The charge can burn inside the SI engine cylinder only in case The fuel air ratio falls in a particular range.That range is called as Ignition limit. In case the charge is too lean(Amount of fuel is too small in mixture than requirement) or too rich (Amount of fuel is too much than requirement).In case we want to burn too lean fuel what to do? Too lean fuel has its own advantage i.e. fuel saving (Less amount of fuel being burnt while combustion). Too lean fuel doesn’t catches fire easily inside the engine cylinder. To burn too lean fuel inside the engine stratified charge engine is used.How stratified charge engine is successful for burning too lean fuel can be learnt by the following video lecture.