Why constant volume combustion gives higher energy than constant pressure?

Question

I have noticed when I have done some combustion equilibrium that enthalpy is higher when doing a constant volume combustion than pressure constant combustion.

Why is it so ? What are the reasons explaining that ?

Is it because at constant pressure combustion the variation of enthalpy is equal to heat transfert $\Delta H = Q $ whereas at constant volume combustion the variation of enthalpy is $\Delta H = Q + V\Delta p$ ?

If so that suppose that heat transfer $Q$ is the same for both constant pressure and constant volume combustions. Is that so ?

Thank you

Answer 1

Combustions of organic stuffs produce a greater number of gaseous moles than in the reactants. When done at constant pressures, a part of the energy produced by the combustion must be used for repelling the outer atmosphere. This is a work and it costs some enegy. This is why a combustion at constant pressure produces less energy than at constant volume.