a diesel smokes under load because there is not enough oxygen in the intake charge for a perfect stoichiometric air-fuel mix.

under ideal conditions, carbon dioxide (CO2) and water vapor (H2O) should be produced, among other things. If there is not enough oxygen, carbon monoxide (CO) and carbon particulates are produced, among other things. It is the carbon particulates that make up the "smoke", since CO is colorless.

for this reason, diesels are a good candidate for force-fed induction systems.