States like California have emission rules for portable generators.
Well, sort of. It isn't like the emission rules on cars though. My Honda generator was purchased in California by the original owner so it has the special California gas cap that has a charcoal filter in it and a mandated lanyard (which I removed because it got in the way) but otherwise it is identical to the 49 state model. Carbureted engine, no EGR, no catalyst, no oxygen sensor, air injection or anything like that. It would never pass the emissions standards cars have to pass.
There are emissions standards for mobile generators (effectively what that unit was), but a) they aren't required in Australia so its unlikely that was the case for the comparison presented, and b) the limits are so much higher per unit of energy/work/whatever metric that its implausible the emissions are similar. Going with same year (2019) emissions limits for Europe:
Euro 6 diesel car limits:
CO: 0.5g/km
HC+NOx: 0.17 g/km
Particulate load: 0.0045 g/km
Particulates: 6×10^11 /km
Euro V nonroad mobile machinery diesel engine 8-19kW:
CO: 6.6g/kWh
HC+NOx: 7.5 g/kWh
Particulate load: 0.40 g/kWh
Particulates: n/a
converting that for their idealised 150wh/km in the best case BMW i3:
CO: 1.0 g/km
HC+NOx: 1.1 g/km
Particulate load: 0.06 g/km
So an order of magnitude worse on pollutants, and 2x on CO.
But is it green? It’s better than putting diesel in a car, says Edwards, because the constant running rate of the gennie uses the fuel more efficiently than idling and accelerating in a car.
So it uses more fuel in an engine with looser environmental restrictions, and is somehow "greener". No, no it isn't.
A fundamentally wrong conclusion by picking a flawed comparison and failing to actually account for environmental externatilites.
Its neither more efficient or greener.
Usually that is the case though. Lower compression engines are less efficient but lower compression is required to prevent the formation of excessive nitrogen oxides unless these are handled in some other way. The flow restriction from a catalytic converter also lowers efficiency.
Theoretically, could be possible, but isn't in practice when compared to a car engine which has been pushed to the limits to keep its emissions down (they're trading off all sorts of other parameters to meet the vehicle emissions standards).
Intuitively I'd say being able to easily just throw an order of magnitude more space at catalytic conversion in a stationary generator than in a car will reduce efficiency impact somewhat. Using consumable based approaches is also less of an issue for a generator being run by people paid to run it instead of our lazy asses who don't want to be bothered with an extra pump.
Again, in theory entirely possible. But not done in practice as there is no incentive to do so.
Less efficient use of fuel in a more polluting engine, certainly not greener.
Could possibly perhaps be greener in the future if people wanted to, yes. But thats just hollow promises.