Spontaneous combustion
Spontaneous combustion or spontaneous ignition is a type of combustion which occurs by self-heating ( increase in temperature due to exothermic internal reactions ) , followed by thermal runaway ( self heating which rapidly accelerates to high temperatures ) and finally , autoignition . 
<triplet> Spontaneous combustion <subj> combustion <obj> subclass of <triplet> combustion <subj> exothermic internal reactions <obj> subclass of <triplet> thermal runaway <subj> exothermic internal reactions <obj> subclass of