Maintenance, Car Care
191
How Emission Control Works When an automobile engine is running, it uses en-
ergy generated through the combustion of a mix-
ture of air and fuel. Depending on whether a car is
driven fast or slowly or whether the engine is cold
or hot, some of the fuel (hydrocarbons) may not
be burned completely, but may be discharged into
the engine crankcase or exhaust system. Addition-
al hydrocarbons may enter the atmosphere
through evaporation of fuel from the fuel tank.
These hydrocarbons (HC), when released into the
air, contribute to undesirable pollution.
In addition, carbon monoxide (CO) and oxides of
nitrogen (NOx) contribute to engine emissions.
They, too, are formed during the combustion proc-
ess and discharged into the exhaust system.
To reduce these pollutants, your Porsche is
equipped with a precisely calibrated fuel injection
system to assure a finely balanced air/fuel mixture
under all operating conditions. Oxygen sensor The oxygen sensor, installed in the exhaust pipe
continuously senses the oxygen content of the ex-
haust and signals the information to an electronic
control unit. The control unit corrects the air/fuel
ratio, so the engine always receives an accurately
metered air/fuel mixture.
Crankcase ventilation Through crankcase ventilation, undesirable emis-
sions from the engine crankcase are not permit-
ted to reach the outside atmosphere. These emis-
sions are recirculated from the crankcase to the
air intake system. From here the emissions mix
with the intake air and are later burned in the en-
gine. Catalytic converters The catalytic converters are efficient ”clean-up“
devices built into the exhaust system of the vehi-
cle. The catalytic converters burn the undesirable
pollutants in the exhaust gas before it is released
into the atmosphere.
The exclusive use of unleaded fuel is critical-
ly important for the life of the catalytic con-
verters. Therefore, only unleaded fuel must
be used.
The catalytic converters will be damaged by:
– push or tow starting the vehicle,
– misfiring of the engine,
– turning off the ignition while the vehicle is mov-
ing or
– driving until the fuel tank is completely empty,
– by other unusual operating conditions. fDo not continue to operate your vehicle under
these conditions, since raw fuel might reach
the catalytic converters. This could result in
overheating of the converters. Federal law pro-
hibits use of leaded fuel in this car.
Fuel Evaporation Control Fuel tank venting The evaporation chamber and the carbon canister
prevent fuel from escaping to the atmosphere at
extreme high outside temperatures, when driving
abruptly around curves and when the car is parked
at an incline or in any other nonlevel position. Vapor control system and storage When the fuel tank is filled, vapors are collected in
the evaporation chamber by a vent line leading the
vapors to the carbon canister where they are
stored as long as the engine does not run. Purge system fWhen the engine is running, the fuel vapors
from the canister will be mixed with fresh air
from the ambient air of the canister. This mix-
ture will be directed to the intake air housing by
the tank vent line, mixed with the intake air and
burned during normal combustion.
