The exhaust system takes the hot gases from the engine and passes them through a system of pipes and silencers to eventually pass out into the atmosphere.
One critical component in modern cars is the catalytic convertor along with the Oxygen sensor(s) which monitor the exhaust content and by sending the appropriate signals to the onboard electronic control unit (ECU) adjust the fuel air mixture.
What is the catalytic convertor
Hot exhaust gases leave your engine and pass through your exhaust, through the catalytic converter, passing through the rear silencer(s) and out into the atmosphere. Inside the catalytic convertor, is an expanded tube containing a massive network or honeycomb of ceramics. This ceramic network has been coated with expensive metal compounds that react with the exhaust gases to eliminate harmful emissions.. Even though the exhaust is passing through the exhaust at high velocity, the molecules that coat the ceramics are able to react in milliseconds, retaining the harmful content until it's converted to something less harmful like Nitrogen, Carbon Dioxide or water vapor.
The precious metal content, of which Platinum is the most active catalyst is widely used but has a high cost factor. Palladium and Rhodium are two other precious metals often used. The use of these expensive metals is the reason for the high cost of the catalytic convertors and incidentally the reason why they are so attractive to the metal thieves. A further cost is that on later vehicles, a replacement has to be type approved and it is illegal for a supplier to fit the low cost and often innefective aftermarket units which are sometimes available on auction sites.
There are a number of factors that affect the performance of your catalytic converter. The converter operates in conjunction with your oxygen sensors to provide the cleanest possible exhaust. Whilst everything is operating as it should, particularly the engine being at its proper operating temp and the air/fuel mixture correct, the system works well. However, a faulty oxygen (O2) sensor will cause major problems with the emissions. Since the O2 sensor can alter your air/fuel mixture, it can destroy your catalytic converter. If the mix is too lean, the converter won't have the right elements to clean the exhaust. The worst problem is if the mixture is too rich, the converter will heat up to the point of melting. If this happens, it will cause a blockage in the exhaust system and the engine will lose power, attempting to push gas through a solid block. If your car burns oil or leaks coolant into the engine, these contaminants can collect on the ceramics inside the converter and cause it to clog or break up.
The compounds coating the inner structure of the catalytic convertor act on the gases without themselves being destroyed and convert these emissions into less harmful gases and water, leaving the gas that comes out of your tailpipe much better for the environment. Because the exhaust emissions have a high content of gases harmful to the environment in a straight through exhaust, the manufacturers of modern cars, under pressure from worldwide governments, devised the catalytic convertor. In the UK, the annual MoT test will examine the output and ensure that it falls within the acceptable levels for your car. A print out with the results of the test will be available for from the gas analyser.
The science of the Catalytic convertor.
Once your catalytic converter reaches its operating temperature, usually between 400 and 600 degrees Fahrenheit, the catalyst compound coating the inner ceramics start to convert by a chemical reaction, the three regulated and harmful emissions into less harmful emissions. The three harmful emissions into the exhaust are Carbon monoxide (CO), Hydrocarbons (or VOCs for Volatile Organic Compounds), and Nitrogen compounds (NOx).
Most of the used air leaving your engine is Carbon dioxide or CO2. But since combustion isn't always perfect or complete, some of the Carbon molecules only pick up one Oxygen to create carbon monoxide, a deadly, odorless gas. The catalytic converter creates a reaction between the CO and its surrounding air particles to create CO2 and H2O (water).
A Hydrocarbon is any compound made of Carbon and Hydrogen that can be burned. These emissions cover a range of harmful emissions, but they are all made up of unburned Carbon and Hydrogen. Hydrocarbons are harmful when breathed and contribute greatly to smog build up in urban areas.
Nitrogen compounds referred to as NOx have caused many an emissions test failure. NOx emissions are basically Nitrogen molecules that have combined with Oxygen and escape the engine unburned. NOx emissions cause smog and acid rain.