In a 4-stroke (am I right in thinking?) air is only drawn in a quarter of the time.

So this gives: 225 l/min, yes?

I'll try a different tack: Ignoring the size/speed of engine:

OK, I will try some maths

...

stoichiometric combustion ratio is 14.7:1

1lt weighs of petrol weighs 0.72kg

Say car gives 12km/litre (~36 miles/UK gallon)

- or 8.64km/kg of petrol

In fifth gear the car travels ~1.9km/min (70 miles an hour)

0.22 kg of petrol consumed in a minute

So 14.7 * 0.22 gives 3.23kg of oxygen needed per min.

Have I messed up somewhere?

The 2nd part of my question means:

Say all the air in the cylinder was inert except the said O2 above,

for instance recycled exhaust gas, could a 2nd injector that surrounded the fuel injector be used to put an oxygen cloud around the fuel vapour at the point of ignition?