system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What does DEF fluid do and why is it only for diesels?
gpt
Diesel engines burn fuel at a much higher temperature and pressure than gasoline engines. The heat allows them to be more efficient, but it also increases the rate at which oxygen reacts with nitrogen in the air to form nitrogen oxide, one of the major pollutants in car exhaust.

The big problem with nitrogen oxide is that you can't use a regular catalyst to remove it from the exhaust. And that's where the DEF comes in: It's a solution of water and urea, a combination that decomposes into highly reactive ammonia gas when you heat it up. They inject it into the hot exhaust before it goes through the cat, and inside the cat the ammonia reacts with the nitrous oxide into harmless water and nitrogen.