Diesel Exhaust Gas

Hannu Jääskeläinen

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Abstract: Exhaust gas is discharged from the engine through the exhaust system. Exhaust gas properties which are important for the exhaust system design include its physical properties, exhaust gas temperature—which depends of the vehicle duty and/or test cycle—and the exhaust gas flow rate.

Exhaust Gas Properties

Engine exhaust gases are discharged into the environment through the exhaust system. The exhaust system includes several specialized components, ranging from mufflers to emission aftertreatment devices. The designer of the exhaust system and/or exhaust system components must know a number of exhaust gas properties.

Compared to the composition of air, the diesel exhaust gas contains increased concentrations of water vapor (H2O) and carbon dioxide (CO2)—the main combustion products. The concentrations of both H2O and CO2 can vary from a few percent, up to about 12% in diesel exhaust. These combustion products displace oxygen, the concentration of which can vary from a few percent, up to about 17% (compared to 21% in ambient air). The main component of diesel exhaust, just as is the case with ambient air, is nitrogen (N2). By comparison, the concentrations of diesel exhaust pollutants are very small—for the purpose of calculating the physical properties of diesel exhaust gas, they can be neglected.

As an approximation, the properties of air can be used for diesel exhaust gas calculations, Table 1 [175]. The error associated with neglecting the combustion products is usually no more than about 2%. In a more rigorous approach, corrections must be taken to account for the actual exhaust gas composition (increased H2O and CO2, decreased O2). An additional difficulty with this approach is the necessity to account for the variable exhaust gas composition, which changes with the engine load factor and the air-to-fuel ratio. Physical properties of mixtures of gases, and methods to calculate them from the properties of components can be found in the literature [363].

Table 1
Physical Properties of Air (p = 101.13 kPa)
T temperature, K; ρ density, kg/m3; h specific enthalpy, kJ/kg; s specific entropy, kJ/(kg·K); Cp specific heat at constant pressure, kJ/(kg·K); µ viscosity, 10-4 Pa·s; k thermal conductivity, W/(m·K)

In addition to the physical properties, the exhaust system designer must know certain other exhaust gas parameters. These include exhaust gas temperature—which is of special importance for the design of catalytic aftertreatment devices, as catalyst performance is a function of temperature—and exhaust gas flow rate. Both parameters are discussed in the following sections.

Engine Exhaust Back Pressure

Another important parameter is the maximum pressure drop through the exhaust system, caused by the hydraulic resistance of exhaust system components. This parameter—commonly referred to as the “engine backpressure”—requires that the engine perform additional pumping work, and has other impacts on engine operation which are discussed in the backpressure paper.