Smoking

[stuart little]

It was said several years ago that the cleanest air you could breath on Oxford Street was to get your mouth around the exhaust pipe of one of the top of the range Saab's.

One would need a mighty big mouth!!!!!!!!!

 

[Sam Kilby]

Assume an engine when running consumes all the oxygen in the air it ingests, and:

• 2.0 litre 4 cylinder engine
• idling speed 700 rpm
• gas exhausted per revolution = 1 litre (4 cycle engine)
• gas exhausted per hour = 4.2 cu m (700 rpm x 60 mins x 1 litre)
• assume volume of garage = 45 cu m (length 6m x width 3m x height 2.5m)
• assume volume of car interior = 5.4 cu m (length 3m x width 1.5m x height 1.2m)

If the percentage of oxygen in the air falls below 20% negative symptoms start to become apparent. Below 15% - acute hypoxia and serious risk of death.

To reduce the air in the car interior to ~15% would require the displacement of ~2cu m of air. It would take ~20-30 minutes. Somewhat imprecise as it depends on air mixing from the garage interior and outside - but likely to be low.

Bit more than a bad headache.

This is exactly the kind of equation that was missing from the original fact poster guy.
The numbers dont mean anything in isolation, and without calculating the volume of a garage, car etc, and how much gas displacement needs to happen, what can you possibly do with just the numbers ?

Also without a strong scientific background you wouldn’t be able to apply standardised known and proven concepts.

For example as a non science whizz I might ask but wouldn’t fresh air just push back in from outside ?
And a scientist would say no, because there is no pressure forcing it to do so. The air will be pushed out of the garage by the gases being generated inside. A small amount would replenish or mix better with outside, usually around the perimeter due to leakage etc

I’m also not sure the maths above is correct, cos I don’t have the tools to be critical of it but it seem to me it shows the start of proving or disproving whatever claim you are making dabop is you want to get deep into the weeds you could look at other things I guess, like studies on dispersal rates of concentrated gas in an enclosed chase, perhaps even c0, co2 etc as I imagine they all behave slightly differently.

I think (and again I’m no scientist) that a lot of the dangerous gas is staying in the garage, so it’s compounding and building up quite quickly ?

 

[Fergie 307]

The other point Dabop does not seem to appreciate is that the engine produces a greater volume of exhaust gas than the volume of air it ingests in the first place.
Partly because of combustion byproducts, but to be fair this is pretty insignificant, about 1.05:1 if I recall correctly, long time since I was doing this stuff regularly!
The main factor is heat. Heat a gas and it expands, pretty basic stuff. And it is readily observable that the gas coming out of your exhaust pipe is warmer than the surrounding air.
Running at speed, say 3000rpm, the ratio by volume of exhaust gas versus air consumed is about 4:1. No idea what it is at idle, never a figure I took much interest in when tuning as it is hardly relevant.
But I think we can reasonably assume it to be around 2:1. You could come up with a more precise figure by measuring the temperature of the gas at the tailpipe and the ambient temperature.
So if you wanted to do a safe experiment you could crack the window of your garage open half an inch, set the engine running and close the door. What are you going to find if you come back after 45min?
You will see gas flowing out of the window. The simple reason being that the exhaust gas is coming out of the pipe under pressure because it is hot, so will immediately start to raise the pressure inside the building. Once the pressure inside is greater than outside, by even the smallest amount, then no fresh air from outside can enter, it is impossible.
This is the bit Dabop seems unable to understand.
So the percentage of oxygen in the building is steadily decreasing. Partly because it is being consumed by the engine, and partly because it is mixing with the exhaust and joining the steady stream of gas leaving through the window owing to the pressure differential.
After a time the oxygen concentration will fall below that required to sustain human life. Further along and it will eventually reach the level where it can no longer sustain combustion, and the engine will stop.
How long the process takes depends on the size of the engine, and the volume of the building. 2l engine small garage I would have thought half an hour or so, maybe less before you are in serious trouble.
One thing I have found is that few people really appreciate just how much air an engine uses. To burn petrol in the engine it has to be mixed with air at a ratio of 14 parts of air to each part of petrol. That 14:1 is by mass, not volume. Petrol has a mass of about .75 kg/l. Air is much less substantial, around 1.2kg/ cubic metre. So to burn 1 litre of petrol you need around 8.5 cubic metres of air, which is a lot.