Idle and Acceleration / Deceleration Emission Controls
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. 23 Tamper-proof idle mixture:
TAMPER RESISTANT IDLE MIXTURE
All carburetors have a tamper resistant idle mixture adjustment, Fig. 23. The CO setting has been made at the factory. Neither removal of the plug or tampering with the mixture screw is required in service except when a major carburetor overhaul, throttle body replacement, or high-idle CO adjustments are required by state or local inspections.
Fig. 24 Tamper-proof choke:
TAMPER RESISTANT AUTOMATIC CHOKE
All carburetors also have tamper-proof choke, Fig. 24. The choke related parts are factory adjusted. Neither removal of the choke cover or tampering with the wax-stroke adjusting screw (WAS) is required in service except when major carburetor overhaul or adjustment of choke calibration related parts is required by state or local inspection.
Fig. 14 Fuel cut-off system:
FUEL CUT-OFF SYSTEM
When the ignition key is turned off the deceleration solenoid, Fig. 14 valve cuts off fuel flow to prevent engine "run-on" (dieseling).
During certain deceleration situations the deceleration solenoid valve reduces the fuel flow to reduce HC emissions and improve fuel economy.
Under normal engine operating conditions the needle valve is held open allowing the engine to run normally.
ENRICHMENT SYSTEM
The enrichment system consists of a metering jet and a solenoid operated on/off enrichment valve which provides additional fuel for the main metering system. Activation of the on/off valve is controlled by the ECU. The duration current is supplied to the solenoid is based on engine operating condition and includes heavy acceration, heavy engine loads, cold start, or warm-up operation.
Fig. 13 Jet mixture system:
JET MIXTURE SYSTEM
The jet mixture system, Fig. 13 supplies fuel to the engine through the jet mixture solenoid valve and jet mixture passage to achieve optimum fuel/air mixture. The The solenoid valve is activated by the electronic control unit (ECU).
If the exhaust oxygen sensor detects a lean mixture condition the ECU increases the duty cycle of the jet mixture solenoid valve to enrich the mixture. If the oxygen sensor detects a rich condition the ECU decreases the duty cycle to lean the mixture.
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.