Spark Timing Emission Control System (Automobile)


Spark Timing Emission Control System

The quality of the spark produced and the timing of the spark are the two ways in which the ignition system can effect exhaust emission distinctly. The quality of a spark indicates its ability to ignite the mixture. The stronger the spark less in the chance of a misfire, which can enhance the production of hydrocarbons. The duration of the spark in particular is significant during ignition of weaker mixtures.
Spark timing affects combustion temperature. Firing the spark at precisely the right instant, generates the maximum possible amount of heat and pressure, and develops the maximum possible engine power. Unfortunately, this heat also creates a large amount of NOx exhaust emissions.
An efficient and high-temperature combustion process occurs with an advanced spark. However, NOx emission levels are also increased at higher combustion temperatures. Later in the combustion process, the exhaust gases are relatively cooler, so they do not heat up the exhaust manifold as much. But, this low exhaust temperature increases amount of HC emis­sions.
Through spark timing control the ignition timing is retarded during idle and low-speed operation, when %he air-fuel mixture is rich. Retarded timing reduces the peak combustion | temperature because ignition occurs at lower ^ cylinder pressure. This reduces the formation of ~ NOx. Also, the greatest combustion temperatures p occur during the end of combustion giving rise to jjj higher exhaust temperatures, which reduce the amount of HC in the exhaust. Therefore, proper timing of the ignition reduces exhaust emissions and meets HC and NOx standards.
Influence of ignition timing on emissions and fuel consumption.
Fig. 17.46. Influence of ignition timing on emissions and fuel consumption.
The spark timing is critical, as it is a com­promise to balance power, driveability, consump­tion and emissions. Figure 17.46 illustrates the influence of ignition timing on emissions and fuel consumption. The production of carbon monoxide is almost dependent only on fuel mixture and is not significantly affected by changes in ignition timing. Electronic and programmed ignition systems have significantly improved the emission levels of today’s engines. Chapter 16 may be referred for further details on ignition systems.
General Motors started introducing auxiliary spark timing control in some 1970 cars, but the widespread use of the system occurred in 1971 to 1974 cars. Each manufacturer developed slightly different spark timing control, suiting to engine requirement and emission standards, but all the systems and devices operate on the same principles.

Early Distributor Controls

Early emission-control system could advance or retard ignition timing under specific engine operating conditions, generally during starting, deceleration, and idle. Subsequently the deceleration vacuum advance valve (Fig. 17.47) was used in the middle to late 1960′s on Chrysler, Ford, AMC, and Pontiac products with manual transmissions. This produced extreme-
ly rich air-fuel mixture during deceleration or gear shifting in a car with a manual transmission. This valve momentarily switches the vacuum to operate the vacuum advance chamber from a low vacuum source at the carburettor to a high manifold vacuum source during deceleration and then comes back to the carburettor low vacuum source. This prevents over-retard during deceleration or gear shifting, which could produce high CO emissions in some engines.
The deceleration vacuum advance valve was most effective against CO emission, but not against HC and NOx emissions and so it was discontinued in the early 1970′s. To face stricter emission limits for HC and NOx, manufacturers developed more effective devices to control all three major pollutants.
The distributor retard solenoid was intro­duced in 1970 and 1971 on some Chrysler models with V-8 engines and automatic trans­missions. This solenoid is attached to the dis­tributor vacuum advance unit (Fig. 17.48) to control its action. The solenoid is activated by contacts installed on a carburettor throttle stop solenoid. When the throttle is closed, the carburettor solenoid is touched by the idle adjusting screw thereby completing the ground circuit. The contacts in the carburettor solenoid allow current to flow in the dis­tributor solenoid windings. The distributor solenoid’s armature is connected to the vacuum diaphragm, and hence movement of the distributor solenoid turns the breaker plate in
the retard direction. When the engine speed increases, the idle adjusting screw breaks contacts with the carburettor solenoid, so that current flow to the distributor solenoid is stopped, and normal vacuum advance comes into action.
The distributors on some 1972 and 1973 Chrysler V-8 engines uses a spark timing advance solenoid that assist better starting by advancing the spark by 7.5 degrees. The solenoid is installed in the distributor vacuum unit (Fig. 17.49) and is energised only when the engine is being cranked using power from the starter relay terminal, which also sends power to the starter solenoid. The starting ad­vance solenoid is not an emission control device. However it permits lower basic timing settings, which provide an ad­vanced timing setting for quicker starting, and thereby help in control emission.
A deceleration vacuum advance valve.
Fig. 17.47. A deceleration vacuum advance valve.
A distributor retard solenoid.
Fig. 17.48. A distributor retard solenoid.
The installation of a distributor advance solenoid.
Fig. 17.49. The installation of a distributor advance solenoid.

Principles of Recent Distributor Control Systems

The most common types of distributor control systems adopted in the early to mid-1970 are (a) vacuum delay valve, and (6) speed-and transmission-controlled timing.

Vacuum Delay Valves.

The vacuum delay valve filters the carburettor vacuum, so that it takes longer time to reach the distributor advance mechanism. Normally, vacuum must stay in the system for 15 to 30 seconds before it acts with the advance mechanism.
In one method of vacuum delay, used in Ford’s spark delay valve (SDV) system (Fig. 17.50A), vacuum passes through a sintered, or sponge-like, metal disc before reaching the distributor. Many GM engines also incorporate this type of spark delay valve. Another method of vacuum delay is used in Chrysler’s orifice spark advance control (OSAC) system (Fig. 17.50B). In this system a restriction is introduced in the vacuum line to cause a delay in vacuum build-up. These vacuum delay valves are often used along with other emission control systems and all operate on one of these two principles.
Vacuum delay valve. A. Uses sintered metal. B. Uses small orifice.
Fig. 17.50. Vacuum delay valve. A. Uses sintered metal. B. Uses small orifice.

Speed-and Transmission-controlled Timing.

These systems prevent any distributor vacuum advance mechanism to act when the car is in low gear or is travelling slowly. A solenoid controls the application of carburettor vacuum to the advance mechanism (Fig. 17.51). A switch reacts to the cars operating conditions and accordingly controls the current flow in the solenoid winding. In a manual transmission, the control switch senses shift lever position and in an automatic transmission the switch normally works from hydraulic fluid pressure.
 Transmission-controlled spark system (simplified).
Fig. 17.51. Transmission-controlled spark system (simplified).
A speed-sensing switch may be connected to the vehicle speedometer cable (Fig. 17.52). The switch signals an electronic control module when the vehicle is moving below a certain speed. The module activates a solenoid, which controls engine vacuum at the distributor.

Speed-sensing switch.
Fig. 17.52. Speed-sensing switch.
Both vacuum-delay systems and speed- and transmission-controlled systems generally use an engine temperature bypass, which permits normal vacuum advance at high and low engine temperatures. Before March, 1973, some system used an ambient temperature override switch, which was mostly discontinued. Later temperature override systems sense coolant temperature or under-hood temperature. The system using these principles is known by many different trade names, and is all somewhat different.

Electronically Controlled Timing

The ignition system must perform accurately to meet emission standards and fuel mileage requirements. Centrifugal and vacuum advance devices often cannot react fast to changes in engine operating conditions. The computer-controlled ignition systems could attain the neces­sary accuracy.
Crankshaft position signals can be tanken directly from the crankshaft.
Fig. 17.53. Crankshaft position signals can be tanken directly from the crankshaft.
In computer controlled systems, various sensors send signals to an electronic control module. These signals may include information on coolant temperature, atmospheric pressure and temperature, throttle position and rate of change of position, and crankshaft position. ICs
in the control module interpret this information and determine the proper ignition timing for each individual spark. The latest systems work with the manufacturer’s standard solid-state ignition systems. Some modifications, however, are incorporated to the standard ignition, because it no longer has to control spark timing, but many parts remain the same.
Two types of computer controlled ignition systems are in use. One type uses distributor shaft rotation for sending a crankshaft position signal to the control module. The other type receives crankshaft position information from a sensor installed near the crankshaft (Fig. 17.53). The sensor, in this type, senses the rotation of a special disc fixed to the crankshaft.
When signals are taken directly from the crankshaft, they are more accurate than those taken from the distributor shaft. The gears or chain driving mechanism for the camshaft and the gears driving mechanism for the distributor shaft have tolerances. Although these toleran­ces are actually very small, but can combine to produce a significant difference between crankshaft position and ignition timing.
The electronic timing-regulation function of all the practical systems is similar, but the electronics are fundamentally different. The Electronic Lean-Burn (ELB) system of Chrysler uses an analogue computer, while General Motor Micro-processed Sensing and Automatic Regulation (MISAR) and Ford Electronic Engine Control (EEC) use digital microprocessors.
In this kind of application, a digital computer can instantly alter timing, for example, from 1 to 65 degrees. An analogue computer has to carry out far more calculations to make such an adjustment. Since an electronic spark advance adjustment takes only few milliseconds, this fact is not practically significant in the automobile. However a digital system is more flexible to build than an analogue system.

Chrysler’s ELB System.

This system was first introduced on some V-8 engines during 1976 and since then its use has steadily increased. The spark control computer is installed on the air cleaner. Early models use two printed circuit boards, one for the ignition schedule module and other for the ignition control module. Computers for all 1978 engines and later systems have only one circuit board, which performs both jobs.
The ignition schedule module receives signals from various engine sensors and interprets them to determine the exact spark timing required. It then directs the ignition control module to advance or retard the timing accordingly.
Seven or at least six sensors (Fig. 17.54) are used to feed following information to the computer:
(a) The start pick up coil located in the distributor provides a fixed amount of advance to
the computer during cranking.
(6) The run pickup coil provides a basic timing signal so that the computer can determine
engine speed. The system for all 1978 and later systems use only one pick up coil to
supply all timing signals to the computer.
(c) The coolant temperature sensor mounted on the water pump housing signals the computer if coolant temperature becomes low.
(d) The air temperature sensor, placed inside the computer, is a thermistor. It provides a varying amount of resistance with change in air temperature. The resistance decreases with the increase in temperature.
(e) The carburettor switch sensor indicates the computer whether the engine is at idle or off idle.
 A circuit diagram of the ELB System.
Fig. 17.54. A circuit diagram of the ELB System.
The remaining two sensors are transducers, used to change mechanical movement to an electrical signal. The transducer contains a coil and a movable metal core. A small amount of voltage, applied to the coil, varies when the core moves inside the coil. The computer receives this varying voltage signal and interprets it.
(f) The throttle position transducers use a core connected to the throttle lever. Core movement signals the computer the position of the throttle plates and their rate of change of position.
(g) The vacuum transducers have a diaphragm exposed to engine vacuum. The change in the position of the diaphragm moves the core and signals the computer of variations in engine vacuum.

General Motors MISAR System.

This system (Fig. 17.55) was introduced on 1977 Oldsmobile Toronados. The control module has a microprocessor and is installed under the instrument panel in the passenger compartment. The module monitors signals from a coolant temperature sensor, a manifold vacuum sensor, a atmospheric pressure sensor, and a crankshaft speed and position sensor.
A circuit diagram of the MISAR System.
Fig. 17.55. A circuit diagram of the MISAR System.
The coolant sensor is a thermostat that provides varying amounts of resistance with changes in temperature and the resistance decreases as temperature increases.
The vacuum sensor, placed in the control module, is a solid state unit. A vacuum line connects the sensor to the intake manifold. A second line connected to the module is open in the engine compartment to provide an atmospheric pressure signal.
On 1977 MISAR systems, crankshaft speed and position signals are supplied by a rotating disc and a stationary sensor on the front of the engine. In 1978, the crankshaft speed and position sensor was moved into the distributor.

Ford EEC System.

Ford in 1978 used its electronic engine control (EEC) system on the Lincoln Versailles. The system controls both spark timing and EGR valve operation. Various sensors provide signals to a digital microprocessor mounted in the passenger compartment (Fig. 17.56).
An electromagnetic pickup fixed at the flywheel end of the crankshaft provides crankshaft position and speed signals. This also serves the purpose of the pickup coil and trigger wheel of the distributor. Other sensors such as a throttle position sensor, a coolant temperature sensor, a barometric pressure sensor, an inlet temperature sensor, a manifold pressure sensor, and an EGR valve position sensor provide various informations to the computer.
The module determines the optimum spark timing and the EGR valve operating mode. A spark timing signal is sent to the Dura-Spark solid state ignition control module.
Ford's electronic engine control system.
Fig. 17.56. Ford’s electronic engine control system.

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