Fast burn rates are usually obtained by use of high air swirl and increasing turbulence in the charge through use of suitable designs of intake valve port and the combustion chamber.
The amount of charge dilution or EGR is usually  limited  to below 15% due to its adverse impact on engine performance causing  power loss, high specific fuel consumption and high unburned fuel emissions.