Smoke and smell reduction are major diesel exhaust problems.

Automotive diesels operate well within the California maximums for unburned hydrocarbons and carbon dioxide. This is not true, however, for the maximum established but not yet mandatory for oxides of nitrogen. Since there is no readily available way to reduce the output of these oxides significantly by engine modifications, control devices such as catalytic mufflers, scrubbers, and dilution means would be required. The problem presented by these oxides needs careful study since some research suggests that photochemical smog might actually be increased by reductions in the concentration of nitrogen oxides.