Soot and Unburnt Liquid Hydrocarbon Emissions from Diesel Engines

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Abstract
A direct injection (OI) and an indirect injection (IDI) diesel engine have been used to study the effect of engine type on soot and unburnt liquid hydrocarbons contained in diesel exhausts as a function of the air/fuel mass ratio, α. The impact of the broadening fuel specifications on diesel emissions has been investigated for both engines burning four different fuels in order to provide a relationship between fuel and particulate composition. Soot and liquid hydrocarbon emissions seem to be affected by the fuel aromatic content and volatiLity in a different way in the two engine types in dependence on the air/fuel ratio. In the D1 engine soot decreases as α increases, while liquid hydrocarbons show an opposite trend. In this engine a lower cetane number and a higher fuel volatility result in decreased soot emission since a higher amount of fuel can burn in "lean". premixed conditions, avoiding soot formation. Thus the enhancing effect of aromatic content on soot formation is masked by the corresponding decrease of the fuel cetane number. Unburnt liquid hydrocarbon emissions increase as the fuel volatility decreases and, for the fuels having the same volatility,just a slight enhancing effect of aromatic content was found. In the ID1 engine soot and liquid hydrocarbons decrease as α increases. The fuel injected can mix only with the available air in the prechamber leading to a very “rich” premixed region where partially pyrolyzed compounds and soot are massively formed independently on the fuel cetane number. In this engine either the fuel aromatic content or volatility increase promote soot and liquid hydrocarbon emissions. The chemical composition of the liquid unburnt hydrocarbons (particulate soluble organic fraction) has been determined, in terms of paraffinic, aromatic and polar compounds, for both engines, founding that it is independent on the fuel composition and on the air/fuel ratio. Slight differences between the liquid hydrocarbon chemical composition from the D1 and the 1DI engine were found and they were useful to explain the different combustion mechanism that occurs in the two engines.

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