Page 73 - Automotive Engineering

CH AP TER 3 .1 Emissions control

There may or may not be a central cylindrical or
conical screen around the inlet from the tank vent. The
function of such a screen is to ensure that the fumes
entering from the tank are initially spread into the carbon
particles, instead of being short circuited directly into the
induction manifold, which could cause driveability
problems when the vehicle is being operated slowly
under very light load.
The canister purge tube may be fitted with either of
several types of valve as alternatives to that in Fig. 3.1-15.
Purging takes place through both the timed and constant
purge orifices. When the depression over the diaphragm
falls and the spring lowers the valve on its seat again, the
flow continues, but at a greatly reduced rate, through the
constant purge orifice.
One of the alternatives is the Rochester Products valve
illustrated in Fig. 3.1-17. When the engine is not running,
the spring holds the valve open, allowing the carburettor
float chamber to vent into the canister. As soon as it is
started, the timed manifold depression seats the valve so
that the float chamber is then vented internally through
ducts in the carburettor body to the air intake.
Fig. 3.1-15 A purge valve canister of the open-bottom type.
The valve in Fig. 3.1-18 performs two functions: it acts
as both a carburettor vent valve, as described in the pre-
valve which is normally held open by a spring. However, vious paragraph, and a purge valve, as in Fig. 3.1-15. When
when the engine is started, closure of the throttle un-
covers a hole just downstream of it and transmits mani-
fold depression to close the diaphragm valve, the venting
of the float chamber then being effected directly to the
carburettor air intake. Yet another option is to take the
float chamber vent to a valve on the purge tube, for both
the purging and diversion of the float chamber venting in
the timed manner just described.

Fig. 3.1-17 The Rochester type 1 canister control valve functions
as a simple vapour vent valve.

Fig. 3.1-18 Canister control valve type 2 performs the functions
Fig. 3.1-16 Canisters of the closed-bottom type will not let water not only of the vapour vent valve but also of the purge valve as in
in at the base, where it could freeze and inhibit the purge action. Fig. 3.1-15.

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