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# Diesel-Fueled Vehicles

Except for particulate matter, exhaust emissions (particularly HC and CO) from diesel engines are quite low compared to gasoline engines. Thus much of the attention on diesel exhaust emissions has focused on particulate and NOx emissions. The particulate matter from diesel exhaust consists of soot, condensed hydrocarbons, sulphur-based compounds, and other oil-derived material. Smoke represents the immediately visible portion of particulate emissions and its opacity depends on the number and size of carbon particles present. The main cause of black smoke is poor maintenance of air fillers or fuel injectors. Fuel quality can also affect smoke emissions, the main factors being fuel density, aromatic content and certain distillation characteristics [T. J. Russell, 1989, ECMT, 1990].

Most of the techniques for reducing particulate and HC emissions from diesel engines improve the combustion efficiency and are fuel efficient but result in higher NOx levels in the exhaust. Common approaches to emission control require a series of diesel engine modifications, including fuel injection, electronic engine controls, combustion chamber modifications, air handling characteristics, reduced oil consumption, turbocharging, injection retard, exhaust gas recirculation, and reduced heat rejection [ECMT 1990].

Efficient combustion through improved mixing of air and fuel results in low emissions of hydrocarbons and smoke. Electronic control of fuelling levels and timing combined with high pressure fuel injection systems can be quite beneficial in this respect. Turbocharging increases NOx emissions but reduces particulates. Charge cooling (cooling the intake air after the turbocharges) directly reduces NOx emissions by reducing peak cycle temperatures and pressures. Injection retard is the most effective way of reducing NOx emissions but it increases fuel consumption and smoke and HC emissions, particularly under light loading. EGR can significantly reduce NOx but may double particulate emissions. Effective control of lubricating oil through engine design to prevent it from entering the engine piston rings, valve guides or turbocharges has been shown to reduce HC emissions by about 50% [ECMT 1990].

In order to achieve low levels of particulate emissions, manufacturers also have turned to the development of exhaust treatment devices, that is, devices added to clean up the exhaust after it leaves the engine. Several types of devices are being evaluated. First, a flow-through catalytic converter designed to operate on low sulfur fuel could reduce the soluble organic fraction (SOF) of particulates by as much as 90% and may also reduce the carbon portion. Second, and probably the most promising of these aftertreatment devices is the trap oxidizer control system. Trap oxidizer systems have demonstrated particulate control efficiencies in some instances of over 90%.

# Trap Oxidizer **Operating Characteristics**

The trap oxidizer system consists of a filter positioned in the exhaust stream designed to collect a significant fraction of the particulate emissions while allowing the exhaust gases to pass through the system. Since the volume of particulate matter emitted is sufficient to fill up and