Evaporative Emission Control (EEC) Systems (Automobile)

17.6.

Evaporative Emission Control (EEC) Systems

The EEC system is a method of controlling HC emis­sions by collecting fuel vapours from the fuel tank and carburettor fuel bowl vents, and directing them into an engine intake manifold. The first evaporative emission control (EEC) was introduced on 1970 cars due to California’s stringent emission law, and was used on all old and 1971 cars. A typical evaporative emission control system is illustrated in Fig. 17.19.
17.6.1.

Common Components of EEC System

The fuel tank filler caps used on cars with EEC sys­tems differ from those used on cars without EEC systems. Most caps in EEC system are incorporated with built in pressure-vacuum relief (Fig. 17.20), so that a vacuum lock may develop due to the fuel expansion or contraction. Fuel tanks are protected in different ways against fuel expan­sion and overflow, caused by heat. An overfill limiter, or temperature expansion tank, was fitted on many 1970-73 EEC systems to limit total filling of the tank. This is installed inside the fuel tank and has small holes, which open it to the fuel area. When the fuel tank appears to be completely full, it holds no more and the fuel gauge reads full although the expansion tank remains virtually empty. This offers enough space for the expansion of fuel and
collection of vapour if the vehicle is parked in the hot sun after filling the tank.
PCV valve operation in case of backfire.
Fig. 17.18. PCV valve operation in case of backfire.
Evaporative emission control system
Fig. 17.19. Evaporative emission control system
The dome shape of the upper portion of the fuel tank incorporated in some later-model cars, or the overfill limit­ing valve installed inside the vapour-liquid separator, eliminates the need for the overfill limiter tank fitted in earlier systems.
Some Ford-built cars use a combination valve, which performs the following tasks :
(i) It isolates the fuel tank from engine pressures and permits vapour to escape from the vapour separator tank to the vapour storage canister.
EEC systems fuel tank caps.
Fig 17.20. EEC systems fuel tank caps.
(ii) It vents excess fuel tank pressure to the atmosphere in case the vapour delivery line is blocked.
(Hi) It allows fresh air to be drawn into the fuel tank to fill the space created by petrol as it is used.
All EEC systems incorporate some type of liquid-vapour separator to prevent liquid fuel from reaching the engine crankcase or vapour storage canister. Some liquid-vapour separators are con­tained within the tank and use a single vapour vent line from the tank to the vapour canister. When the separator is not built into the tank (Fig. 17.21) it is usually installed on the outside of the tank or on the frame near it. In this case, vent lines extend from the tank to the separator and are arranged to vent the tank, irrespective of whether the car is on a level surface or not. Liquid fuel entering the separator returns to the tank through the shortest line.
Liquid-vapour separator is mounted separately from the tank.
17.21. Liquid-vapour separator is mounted separately from the tank.


Carburettor Venting.

Carburettors must be vented to maintain atmospheric pressure in the fuel bowl, to have the
pressure differential required for precise fuel meter­ing. Both internal and external vents are used.

Internal Vents.

Carburettors are internally vented through the balance tubes connecting the fuel bowl to the air-horn (Fig. 17.22). The internal vent or balance tube maintains atmospheric pressure on the fuel in the bowl so that the fuel flows from the bowl, through the circuit and jets, to the lower-pressure area created by the carburettor venturi. The balance tube also helps to compensate for an air pressure drop caused by a dirty air cleaner so that overly rich air-fuel mixtures are prevented. This tube also allows vapours from the fuel bowl to collect in the air cleaner when the engine is not running. This helps in controlling evaporative emissions.
 Internal carburettor vents.
Fig. 17.22. Internal carburettor vents.

External Vents.

Many carburettors have external vents for the fuel bowl. They release vapours from the fuel bowl to prevent the build-up of vapour pressure, which could cause percolation. These vents opened directly to the atmosphere on older cars having no EEC systems. On 1970 and later cars with EEC systems, the external carburettor vents are connected to the vapour storage canister by a rubber hose. External vents are often operated by carburettor linkage (Fig. 17.23) so that they are closed when the throttle is open and open when the engine is shut off or idling.

Vapour Storage.

The EEC system collects petrol vapours from the fuel tank and carburettor. These vapours are stored either in the engine crankcase, or in a charcoal-granule-filled canister, until they are fed into the engine intake system when it is running.

Engine Crankcase Storage.

The 1970-71 Chrys­ler, AMC, and some 1970 Ford-built models sold in California, used the crankcase as a vapour storage place for the first time. When the engine is started, the stored vapours are drawn from the crankcase through the PCV system into the engine, where they are burned.

Vapour Canister Storage.

This method of fuel vapour storage was introduced on all 1972 domestic cars in USA and since then is used on most cars in USA. The canister is placed under the hood (Fig. 17.21) and is filled with activated charcoal granules, which hold up to one-third their own weight in fuel vapours. The fuel tank is connected to the canister through a vent line. Carburettor fitted with external bowl vents are also vented to the canister.
Activated charcoal is used to trap vapour because of its large surface area. Each gram of activated charcoal has a surface area of 1,100 square metres. Typical canisters hold either 300 or 625 grams of charcoal (Fig. 17.24), which has a surface area equivalent to that of 80 or 165 football fields. Fuel vapour molecules are attached to the carbon surface by absorption force. Since this attaching force is not strong, the molecules are removed easily by flowing fresh air through the charcoal.
External carburettor vent (operated by a link from the carburettor throttle shaft).
Fig. 17.23. External carburettor vent (operated by a link from the carburettor throttle shaft).
A typical vapour storage canister.
Fig. 17.24. A typical vapour storage canister.
Purging the vapour storage canister.
Fig. 17.25. Purging the vapour storage canister.

Vapour Purging.

When engine is running, the stored vapours are drawn from the canister to the engine through a hose connected to either the carburettor base or the air cleaner (Fig. 17.25). This process is known as “purging” the canister. The flow rate and purge method should be such that (i) they must reactivate the charcoal, and (ii) they must have little effect on the air-fuel ratio and driveability. Purging of the canister can be accomplished in the following three ways.

Constant Purge.

In this method, the purge air rate through canister remains constant, irrespective of engine air consumption. By teeing into the PCV line at the carburettor, the vapour from the canister is drawn by using intake manifold vacuum. Even though manifold vacuum fluctuates, an orifice in the purge line maintains a constant flow rate. Figure 17.26 illustrates both a constant purge connection
and a variable purge connection. From the canister, the variable purge hose runs to the air cleaner, whereas the constant purge hose runs to the intake manifold.

Variable Purge.

In this case, the amount of purge air drawn through the canister is proportional to the amount of fresh air drawn into the engine. Hence the more air the engine sucks in, the more purge air is sucked through the canister. Purge air is drawn through the canister either by the pressure drops across the air filter, or by the velocity of the air moving through the air cleaner snorkel. Both the schemes are presented in Fig. 17.26.

Two-stage Purge.

If the air-cleaner purge flow is not sufficient, a purge valve (Fig. 17.27) is used, which is operated by ported vacuum signal. The valve opens a second passage from the canister to the intake manifold. A carburettor purge port also may be used with the constant purge system. This port is located above the high side of the carburettor throttle plate so that there is no purge flow at idle but the flow increase as the throttle opens.
 EEC system with a constant and variable purge.
Fig. 17.26. EEC system with a constant and variable purge.
Two-stage purge arrangement.
Fig. 17.27. Two-stage purge arrangement.

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