The exhaust gases emitted by a 4‑cycle spark ignition
engine, operating at 100% efficiency and running on a mixture of air (oxygen)
and any type of hydrocarbon fuel such as gasoline, would consist of water vapor
and C02 (carbon dioxide). However, the typical automotive engine does not
operate at 100% efficiency, and the air/fuel mixture when burned produces
exhaust gases which contain various pollutants. Among these are: CO (carbon
monoxide), a colorless, odorless, highly poisonous gas; HC (hydrocarbons) made
up principally of minute particles of unburned gasoline; these particles react
photochemically with sunlight to produce smog; and NOx (oxides of nitrogen)
which combines with water in the atmosphere to help produce so-called acid rain,
a dilute nitric acid which is damaging to vegetation. NOx also aids in the
production of ground-level ozone which is damaging to sensitive lung tissues,
and threatening to the health of very young people, and all those with upper
respiratory problems. Harmless gases are also emitted in the automotive exhaust
stream. Most of the nitrogen content of the air which passes through the engine
is unchanged. Any oxygen not used in the combustion process is also emitted
unchanged, along with C02 and water which also are byproducts of the combustion
process. An air/fuel ratio of 14.7:1 (known as the stoichiometric ratio) is
considered to be the ideal to achieve from an emissions point of view. This
ratio is the baseline value for measuring the oxygen content of the exhaust, and
is often referred to as Lambda value = 1.
CO is a result of incomplete combustion due to insufficient
air in the air/fuel mixture. The level of CO emissions (usually measured as a %-age
of the total) is almost entirely dependent on the balance of the air/fuel ratio.
The lowest emissions being consistent with excess air. In this condition,
further weakening the air/fuel ratio has no effect on CO levels.
When the mixture is fuel-rich, the CO emissions will be
high. Variations of ignition timing will have only a marginal effect on CO
HCs are measured in ppm (parts per million). Their presence
in the exhaust stream is a result of unburned or partly burned fuel, and engine
oil. Unburned fuel originates from areas of improper combustion in those areas
of the combustion chamber which are difficult for the sparkignited flame front
to reach. HC emissions are also caused by blow‑by, where unburned air/fuel
mixture escapes past the piston rings into the crankcase. Current engine design
restricts the amount of escape to the atmosphere by recirculating engine fumes
back into the intake manifold. Unlike CO emissions, HC emissions increase during
both rich and lean air/fuel conditions. When the air/fuel mixture is rich in
fuel, the combustion may be incomplete, therefore allowing the presence of
unburned HCs in the exhaust stream.
HC emissions increase in proportion to ignition advance,
except at very lean air/fuel ratios. Factors such as poor mixture distribution,
ignition misfires and low engine temperatures, will all cause significant
increases in HC and CO emissions.
Evaporation from the gas tank also releases HCs into the
atmosphere. Gas tank fumes are passed to a carbon canister which absorbs the
fumes and then recycles them for combustion in the engine, when conditions
permit. This process is controlled by the ECM (engine control module), via an
EVAP (evaporative emission) canister purge valve. Purge tests are now becoming
part of the enhanced emission tests in some states, as are gas tank cap pressure
NOx emissions rise and fall in a reverse pattern compared
with HC emissions. As the mixture becomes leaner, more HCs are burned. But the
free oxygen present combines with nitrogen, especially at high engine
temperatures and at high pressures in the combustion chamber.
NOx emissions also increase in proportion to ignition
advance, regardless of variations in the air/fuel ratio.
NOx emissions can be significantly reduced by the technique
of EGR (exhaust gas recirculation). Since NOx formation is encouraged by high
combustion chamber temperatures and pressures, EGR works to divert exhaust gases
from the exhaust manifold into the intake duct, either through internal
passages, or via external stainless steel piping. This addition of exhaust gases
cools the incoming charge. The process is controlled by either a vacuum-operated
EGR valve, or as in some late‑model vehicles, by an electrically-operated
To measure NOx emissions accurately requires a dynamometer
and a 5‑gas analyzer. In those states now implementing enhanced emission
programs, the measurement of NOx will be added to the usual emission test
measurements of CO and HC.
Oxygen in the exhaust stream is the result of excessive air
(leaning out) in the air/fuel ratio. As the air/fuel mixture becomes higher in
air and lower in fuel, it may be referred to as going out of stoichiometric.
Since the engine does not produce oxygen, the combustion process uses what
oxygen there is in the air taken in with the gasoline. Therefore, any oxygen
emitted with the exhaust gases will have passed straight through the engine.
Any misfire will cause the 02 content to rise sharply,
since the air is not used in the combustion process. Residual oxygen in the
exhaust, ahead of the catalytic converter, is sensed by the 02 sensor, and the
signal generated is used to adjust the air/fuel mixture back into the ideal
stoichiometric ratio of 14.7:1.
The catalytic converter is normally installed between the exhaust manifold and the first muffler. Some modern cars will incorporate a pre-catalyst into the exhaust manifold itself. It has a steel outer casing and an interior ceramic matrix. This matrix is coated (textured) with aluminum oxide to increase the surface area by several thousand times. The resultant surface area is in turn coated with a very small amount of rhodium and platinum.
The platinum coating accelerates the oxidation of HCs to CO2 and H20 (water). The rhodium coating breaks down NOx emissions to Nitrogen and Oxygen. The Oxygen combines with CO to produce CO2. Neither coating is affected by this process, but each acts as an accelerating agent to provide a suitably quick reaction as exhaust gasses flow through the converter.
The working life of a catalytic converter is "greater than 50,000 miles." There is no quantifiable upper limit on how long a catalytic converter will function correctly under ideal combustion conditions. Converter life can be drastically shortened if an engine is run with leaded fuel or contaminated by oil from a badly worn engine. Misfiring or over-rich mixtures can also lead to shortened catalytic converter life, as the high temperatures of the converter will tend to ignite unburned fuel in the exhaust stream. If these combustion temperatures exceed 2550 degrees F the ceramic core of the converter will melt.
The proper functioning of a catalytic converter can be checked using a surface temperature probe. The before-converter exhaust temperature should be about 122 degrees F cooler than the after-converter temperature. If there is little or no temperature change between the two readings, it is a sign of a damaged or contaminated catalytic core. The normal working temperature of a catalytic converter is between 752 degrees F and 1442 degrees F.