Catalyst Operating Conditions

The installed position of the catalyst requires fundamentally contradictory design criteria. For superior starting when the engine is cold, the overall pipe length from the cylinder head to the catalyst should be as short as possible. In contrast, the temperature of the catalyst should be kept low during high engine performance to ensure a long life. This can be achieved when the pipe is as long as possible.

Gas-Dynamic Processes. The exhaust under high pressure in the combustion chamber causes a pressure wave when the exhaust valve is opened that makes the exhaust pulsate at a high amplitude. According to acoustic theory, the pressure amplitude advances at the speed of sound through the exhaust line and is reflected at the open pipe end as a negative pressure amplitude. If it is at the exhaust valve at the right time, the negative pressure amplitude can support the charge cycle by extracting residual gas from the combustion chamber.

Real exhaust systems have different reflection sites in the exhaust line from the cylinder head to the entrance in the catalyst housing because of the individualized pipe branching.

Figure 10-42 schematically illustrates the pressure wave for cylinder 1 in an exhaust system from Fig. 10-41. After passing along the exhaust path l₁ the positive pressure curve meets the first reflection site where the pressure pulse is divided according to the design of the pipe branches and the pipe cross sections of the exhaust manifold and Y pipe. At a correspondingly sharp branching angle, a small amount of the pressure pulse with a primarily positive amplitude passes through exhaust line L₁ of cylinder 4 and is reflected from the closed exhaust valve as a mainly positive pressure pulse. Another part of the pressure pulse is reflected from the pipe branch as a vacuum pulse and returns against the main direction of flow to cylinder 1.

Fig. 10-42. Left: Schematic representation of the reflection sites. Right: Pressure characteristic in the exhaust manifold (100 mm after the exhaust valve)

The majority of the original pressure passes along exhaust path l₂ of the Y pipe up to the pipe branch at the manifold where a division of the positive pressure pulse occurs similar to the transition from the exhaust manifold to the Y pipe. The remaining portion of the original pressure pulse that passes along exhaust path l₃ is reflected at the transition to the catalyst housing as a vacuum.

The rise of the positive pressure triggered by the opening exhaust valve starts at Al. The rise in pressure to the maximum A2 depends mainly on the function of the lifting valve. The further course of the pressure curve from A2 to A4, the maximum of the reflected vacuum characteristic, depends on the design of the exhaust system. The characteristic length l_c that remains constant for the respective exhaust system independent of the working point can be calculated from the crank angle a_c that extends from A2 to A4 by considering the rpm and speed of sound.

The pressure curve from A4 to A5 is characterized by the overlapping wave movements in the exhaust system. The basic characteristic is similar for any respective exhaust systems and is nearly independent of the working point. At A5, the pressure of cylinder 4 starts to rise at the measuring sensor after passing through l₁ of cylinder 4 and l₁ of cylinder 1 up to the measuring sensor after EO of cylinder 4.

The location and characteristic curve of the pressure from A₃ to A₅ and the characteristic length l_c strongly influence the engine properties. A minimum pressure during valve overlap is always advantageous.

Fig. 10-43. Pressure characteristic in the exhaust manifold (pressure sensor 100 mm after the exhaust valve)

The characteristic length l_c essentially depends on the exhaust pipe lengths l₁ and l₂, the ratio of the Y pipe diameter to the exhaust manifold diameter d₂/d₁, as well as on the design of the transition from the exhaust manifold to the Y pipe. As the sum of l₁ + l₂ increases and the diameter ratio d₂/d₁ decreases, the characteristic length l_c increases since the main reflection site is farther from the inlet port. This is also shown by the following experimentally determined pressure characteristics of three different exhaust system variants (Fig. 10-43).