Abstract
The purpose of this paper is to clarify the mechanisms of unburnt hydrocarbon (HC) emissions from a small direct - injection (DI) diesel engine. HC emission levels of small DI diesel engines are considerably higher than those of corresponding indirect - injection (IDI) diesel engines, even when sacless injection nozzles that are effective in reducing HC emissions are installed on them. In this study, analytical engine tests were performed to evaluate the relative significance of various potential sources of HC emissions from a small DI diesel engine equipped with sacless type injectors. The results suggest the major sources of HC emissions to be 1) over-lean mixture formation beyond the lean limit of combustion during the ignition delay period, 2) spray impingement on the combustion chamber wall, the phenomenon of which may be closely related to the spray induction length, 3) spray droplets of relative larger size due to poor spray atomization in the final injection stage, and 4) flame quenching or incomplete combustion in the narrow clearance between the piston top and the cylinder head.
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