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I don't know, run a full size modern diesel car in a garage for a few seconds then compare it to a 2 stroke running for the same amount of time. The 2 stroke will certainly smog up the place quicker.

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Yep. The scooter might output less CO2, but due to the primitive nature of the 2-stroke engine, it's basically chucking a rich aerosol of unburned fuel and lubricating oil out the exhaust. The X5, by comparison, has a catalytic converter to get rid of carbon monoxide, nitrogen oxides and unburned fuel, plus a particulate filter to get rid of soot; it will also have secondary emission control systems to stop fuel vapours escaping from the fuel tank and crankcase.

2-strokes have remarkable power-to-weight ratios and are pleasingly mechanically simple, but goodness me they're dirty.

those are just the visible gases - you can't see carbon monoxide etc.

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