Energy Relationship
Here we see how Heat, Work, Internal Energy, and a change in Enthalpy are all related. Together they describe a system's energy state.
Combustion Chamber
During combustion, the combustion chamber can be divided into two regions, the burned region, and the unburned region. As you can see below, as the flame propagates, more and more of the chamber is classified as burned. When combustion begins, a change in pressure is felt, throughout the chamber, moving at the speed of sound. There is little pressure change after this initial pressure jump. Flame propagates at a much slower rate than sound. Therefore, you can assume constant pressure during flame propagation.
