Originally Posted by Christopher
Carbon monoxide is a result of incomplete combustion.
That is a true statement. Consider, however, that you are starting with a Hydrocarbon molicule. If it combusts to the point of forming CO, then it has started to burn. The major way in which combustion can be stopped at that point is through a lack of available oxygen.
Diesels are designed to over supply oxygen (lean burn) under all conditions. (barring hop-up modifications, poor maintainance, malfunctions, etc.) Thus the CO production is almost nil.
There is a very small amount, as the other way of putting out the fire (cooling) is possible, but specifically minimized in the design process. To elaborate, the injector spray pattern is shaped to avoid fuel impinging on the piston top or cylinder wall. This is specifically determined in the design testing phase by looking for CO in the exhaust stream. If CO is detected, injection rate, timing, or injector spray pattern is altered to minimize it.
Having lots of extra air does provide lots of nitrogen and left-over oxygen, so diesels tend to have issues with NOx levels rather than CO.