To reduce diesel odor its first essential that a simple but accurate method of measuring odor level be available. Various approaches were considered but found to give poor correlation to the odor intensity as assessed by professional perfumers. However a method utilizing a cold trap and measurement of the odor components' pH correlated very closely to the human olfactory sense. This method proved capable of accurately measuring even small changes in odor level and was applied to a study of the effects on odor Levels of changes in engine specifications and operating conditions. The results indicated that squish area geometry greatly affects diesel odor.