Schematic diagram of the design of the three-way catalytic converter |
1 | Ceramic monolith |
2 | Holding mat |
3 | Housing (insulation) |
4 | Substrate coating (washcoat) with components containing rare metals |
P49.10-2419-76
Overview
This document contains information on:
General
Function requirements
Function
General
The chemical catalytic process takes place in catalytic converters and ensures exhaust gas cleaning. The catalytic process denotes the change in the kinetics of a chemical reaction by means of a catalytic converter. The objective is to set it in motion, to accelerate it or to steer it in a favored direction. In addition to the unit in which the catalytic process takes place, a catalytic converter also denotes the substance which brings about the catalytic process.
Function requirements
Engine management ON (circuit 87M)
Engine in operation
Catalytic converters at operating temperature (approx. 300 °C)
Fuel/air ratio in exhaust gas stoichiometric
Function
Catalytic converter units are located in the exhaust system downstream of the exhaust manifold. Due to the design of the catalytic converter, it is possible to induce and accelerate a catalytic process in the exhaust gas. Through oxidation or reduction, the catalytic converter turns the harmful exhaust gas emissions hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx) into the components carbon dioxide (CO2), water (H2O) and nitrogen (N2). The catalytic process achieves an optimum conversion ratio in a stoichiometric fuel/air ratio (λ = 1), i.e. when complete combustion takes place. The catalytic process achieves a conversion rate of more than 50 percent at temperatures of between 250 °C and 300 °C.
A stoichiometric fuel/air ratio describes the mass ratio between air and fuel in combustion. For theoretically complete combustion, an air mass of 14.7 kg to 1 kg of fuel is required. The formula symbol λ is assigned to this ratio. If the incoming air mass corresponds to the air mass theoretically required for the combustion of the fuel, this produces the value = 1.
The catalytic converter consists of a stainless steel housing, a mat mount, a substrate and a catalytically active layer. The substrate is made of a cylindrical or oval, ceramic or metallic honeycomb body (monolith), which has a large number of thin-walled channels. The monolith has a substrate coating (washcoat) applied that is made from aluminum oxide (Al2O3) and catalyzers (promotors). This increases the effective surface area of the catalytic converter by 7000 times. The rare metals platinum (Pt), palladium (Pd) and rhodium (Rh) are contained in the substrate coating. Platinum triggers the chemical process of oxidation and rhodium that of reduction. Depending on the displacement of the engine and the exhaust emissions standards to be met, the quantity of rare metals is between 1 and 10 g. The monolith is mounted in a metallic housing (canning) using special layered matting. The housing of the catalytic converter provides options for connecting temperature sensors, oxygen sensors or NOx sensors in the vehicle’s exhaust system.
The optimum operating range and temperature range of a catalytic converter is 300 °C to 950 °C. The operating temperature of 300 °C or higher can be achieved through:
Installation close to the engine
Catalytic converter heater
Air-gap insulated exhaust manifold
Cancelation of the ignition timing
Secondary air injection
Spark-ignition engine: Above 950 °C, thermal aging of the catalytically active layer sets in with catalytic converters in exhaust systems.
Temperatures of higher than 1000 °C can destroy the catalytic converter. Such values are reached by misfires, for example. Here, unburnt hydrocarbons get into the catalytic converter and combust there with the stored residual oxygen.
The following catalytic converters are used in exhaust systems depending on the fuel type:
In diesel engines:
Oxidation catalytic converter
SCR catalytic converter
In spark-ignition engines:
Three-way catalytic converter
SCR catalytic converter
Additional basic functions | |||
Catalytic converter monitoring, basic function | Engine 139, 176, 177, 254, 256, 260, 264, 279, 282 | ||
Components | |||
Three-way catalytic converter, basic function | Engine 139, 176, 177, 254, 256, 260, 264, 279, 282 | ||
Oxidation catalytic converter, basic function | Engine 608 in model 118, 177, 247 | ||
SCR catalytic converter, basic function | Engine 608 in model 118, 177, 247 |