The uppermost curve is the combustion temperature in degrees Celsius. The blue curve is the cylinder pressure. In this instance, it peaks at 1,536 psi. Note the spikes in the curve near the peak. This is the result of an inconsistent burn and/or a chemical reaction, such as the generation of oxides of nitrogen (NOX) absorbing a portion of the combustion heat. The lower curve is the cylinder pressure that would have occurred if the plug had not fired the charge.