Diesel is not lower emissions – its lower CO2
Not true unfortunately – a diesel engine will produce less CO and HC (as well as CO2) than a petrol engine but, as mol says, will produce more NOx and PM.
However the emissions limits now are so tight and so close together that it is really only CO2 that you can easily call between the two. Moving to Euro 6 cars are going to be pretty damn clean in the grand scheme of things.
Interestingly for this thread, from Euro 6 gasoline engines will also need to pass a Particulate Mass test (i.e. soot) the same as diesels – can’t see this bringing the need for DPFs unless the calibration engineers are crap, but the particulates released from diesels and gasoline engines will be, at maximum, the same from Euro 6 onwards (0.005 g/km).