Studies on autoignition and combustion wave propagation in a spatial reactivity gradient environment constructed using the shock-converging method were conducted to obtain a new understanding of combustion modes and their mutual transition mechanism. Autoignition induced by the converging cylindrical shock wave and weak detonation can be observed by experiment and numerical simulation. Research indicates that for an appropriate spatial reactivity gradient, the velocity of the weak detonation wave can decrease and adjust to the local sound speed. This leads to the transformation of the weak detonation into the self-propagating Chapman-Jouguet (CJ) detonation. The speed of the spontaneous reaction wave characterizing the end of the chemical reaction is never less than the local sound speed; hence, there is no overdriven phenomenon throughout the weak-to-CJ detonation transition. This conforms to the fact that the overdriven phenomenon is not a necessary condition to initiate the detonation, and the combustion has higher thermal efficiency compared with isochoric combustion. Further research indicates that CJ detonation propagating in the preheated reactant can also transform into weak detonation for an appropriate spatial reactivity gradient. The initial CJ detonation degrades into a shock that decays and finally disappears. The wave-eliminating effect of the CJ-to-weak detonation transition can ensure the flow field quality and thermal efficiency of the combustion. In summary, performing a controllable weak detonation between isochoric combustion and CJ detonation in an engine has significant practical importance for improving combustion thermal efficiency and knock suppression. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.