I happen to think the the OEM'S either cut corners, or weren't really trying very hard on the economy front.
No arguments there.
If fuel was $20 per gallon they might, but there would be rioting in the street before they would get a chance to change their ways.
Just a couple of points, catalytic converters need to operate on the stoichometric point so as to convert the oxides of Nitrogen back to Nitrogen and Oxygen, the reaction is endothermic so the heat has to come from unburned fuel in the exhaust. Not enough heat and too much oxygen and they dont work.
An unfortunate trade off for a clean exhaust.
Use of propane fueled cars in Australia has noted the improvement in exhaust emissions which could be attributed to better mixing of the fuel, but the improvement isn't a lot, I cant quantify by how much.
You could try raising the fuel pressure substantially to improve atomisation, a trick used on diesels. Once again its hard to quantify the amount of improvement this represents.
In the diesel world; as most diesels went to direct injection at same time as raising fuel pressure. The associated deletion of the pre chamber represents a gain in thermal efficiency and combustion chamber shape also represented an improvement in volumetric efficiency.
Typical unitary direct injection gave a 10-15% efficiency improvement over pre chambers. IMO most of that would be the lack of a pre chamber which has a high surface are to volume ratio.
Other tricks like using propane as a supplementary fuel on diesels were meant to improve fuel burn, actual results indicated the associated efficiency gain was less than 10%
It would be an interesting experiment, sure....however I think there would be too many trade offs using things like carburetors to improve atomisation.
You might be able to come up with a good system that would work on a constant speed stationary engine. The variable speed and power output of most car engines needs the accurate control of electronics to ensure the fuel delivery amount is more accurate.
I think the manufacturers could have discovered that on a cost efficiency basis is was cheaper to control the fuel injection timing and amount (in gasoline engines) than it has been to attempt to make it vapourise any better
Gasoline Direct injection uses a higher pressure (not sure how much) which would lead to better atomisation although the exposure time is much less.
If the particulate emissions are anything to go by maybe the mixing is much worse than port fuel injection.
Its probably still more about emissions control than fuel efficiency.
As I said, could be interesting; if it were me I would start with propane as you are most of the way there as it would be injectable as a gas and something you could control with electronics. You could mathematically compensate for the lower energy density and this would give you a clue how much improvement you could expect from Gasoline if it was fully atomised when it entered the engine.