An apparatus for monitoring combustion and/or controlling operation of an internal combustion engine includes a processor to receive input from a crank angle sensor and a combustion chamber pressure sensor. The processor receives a first pressure signal during a first thermodynamic volume during the compression phase and a second pressure signal during a second thermodynamic volume corresponding to a portion of expansion phase, where thermodynamic volumes are equal or equivalent. The processor determines an indication of combustion quality of each combustion event, such as poor fire or misfire, based on a comparison of the difference between the first and second pressures to a threshold value associated with each volume.