29
Jul
Since its inception more than a century ago, the internal combustion engine has gone through several iterations, with incremental improvements targeting power output and overall efficiency. Engines have been downsizing, while power and torque numbers climbing. Moreover, high fuel prices and stringent emissions regulations have meant that cars now use less fuel and release fewer toxic gases into the atmosphere.
A significant step in this direction was mandating the use of catalytic converters in the early 1980s. The parts deal with some of the most harmful toxic gases, including hydrocarbons, carbon monoxide and nitrogen oxides.
A catalytic converter is integral to all petrol and diesel cars today. It is located in the exhaust system and converts toxic combustion and exhaust gases into less harmful compounds. These emission control devices play a key role in reducing air pollution, promoting clean air quality, and reducing the effect of toxic combustion by-products on human health.
To understand the science behind how a catalytic converter works, first, let’s break down the individual parts. Converters are made of durable, ribbed stainless steel outer casings that prevent expansion and distortion. This also forms channels that protect the internal catalyst cushioning mat from direct exposure to hot combustion gases.
The main component of converters is a monolithic free-flowing substrate. This consists of a mix of precious metals and catalysts, such as platinum, palladium, and rhodium, which initiate a chemical reaction when in contact with exhaust gases. The materials are dispersed in a rough, irregularly shaped washcoat, made of aluminium, silicone or titanium oxides.
A mat cushions the substrate, helping with proper alignment, and sealing it from the body to ensure all exhaust gases go through the catalyst. To protect the undercarriage from high exhaust gas temperatures (400 Celsius) the parts are wrapped in metal heat shields. And to ensure proper fuelling, timing and engine management, oxygen sensors located before and after the catalytic converter measure oxygen levels before and after catalysis. This measures converter efficiency and provides valuable data regarding the combustion process.
Catalytic converters deal with toxic substances through a series of chemical reactions. This includes oxidation, reduction and acid-base reactions. Oxidation happens when the catalysts transform unburned hydrocarbons and carbon monoxide (from partial burns) into carbon dioxide (CO2) and water. More worrying are remnants of nitrogen oxides, toxic by-products that emerge when air is mixed fuel, and then combusted at high temperatures. These are major pollutants, and high exposure can be fatal. The compounds are broken down into nitrogen and oxygen. Both processes are examples of acid-base chemical reactions. With up to 90 per cent of harmful substances broken down, oxygen, nitrogen and water exit the exhaust tips.
There are several types of catalytic converters, They differ in construction and compatibility with different engines. The main options include three-way catalytic converters, oxidation types, and diesel particle filters (DPFs).
These are the most common types used in petrol cars. They consist of separate oxidation and reduction catalysts that convert carbon monoxide into carbon dioxide and break down unburnt fuel into carbon dioxide and water. More stringent emissions regulations have meant they also deal with nitrogen oxide and dioxide, converting these pollutants into less harmful compounds.
These are an older take on three-way variants and are effective only at burning off carbon oxides and hydrocarbons. They appear in older petrol cars and some diesel engines.
DPFs are specific to diesel engines. They’re designed to remove diesel particulates or soot by trapping particulate matter in a porous filter. Over time, this can become clogged. Burning off soot, in what is known as regeneration, involves higher than usual exhaust temperatures, achieved when driving for longer periods at high speeds, or using other heat sources to break down pollutants.
These exhaust system parts also differ in where they’re located. Since they require high temperatures to initiate effective chemical reactions, catalytic converters are mounted as near the exhaust manifold as possible. Individual makes and models will differ as to where the converter is located, with space restraints resulting in many manufacturers installing them in the undercarriage.
When looking for a replacement part, consider the following:
