During normal driving, including some acceleration, fuel is supplied along with air to the manifold of the engine. At all such times, some of the fuel wets the walls of the intake manifold and a quantity of fuel is usually present in a liquid form. This condition exists even though the pressure in the manifold is substantially below atmospheric e.g. a vacuum of 15 to 18 inches mercury at cruising conditions.
When a vehicle driver removes his foot from the gas pedal, the throttle closes to the curb idle position and the manifold vacuum becomes much greater such as, for example, 25 inches of mercury. This sharply reduced absolute pressure causes at least a portion of a liquid fuel in the intake manifold to flash to the vapor stage. Since air flow has been sharply reduced to a curb idle quantity, and since idle fuel is still being supplied by the idle fuel passage of the carburetor, the "vaporized" fuel results in an excess amount of fuel being supplied the engine. Such an excess can be detrimental in the sense that emissions are increased during such a transient condition. Such increased emissions are primarily unburned hydrocarbons (HC) and carbon monoxide (CO). Oxides of nitrogen (NOX) probably are not affected.
Some emission control devices being considered for future automotive use require a constant air/fuel ratio delivered to the engine under all driving conditions. Rapid increases in intake manifold vacuum result in a temporarily rich air/fuel mixture due to the evaporation of fuel which has previously condensed on the walls of the manifold. It is desirable to lean this temporarily rich condition.