1. In the carburated engines I am familiar with, over half of the vaporization takes place in the cylinder. I'm referring to normal operating speed here, not idle.

 

2. The primary advantage of finer atomization is enhanced droplet suspension into the cylinder. The larger droplets tend to fall out of suspension and also run into the intake runners at turns. This results in a river of fuel running along the floor of the runner. Large droplets and puddles in the combustion chamber absorb heat slower and hence don't vaporize and burn as well. Thus excess fuel is consumed, displacing working fluid and increasing hydrocarbon polution.

 

One of the ways I used to tune the fuel distribution on carburated race engines was to glue small dams made from slivers of tongue depressors to the floor of the intake runners of the richest cylinders. This would reduce the liquid fuel flow on the runner floor yet had negligable effect on the air flow because it was well down into the boundary layer.

 

Port fuel injection gets around these problems.

 

3. Assuming all of the fuel can still be vaporized and combusted, it is prefferable to vaporize all of the fuel in the cylinder. The density of Gasoline is about 700 Kg/cm (kilograms per cubic meter) in the liquid phase and about 3.5 kg/cm in the vapor phase. Using the 70 deg. F change in air temperature given by Gary for vaporization temperature drop of the air and the ideal gas equation and assuming intitial air temperature of 350 Kelvin and a 14 to 1 air fuel ratio, we get the following equations:

 

Specific volume liquid fuel mixture =  (1 / 700 + 14 / 0.968) / 15 = 0.9643 cm/kg

 

Specific volume gaseous fuel mixture = ( 1 / 3.5 + 14 / 1.092) / 15 = 0.8714 cm/kg

 

Thus, 10.7 % more fuel mixture will fit into the cylinder if the fuel is in liquid form.

 

4. With respect to detonation margin, the vaporization of fuel in the intake of a carburated engine has no benefit compared to that of an injected engine, since the fuel must still be voprized before the combustion event can be completed. (If anything, a lower temperature in the intake runner will absorb additional heat from the runner wall on the way to the cylinder, resulting in nominally higher combustion chamber temperatures) Therefore, a given mass of fuel mixture results in essentially the same peak combustion temperature whether vaporized in the runner or the cylinder. The difference in the temperature curves around the time of ignition are minimal.