The internal volume of a compact car (Volkswagen Gold/Golf/Beetle/Mini Cooper) is approximately:
2.8 - 3.1 m
3 (cubic meters) or 3,000 Liters in Metric (100-120 Sq Cubic Feet (SCF) or 800 Gallons in US/Imperial)
1 Liter of LN2 weighs to 0.8 Kg (1.78 Lbs), and (in freezing liquid cryogenic state) the volume of a quarter US gallon.
If we decanted Liquid Nitrogen outside from its insulated Dewar container into a heavy duty Styrofoam cooler/container, 1 Liter of liquid nitrogen would quickly evaporate into pure Nitrogen (N2) gas at a dangerously rapid expansion rate of 700:1 - evaporating into 700 Liters of N2 gas, and displacing 0.696 m
3 cubic meters or 24.592 SCF of space.
10 Liters of LN2 equates to 8 Kg (~18 Lbs) in weight and 2.64 Gal in liquid volume and would evaporate to produce 7,000 Liters of N2 gas, occupying/displacing 6.46 m
3 or 245.9 SCF of space!
So if a compact car has an internal volume of roughly 3,000 liters, a small 2.5 gallon Styrofoam cooler filled with 10 Liters of liquid nitrogen would expand/evaporate to produce 7,000 liters of N2 gas! Where's all that extra gas going to go? It'll vent and escape somewhere somehow, but not without venting out and exhuming the original atmosphere out with it too altogether.
Imagine 20L of LN2? Eventually the car will become highly N2 pressurized and hissing out gas from any/every opening until all residual gas is exhausted, leaving only pure N2 still evaporating and expelling.
So 10 L of LN2 alone left to evaporate in a Styrofoam cooler in the trunk or somewhere in a small car would be enough to displace more than double the entire confined internal atmosphere/space capacity of a small car. If there is no exhaust vent or means for the N2 + Composite Atomospheric Gas to escape/exhaust from, it would simply pressurize the vehicle more than twice atmospheric pressure ...
Like putting a huge brick of dry ice into a sealed container - as the dry CO2 sublimates from solid-gas, the pressure would gradually increase until explosion (unless a hole or exhaust vent were made for for the excess gas build-up to escape from. Except Nitrogen does not activate the body's hypercapnic respiratory response alarm system, and you'd pass out in a few breaths or two without the feeling of suffocation/panic as N2 tricks your brain into thinking you're breathing in regular air. CO2 would not.
- A 20.9% oxygen composite proportion mix by volume is the standard acceptable under normal conditions humans normally breathe in.
- Below 19% Some adverse physiological effects occur, but they may not be noticeable.
- Between 15-19% Impaired thinking, coordination, attention and intellectual performance without awareness occur along with increased pulse and breathing rate.
- Between 10-12% oxygen mix high likelihood of fainting within a few minutes without warning. Respiration increases, lips turn blue, and judgment is impaired.
- At 8-10% fainting and unconsciousness begin to occur
- Around 10% Inability to move. Fainting and immediate loss of consciousness without warning. Coma.
- At or below 10% death occurs in 8 minutes at 6 percent to 8 percent oxygen; recovery is possible after 4 to 5 minutes if oxygen is restored.
You can use this calculator to calculate the quantity of LN2 and confined space metric/cubic feet to calculate various oxygen depletion rates.
https://www.nottingham.ac.uk/safetyCalculators/Calc-cryo.htm
LN2 It is easily available at any local welding ship, ice cream supply store, chemistry stock room etc...and costs less than bottled water at $1/Liter (in industrial quantities, but in realistically ~$4/L in small retail quantities). You'd only need a $120-250 cryogenic Dewar container, which you can purchase on eBay.
If you're in the UK you may have to do a lot of calling around and digging, but eventually you'll find a source for small quantities if you look hard enough. Of course, unless you're willing to go through the hassle of buying the larger MOQs at a larger scale 20-40L + and renting out a pick up truck for transport.
If you're in the US - it's a (dangerous joke) to procure. There's even a YouTube video of a guy buying LN2 using a broken Dewar without a proper cap, filming as the industrial cryo gas fill-up laughs around while he's filming (basically no questions asked) and they're even helping him load the unsecured Dewar to the back of his minivan with his little kids inside (easy recipe for disaster). I've filled up my Dewars at over 12 welding shops around the state and they don't give a damm. They're a business, and you're a paying customer. All they care about is profit.
UK folks may have it harder, but not impossible.