To clarify further the role of the coupling between heat-release fluctuations and pressure fluctuations in producing and driving an instability, it is useful to make a comparison with the operation of an internal combustion engine (ICE). In an ICE, a higher thermal efficiency is achieved by releasing the heat via combustion at a higher pressure. Likewise, a stronger driving of a combustion instability happens when the heat is released at a higher pressure. But while high heat release and high pressure coincide (roughly) throughout the combustion chamber in an ICE, they coincide at a particular region or regions during a combustion instability. Furthermore, whereas in an ICE the high pressure is achieved through mechanical compression with a piston or a compressor, in a combustion instability high pressure regions form when a standing acoustic wave is formed. The physical mechanisms producing the above heat-release fluctuations are numerous. Nonetheless, they can be roughly divided into three groups: heat-release fluctuations due to mixture inhomogeneities; those due to hydrodynamic instabilities; and, those due to static combustion instabilities. To picture heat-release fluctuations due to mixture inhomogeneities, consider a pulsating stream of gaseous fuel upstream of a flame-holder. Such a pulsating stream may well be produced by acoustic oscillations in the combustion chamber that are coupled with the fuel-feed system. Many other causes are possible. The fuel mixes with the ambient air in a way that an inhomogeneous mixture reaches the flame, e.g., the blobs of fuel-and-air that reach the flame could alternate between rich and lean. As a result, heat-release fluctuations occur. Heat-release fluctuations produced by hydrodynamic instabilities happen, for example, in bluff-body-stabilized combustors when vortices interact with the flame (see previous figure). Lastly, heat-release fluctuations due to static instabilities are related to the mechanisms explained in the next section. Static instability or flame blow-off refer to phenomena involving the interaction between the chemical composition of the fuel-oxidizer mixture and the flow environment of the flame. To explain these phenomena, consider a flame that is stabilized with swirl, as in a gas-turbine combustor, or with a bluff-body. Moreover, say that the chemical composition and flow conditions are such that the flame is burning vigorously, and that the former is set by the fuel-oxidizer ratio (see air-fuel ratio) and the latter by the oncoming velocity. For a fixed oncoming velocity, decreasing the fuel-oxidizer ratio makes the flame change its shape, and by decreasing it further the flame oscillates or moves intermittently.