Example 2.1:

Using the Eq. 2.18 based on initial rate of NO formation estimate whether during typical SI engine combustion the kinetically formed NO could reach the level of equilibrium concentrations

Solution

For the charge that burns early in the cycle the peak burned gas temperatures of 2700 K or higher could be obtained.  At full load   maximum cylinder pressure is of the order of 30 – 40 atm. Under real engine conditions, the rate of NO formation changes with time as the temperature and pressure change with time during the cycle and also the NO concentration (Eq. 2.11). However, for an approximate analysis let us assume that the average temperature and pressure of the charge elements burnt early are 2700 K and 35 atm
At T= 2700 K and P = 35 atm for an early burn  charge element

For an engine operating at 4500 rpm, it would take 10.3º CA to reach  equal to equilibrium NO concentrations. This time period is well within the typical combustion duration being in the range 30- 40º CA
For the charge elements burning later in the cycle the temperatures reached may be around 2300 K and pressure may be down to 20 atm. At these conditions,
 For a late burn element :

For a late burn element on the other hand it needs about 4.07 ms i.e., 110º CA which is too long a period in the engine cycle. Due to expansion, the burned gas temperatures would have fallen  by then to further low levels of around  1300-1400 K  and in the late burn elements the kinetically formed NO would never reach equilibrium concentrations. The NO formation in the late burn elements is frozen at a value higher than that predictted by the equilibirium considerations .This is demonstrated later in this module in Fig. 2.7.