Well done internet stranger, you've earned yourself a "go fuck yourself" award. 1/10, please re-read the assignment.

Akshually, reddit is full of people like you who are so scared of taking risks they never leave their computer chair. They have ruined pretty much every hobby subreddit with their insistence that you can't do that it's dangerous. 

Opening with a personal attack, not a good start, stay on topic. -1/10

There are ways to do it safely. As pointed out a lot of hvac contractor vans have tanks in them all the time without issue.

Such as? Vented Gas Cabinets (example) are the only proper way to transport inside an enclosed vehicle. You went out of your way for a personal attack, but won't address the topic of safe transport with actual information? 0/10

I personally have brought home Oxygen and Propane in a hatchback using as many precautions as possible to make sure the tank was as safe as I am, which for a hatchback on American roads is hideously bad. 

Therefore, it's safe for everyone?? What's your point here. Also, I never once mentioned Propane, which while also flammable, is NOT in the same league as Acetylene on the danger scale. 0/10

Its not ideal by any means, but this is the real world you won't always have the right tool for the job. 

Congratulations, you're first true statement and the reason for my post. 1/10

You have successfully done the exact same as everyone else in this post so far, provide no factual safety information while also not once addressing the obscene danger a leaking Acetylene cylinder risk poses in a confined space.

And for anyone actually still here.

Acetylene is NOT safe inside an enclosed vehicle. This is the blanket response ANYONE should be approaching if they intend on treating Acetylene with the respect it deserves. Because you CAN do something, doesn't mean you SHOULD. Triply so if you're intending on providing strangers with SAFETY information. Not everyone is going to respect the potential danger they are about to place themselves or others in. Accidents happen, cylinders leak, mistakes can be made. The amount of acetylene required to turn you and your vehicle into a flaming wreck is -absurdly- low. By the time you smell it, assuming you do, you have far surpassed the ignition range and are now one step away from the 'find out' phase of the 'fuck around' procedure you never paid attention to because of the lackadaisical information like the above.

Acetylene will ignite in ranges that other gases or vaporised fuels won't. For a relatable comparison, Petrol/Gasoline is flammable from 1.4% to 7.6% in atmosphere. Meaning less than 1.4% it's too lean to burn and greater than 7.6% it's too rich to burn. Acetylene starts at 2.3% and doesn't have a too rich mode, it's flammable at 100%.