As a result of increasing concerns over air quality, environmental legislation has led to more stringent emissions limits for diesel engines and vehicles. This has affected both engine manufactures and fuel suppliers. Whereas in the US, only the fuel requirements for heavy-duty diesel engines are of key interest, in Europe light-duty diesel applications are also important since diesel-powered passenger vehicles are accepted by customers and their market penetration has increased rapidly.
This paper gives an update of Shell's ongoing research on correlations between diesel fuel quality and particulate emissions in both heavy- and light-duty applications.
In heavy-duty testing (both steady-state and transient), sulphur is the dominant fuel property affecting particulate emissions. After sulphur correction, fuel effects are small and can best be described by a combination of cetane number and density. In steady-state testing, cetane number effects are more consistent whereas the role of density is dependent on engine technology. In transient testing, the key fuel property after sulphur correction is density.
In light-duty testing, particulate emissions from no-catalyst vehicles are best described by a non-linear density function. For catalyst vehicles, fuel quality effects are much less pronounced and, in contrast with their no-catalyst counterparts, there is no single fuel property which adequately describes particulate emissions.
An exhaust oxidation catalyst selectively decreases the particulate hydrocarbon fraction, leaving the fixed carbon fraction unaffected. This overall particulates reduction mechanism explains why particulate emissions from catalyst vehicles are less sensitive towards changes in fuel quality.
An attempt has been made to explain the differences observed between particulate emissions from heavy- and light-duty engines. It is tentatively concluded that differences originate mainly from intrinsic differences between the heavy- and light-duty test cycles.