A method of rapidly removing and quenching more than 75% of the cylinder content of a running diesel engine has been developed and tested. The resulting data give total nitric oxide in the cylinder versus crankangle. The data show that nitric oxide forms rapidly reaching a peak value in 20° CA or less. For higher loads and advanced timing the concentration overshoots the exhaust value by 14% before decreasing to the exhaust value. A few comparisons are made between the data and four existent models.