Comparison of energy release from different types of fuels
This is essentially a question concerning the trend of molecules in the same homologus series that have differing chain lengths. When we increase chain length, the number of C-C bonds increase in proportion to C-H bonds. Looking at the bond energies, the bond energy of a C-C bond is $347 kJ/mol$, while for C-H it is $414 kJ/mol$. This means to break a C-C bond compared to a C-H bond, less energy is required. Also when increasing the number of C-C bonds, while producing the same products ($CO_{2(g)} + H_2O_{(l)}$), more carbon dioxide is produced in proportion to the amount of water. The enthalpy of formation of carbon dioxide, results in it releasing more energy than that of water. Taking this into account with the lower energy requirement to break the reactants, the chain length is generally directly proportional to the energy produced. Thus, the COMPLETE combustion of Butanol would have a higher energy out put than Methanol and likewise Butane &gt; Methane.
It is important to note that, as the compounds get more energy dense, i.e release more energy upon combustion, that it becomes harder for the compound to reach complete combustion; what I mean is that more oxygen is required for the reaction to release the full energy output. If not enough oxygen is supplied to the system soot ($C_{(s)}$) &amp; Carbon Monoxide ($CO_{(g)}$) will be produced and the energy produced will be less than the energy produced in the complete combustion. (The degree to which this works, depends on how many moles of oxygen is supplied).