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Blue smoke and HC emissions from a direct injection diesel engine increase gradually during long idling operation (for a few hours). The extent of this increase depends on the injection nozzle specification and engine operating conditions. The accumulation of carbon deposits on the nozzle tip and combustion chamber wall will depend on these factors. Since the carbon absorbs fuel well, low volatility components can not evaporate during the combustion period and the unburned fuel emissions increase over time. This tendency changes according to fluctuations of spray shape and cylinder to cylinder fuel quantity variations.

Exhaust odor of DI diesel engines is worse than that of gasoline engines, especially at low temperatures and at idling. As the number of passenger cars with DI diesel engines is increasing worldwide because of their low CO2 emissions, odor reduction research of DI diesel engines is important. Incomplete combustion is a major cause of exhaust odor. Generally, odor worsens due to overleaning of the mixture in the cylinder and due to fuel adhering on the combustion chamber walls. To confirm this, the influences of different engine running conditions and fuel properties were investigated. The reason for the changes in exhaust odor with injection timing is evaluated by considerations of optimum positions of the maximum heat release. With n-heptane, a low boiling point fuel, odorous emissions increase because of overleaning of the mixture.