Until the passage of the 1990 law, smoke and particulates were not regulated. Prior Clean Air Acts made on-road vehicles in excess of 26,000 pounds GVW virtually exempt from the emissions control program. Generally, diesel engines produce high levels of NO x emissions and significant smoke and particulate material levels. Hot Smoke results from incomplete combustion during heavy engine loads. Hot smoke also results from an over-rich fuel mixture.
Insufficient oxygen prevents the diesel fuel from completely oxidizing.
Traditional engine adjustments do nothing to hot smoke production. Fumigation reduces most hot smoke production resulting in reduced pollution and increased vehicle mpg. Cold smoke produces the same emission pollutant particulate matter. Fumigation does not abate cold smoke. Mandated reductions have forced manufacturers to reexamine the diesel engine combustion process to discover answers.