system
Given the following post, respond as if you're commenting on Reddit:
human
Engineers: Can we solve greenhouse emissions by solidifying them at the point of creation?

I'm curious if it's possible to collect the dangerous emissions of cars and lump them together into a solid rather than as greenhouse gases.  Basically, if the gaseous emissions of fuel consumption are the problem, is there a way to convert the emissions to a solid (or liquid) before expelling them out of the exhaust system and thus side-step the issue?

The ELI5 version of this question would be:  Why don't cars poop?
gpt
Our feces is all the things we don't digest. The fuel part of food gets turned into energy sources the body can use, which are used up and produce energy, water, and CO2 as byproducts.

Similarly, when gasoline is burned in the presence of oxygen, it produces energy (heat), water, and CO2 (along side with other side reactions such as NOx compounds). There is some soot which is not combusted would be the automobile equivalent of feces, but modern vehicles are quite efficient and the fuel is a relatively pure compound, so there isn't much in the way of other side products.

CO2 is a very stable molecule and doesn't react with much; and the reason we get so much energy out of burning hydrocarbons is that CO2 is such a stable, low-energy molecule and we get to use the energy difference for heat engines. You can catalyze CO2 back into carbon and oxygen, but it requires an energy input to do so, which defeats the purpose of burning it in the first place. You could conceivably burn hydrocarbons in the presence of some other oxidizer to produce a solid waste product, but then you have to carry your oxidizer with you and will probably suffer a performance loss and it will be more expensive than using atmospheric oxygen for free. You could capture the CO2, filter it out, compress it and store it as a liquid, but this also requires energy to do and is expensive.

So there are physically a number of things you can do to prevent CO2 emissions from entering the atmosphere, but they are not practical to implement.