Idle and Acceleration / Deceleration Emission Controls

Fig. 1 Coasting air valve system.:

COASTING AIR VALVE

A coasting air valve (CAV), Fig. 1 is used to decrease HC emissions produced during vehicle deceleration. The CAV is activated by carburetor ported vacuum and supplies additional air into the intake manifold below the throttle plate.

Activation of the CAV is suspended by opening the solenoid valve when the engine speed sensor detects engine speeds at or below the specified value.

To improve driveability during vehicle deceleration, the opening of the CAV is delayed by the vacuum delay valve.

Fig. 13 Air switching valve system diagram:

AIR SWITCHING VALVE

The air switching valve, Fig. 13 is used to improve fuel economy and reduce HC emissions during vehicle acceleration. Manifold vacuum is used to operate the air switching valve which supplies air into the idle circuit fuel passage after the primary pilot jet. This leans the fuel mixture by reducing fuel flow through the idle circuit. When the engine speed sensor detects operation below a minimum rpm level it opens a solenoid valve that prevents operation of the ASV thus improving low speed driveability.

Fig. 14 Air switching valve On/Off switching hysteresis:

The actual engine speeds that causes ASV operation to turn on and off are slightly different to prevent surging and stalling. This effect is called hysteresis, Fig. 14.

Fig. 74 Dash pot adjustment:

DASH POT

The carburetor is equipped with a dash pot, Fig. 74 which delays the throttle valve closure to its normal idling position to reduce HC emissions during deceleration.

Fig. 12 Deceleration spark advance system:

DECELERATION SPARK ADVANCE

To reduce HC emissions during deceleration the ignition timing is advanced by a deceleration spark advance system, Fig. 12. Ignition advance is usually controlled by carburetor ported vacuum as well as engine speed. The deceleration spark advance system uses a solenoid valve and engine speed sensor to apply manifold vacuum during deceleration to increase spark advance. During deceleration the vacuum source for the vacuum advance is switched from ported vacuum to manifold vacuum. When the engine speed sensor detects engine speed at or below a specified value the vacuum source is once again switched to ported vacuum for smooth idle performance.

Fig. 85 Jet valve system operation.:

JET VALVE

In addition to the intake valve and exhaust valve, a jet valve has been provided for drawing jet air (super lean mixture or air) into the combustion chamber. The jet valve assembly consists of the jet valve, jet body and spring, and is screwed into the jet piece which is press-fitted in the cylinder head with its jet opening toward the spark plug.

A jet air passage is provided in the carburetor (or throttle body), intake manifold, and cylinder head. Air flows through the two intake openings provided near the primary throttle valve of the carburetor, goes through the passage in the intake manifold and cylinder head, then flows through the jet valve and the jet opening into the combustion chamber.

The jet valve is actuated by the same cam as the intake valve and by a common rocker arm so that the jet valve and intake valve open and close simultaneously.

On the intake stroke, Fig. 85, the fuel-air mixture flows through the intake valve port into the combustion chamber. At the same time, jet air is forced into the combustion chamber because of the pressure difference produced between the two ends of the jet air passage (between the jet air intake openings in the carburetor throttle bore and the jet opening of the jet piece) as the piston moves down.

When the throttle valve opening is small during idling or light load, a large pressure difference is produced as the piston goes down, causing jet air to flow into the combustion chamber rapidly. The jet air flowing out of the jet opening scavenges the residual gases around the spark plug and creates a good ignition environment. It also produces a strong swirl in the combustion chamber which continues throughout the compression stroke and improves flame propagation after ignition, assuring high combustion efficiency.

When the throttle valve opening is increased, more fuel-air mixture is drawn in from the intake valve port so that the pressure difference is reduced and less jet air forced in.

The jet air swirl dwindles with increase of the throttle valve opening, but the intensified inflow of normal intake fuel-air mixture can satisfactorily promote combustion.

Fig. 4 Jet air control valve:

JET AIR CONTROL VALVE

The jet air volume control system, Fig. 4 consists of a jet air control valve and a thermo valve.

During engine warm-up operation with choke valve closed the jet air control valve is opened by carburetor vacuum to decrease HC/CO emissions. This results in addditional air flowing into the jet air passage leaning out the excessively rich fuel/air mixture.

If additional jet air is supplied into the jet air passage immediately after starting a cold engine vehicle driveablity problems will result. To prevent these drivability problems the thermo valve suspends jet air valve actuation when engine coolant temperature is below 55 degrees C or 131 degrees F.