Abstract
To design diesel engines with low environmental impact, it is important to link health and climate-relevant soot (black carbon) emission characteristics to specific combustion conditions. The in-cylinder evolution of soot properties over the combustion cycle and as a function of exhaust gas recirculation (EGR) was investigated in a modern heavy-duty diesel engine. A novel combination of a fast gas-sampling valve and a soot particle aerosol mass spectrometer (SP-AMS) enabled online measurements of the in-cylinder soot chemistry. The results show that EGR reduced the soot formation rate. However, the late cycle soot oxidation rate (soot removal) was reduced even more, and the net effect was increased soot emissions. EGR resulted in an accumulation of polycyclic aromatic hydrocarbons (PAHs) during combustion, and led to increased PAH emissions. We show that mass spectral and optical signatures of the in-cylinder soot and associated low volatility organics change dramatically from the soot formation dominated phase to the soot oxidation dominated phase. These signatures include a class of fullerene carbon clusters that we hypothesize represent less graphitized, C5-containing fullerenic (high tortuosity or curved) soot nanostructures arising from decreased combustion temperatures and increased premixing of air and fuel with EGR. Altered soot properties are of key importance when designing emission control strategies such as diesel particulate filters and when introducing novel biofuels.
Highlights
Diesel engines are a major source of air pollution as they emit large amounts of fine particulate matter and nitrogen oxides (NOx) to the atmosphere
We show that exhaust gas recirculation (EGR) decreases both soot oxidation and soot formation rates which results in increased soot and polycyclic aromatic hydrocarbons (PAHs) emissions
The start of injection (SOI) occurred earlier when EGR was applied and the start of combustion (SOC) was delayed, which increased the time allowed for premixing of air and fuel by approximately 0.8 CAD when the inlet O2 concentration was changed from 21% to 13%
Summary
Diesel engines are a major source of air pollution as they emit large amounts of fine particulate matter and nitrogen oxides (NOx) to the atmosphere. During the soot formation phase, nonrefractory mass spectra of low volatility organics at 15% and 13% inlet O2 concentrations were, in addition to aliphatic-like fragments (CxHy>x+), composed of substantial fractions of aromatic-like fragments (CxHy≤x+), oxidized organic fragments (CxHyOz+), and PAHs. Figure 4(a−f) shows a more detailed analysis of the incylinder soot evolution and comparison with the exhaust.
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