Cub Cadet MTD 70 Series - Manual (Page 107)

Exhaust
101
Catalytic converter muffler
The CARB (California) compliant engines use a cata-
lytic converter muffler. This muffler also has an air injector
to help with the catalytic reaction in the muffler. The Cata-
lytic muffler can be identified by the presence of this air
injector. See Figure 8.5.
NOTE: The catalytic converter muffler is removed/
replaced by following the same procedures as the
conventional muffler.
The catalytic converter is most efficient when it receives
the exhaust of an engine running slightly above the stoi-
chiometric point. The catalytic converter breaks down the
exhaust, through chemical reactions, to reduce the nitro-
gen oxide, carbon monoxide and the hydrocarbon emis-
sions. The air injector adds oxygen to the converter so that
it can complete its chemical reaction inside the muffler.
NOTE: Stoichiometric point is defined as the point where
the mass ratio of fuel to air is chemically balanced.
For gasoline, the stoichiometric point is approxi-
mately 14.7 to 1.
The air injector works as follows:
• Between the exhaust pulses of the engine, there is
low pressure in the exhaust.
• During these periods of low pressure, air is drawn
in to the converter through a reed valve in the air
injector. See Figure 8.6.
• This air provides the oxygen needed for the cata-
lyst to process the next power pulse.
• The reed valve also stops the exhaust from by-
passing the catalytic converter and escaping
through the air injector.
The air injector has an inlet screen. This screen acts as
a filter to keep debris from entering the converter, should
this screen become blocked, the converter will not function
properly.
Figure 8.5
Air injector
Figure 8.6
Reed valve
Inlet screen