A theoretical prediction of ignition modes in shock tubes relevant to engine conditions is proposed and validated with a wide range of shock tube experiment data. The predictive Sankaran number, Sa p, is adapted to distinguish between the weak and strong ignition modes. The non-ideal temperature and pressure rise inherently occurring in combustion devices is considered in the formulation of Sa p. The Sa p criterion is then validated by the experimental data in shock tubes for a number of fuels exhibiting negative temperature coefficient (NTC) and non-NTC behavior. It is demonstrated that the Sa p criterion can accurately predict the weak and strong ignition modes regardless of the NTC and non-NTC fuels over a wide range of pressure and temperature. Sa p = 1 serves as a reliable marker to delineate the boundary between the strong ignition (Sa p < 1 ) and weak ignition (Sa p > 1 ). As inspired by the newly-developed Sa p criterion in shock tube, it strongly suggests that the sensitivity of ignition delay variation in non-constant volume reactors such as the polytropic compression/expansion heating effect in an internal combustion engine and in a rapid compression machine (RCM) should be incorporated in evaluating an ignition criterion to better predict the ignition modes.