shell auto ignition

cw

hi , i am trying to solve a case in starcd where in i am using shell auto ignition (i want fuel to ignite only when it reaches certain temperature zone) . i have defined a single step reaction with C8h18 . its an exothermic reaction.

my observation is

Products are formed immediately once comes in contact with oxygen (like in PPDF method). I was expecting after the "start conbustion Time " is reached it should occur @ high temperature zones only.

I dont see reactants (fuel, scalar ) reducing. If i plot fuel and products they show high value @ same location.

Temperature is not increasing as a result of reaction. there is no increase in domain rather it reduces.

Can any body suggest any thing about it ? More over can any body suggest some method for temperature specified auto ignition where in fuel will ignite @ specified temperature.