Idle and Acceleration / Deceleration Emission Controls

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.