Emissions Tuning Fundamentals: Catalytic Converter Types & Operation
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Catalytic Converter Types & Operation
03.17
| 00:00 | Catalytic converters, often called cats for short, weren't the first emissions control device and they certainly aren't the only ones used today but they play a huge role in exhaust aftertreatment. |
| 00:10 | Catalysts, as the name suggests, promote certain chemical reactions which reduce harmful emissions. |
| 00:16 | Specifically, the cat turns harmful substances into less harmful ones. |
| 00:21 | Let's start with two way catalysts which focus on oxidation. |
| 00:24 | I won't go deep into the chemical reactions but two way catalysts get their name for promoting two specific behaviours. |
| 00:31 | First, oxygen is used to turn harmful carbon monoxide into less harmful carbon dioxide which is also a byproduct of human breathing. |
| 00:40 | Second, when the catalyst oxidises partially burned and unburned hydrocarbons left over from the combustion process, it turns them into carbon dioxide and water. |
| 00:49 | Oxidising hydrocarbons in a catalytic converter is effectively another round of combustion and this can generate considerable heat. |
| 00:56 | If you measure exhaust gas temperature, it's quite hot right at the exhaust port, then generally cools as it moves away from the combustion chamber. |
| 01:05 | But if you place EGT probes right before and after a catalytic converter, you'll often find cat outlet temps are higher than the inlet temps. |
| 01:13 | The temperature of the exhaust as it reaches the cat is a significant factor in catalyst temperature and this affects both its effectiveness and its reliability. |
| 01:22 | If a catalyst gets too hot, it can permanently fail. |
| 01:26 | It's imperative to avoid this and we'll talk about that more as we get into the tuning process. |
| 01:32 | Two way cats are still used to treat diesel engines today and are referred to as DOCs which stands for Diesel Oxidation Catalyst. |
| 01:40 | They used to be common on gas engines but you may have noticed two way cats don't help curb one of the major emissions that regulatory bodies are concerned with, NOx. |
| 01:49 | As oxides of nitrogen became a greater concern in the 1970s, a second type of catalytic converter was created to address it. |
| 01:57 | These new cats were fitted in addition to the existing two way cats but having extra converters was costly and harder to package. |
| 02:05 | Then in the late 70s and early 80s, three way cats were created. |
| 02:09 | They perform the two main duties of the two way oxidiser cats plus they handle NOx, hence the name three way catalyst. |
| 02:16 | Nitrogen oxide and nitrogen dioxide are turned into carbon dioxide and nitrogen. |
| 02:22 | More recently, four way catalysts were created which add the ability to clean up particulate matter but they haven't been widely adopted for road use at the time of filming this course. |
| 02:32 | Because catalysts are so important to emissions, we'll go over some additional cat topics for you in the next module but first let's recap what we've learned here. |
| 02:41 | Two way cats were used on older gas engines and are still used on many diesel engines today. |
| 02:46 | They focus on using oxygen to turn harmful emissions into more benign ones. |
| 02:51 | The two way cat focuses on cleaning up carbon monoxide and hydrocarbons. |
| 02:56 | Three way cats add on the ability to curb oxides of nitrogen and are found on most gas vehicles sold since around 1980. |
| 03:03 | Now, that you know the basic types of cats and what they do, let's discuss why they vary so much in size, shape and cost and help you determine what's the best for your projects. |
