Hi.

Choking a carburetor to start an engine increases the fuel amount or ratio to air.   It is done because the ambient air is cold or because the engine metal block is cold ?

3 hours ago, Externet said:

Hi.

Choking a carburetor to start an engine increases the fuel amount or ratio to air.   It is done because the ambient air is cold or because the engine metal block is cold ?

Engine block. Cars run happily in winter on normal choke settings, once they have warmed up, 

Well, engine block and other temperatures (fuel temp for one) are the most important factors. Fuels like gasoline tend not to atomize as well at lower temperatures, instead forming droplets that can stick to surfaces instead of float around in the combustion chamber. Adding more fuel ensures that at least some properly atomized fuel fills the chamber up enough for the sparkplug to DEFINITELY ignite it (which then ignites any droplets as well, this is the cause of many types of sputtering when cold starting an engine)

Naturally aspirated (Otto) engines are usually designed to operate on a 13:1 mixture of air/fuel (for gasoline) mostly because achieving complete combustion requires around 13 units of air (by weight) per unit of fuel (again, assuming a gasoline Otto type engine).

Air temperature being lower is beneficial to internal combustion engines, technically. This is because, compared to warmer air, there are more oxygen atoms contained inside a given volume of (colder) air. On a naturally aspirated engine this effect is not really noticeable without special measuring equipment, however on supercharged engines (where the intake air is usually heated up quite a bit by the supercharger) this can make a dramatic difference. This is the reason intercoolers exist. Height above sea level does the same thing since it also affects the number of oxygen atoms for a given volume of air.

 

A group B rally car running Pikes Peak (google it) would actually lose almost a tenth of its power output (for group B this could equal 100hp!) at the top compared to the start.

 

Scientia es potentia.