NO Formation in SI Engines

For computation of rate of formation of NO using Eq. 2.11 data are required on;

Thermodynamic Combustion Models

Temperature of the burned gases and subsequently the equilibrium composition for a given engine and air-fuel ratio may be computed using a thermodynamic model from 

Thermodynamic combustion models for SI engines are zero-dimensional and are only time dependent. Space coordinates of the combustion chamber are not taken into account. Two types of thermodynamic combustion models are used:

• One Zone Combustion Model: It is the simplest form of thermodynamic combustion models where the burned gas after combustion is assumed to mix instantaneously with gases  burned earlier and the unburned gases so that all the cylinder gases at a given instant is uniform in composition and temperature. This model is too simplistic and unreal. It is unable to predict  Excepting gross engine performance parameters such models are unable to predict engine performance and emissions with an acceptable degree of accuracy.

• Two Zone Combustion Models: Two zone models consist of an unburned mixture zone and a second zone consisting of the burned gases. The unburned and burned zones are separated by a thin reaction zone (flame front) of negligible thickness and hence the mass of charge in the flame front can be neglected.  

  Two zone fully mixed model: This model assumes that  the burned gases produced on combustion of the  charge element during the given time period instaneously mixes with the burned gases produced earlier. Thus, all the burned gases at a given instant are uniform in temperature and composition. The unburned gases are in a separate zone and obviously at a different and much lower temperature. The pressure in the entire cylinder is however uniform. 

  Two zone unmixed model: At the extreme is an unmixed multi-zone model where no mixing occurs between the burned gases produced by the mixture elements that burn at different instants in the cycle. The unmixed model predicts that a temperature gradient exists in the burned gases. The difference in temperatures of an early burnt element (near spark plug) with a late burn element at the far end of the combustion chamber of around 400 K have been experimentally measured supporting that the burned gases are not uniform in temperature and composition  supporting the unmixed combustion model.  Although, the actual situation in the combustion chamber may be somewhere between the fully mixed and unmixed models, but the unmixed model is more realistic.