Smoke, odor, and other emissions of concern in environmental pollution from four diesel engines were studied experimentally; engine design, operational mode, and fuel characteristics were considered as variable factors influencing the emissions. The engine included a turbocharged 4-cycle unit, normally aspirated 4-cycle units, and an air-scavenged 2-cycle engine. All were direct injection, truck-type power units. Fuel characteristics differed widely among eight fuels used in the study, with principal differences occurring in sulfur and aromatic content.Results of the experimental study showed that emissions levels in all categories are markedly influenced by engine operation. Within the group of engines tested, generally high emissions of unburned hydrocarbon are associated with the 2-cycle design, high smoke levels with the 4-cycle normally aspirated engines, and high emissions of NOX and oxygenates with the turbocharged 4-cycle engine. Exhaust odor intensity levels were roughly equivalent for all engine types. Emissions in all categories were surprisingly insensitive to variation in fuel characteristics, with the exception that smoke levels were significantly reduced with addition of a smoke suppressant additive to the fuel. Neither of two odor inhibitors was found effective.