AIR INJECTION INTO EXHAUST SYSTEM

Air injection is a method of reducing exhaust emissions by injecting air into each of
the exhaust ports of an engine so that air mixes with the hot exhaust and oxidises
HC and CO

The basic air injection system constitutes the following components :

(i) Air supply pump with filter.
Hi) Air manifolds and nozzles.
iiii) Anti backfire valve.
(iv) Check valve.
(v) Connecting hoses.

Fig. 17.28. Typical late-model air injection system.

Secondary air injection system, which pumps air into the engine's exhaust
system when it is cold. The oxygen contained in this air allows the remaining
fuel in the exhaust gases to combust, helping reduce emissions and warm up
the catalytic converter.

Development
The mechanism by which exhaust emissions are controlled depends on the
method of injection and the point at which air enters the exhaust system, and
has varied during the course of the development of the technology.
The first systems injected air very close to the engine, either in the cylinder
head's exhaust ports or in the exhaust manifold. These systems provided
oxygen to oxidize (burn) unburned and partially burned fuel in the exhaust
before its ejection from the tailpipe. There was significant unburned and
partially burned fuel in the exhaust of 1960s and early 1970s vehicles, and so
secondary air injection significantly reduced tailpipe emissions. However, the
extra heat of recombustion, particularly with an excessively rich exhaust
caused by misfiring or a maladjusted carburetor, tended to damage exhaust
valves and could even be seen to cause the exhaust manifold to incandesce.
As emission control strategies grew more sophisticated and effective, the
amount of unburned and partially burned fuel in the exhaust stream shrank,
and particularly when the catalytic converter was introduced, the function of
secondary air injection shifted. Rather than being a primary emission control
device, the secondary air injection system was adapted to support the efficient
function of the catalytic converter. The original air injection point became
known as the upstream injection point. When the catalytic converter is cold, air
injected at the upstream point burns with the deliberately rich exhaust so as to
bring the catalyst up to operating temperature quickly. Once the catalyst is
warm, air is injected to the downstream location — the catalytic converter
itself — to assist with catalysis of unburned hydrocarbons.

Methods of implementation
Pumped air injection
Pumped air injection systems use a vane pump called the air pump, AIR pump,
or colloquially "smog pump" turned by the engine via a belt or electric motor.
The pump's air intake is filtered by a rotating screen or the vehicle air filter to
exclude dirt particles large enough to damage the system. Air is delivered
under light pressure to the injection point(s). A check valve prevents exhaust
forcing its way back through the air injection system, which would damage the
pump and other components.
Carbureted engines' exhaust raw fuel content tends to spike when the driver
suddenly releases the throttle. To prevent the startling and potentially
damaging effects of the explosive combustion of this raw fuel, a diverter valve
is used. This valve senses the sharp decrease in intake manifold vacuum
resulting from the sudden closure of the throttle, and diverts the air pump's
outlet to atmosphere. Usually this diverted air is routed to the engine air
cleaner or to a separate silencer to muffle objectionable pump noise.

Aspirated air injection

Air injection can also be achieved by taking advantage of the negative
pressure pulses in the exhaust system at engine idle. A sensitive reed valve
assembly called the aspirator valve is placed in the air injection pumping,
which draws its air directly from the clean side of the air filter. During engine
idle, brief but periodic negative pressure pulses in the exhaust system draw air
through the aspirator valve and into the exhaust stream at the catalytic
converter. This system, marketed as Pulse Air, was used by American Motors,
Chrysler, and other manufacturers beginning in the 1970s. The aspirator
provided advantages in cost, weight, packaging, and simplicity compared to
the pump. Also, since there is no pump requiring engine power, parasitic
losses associated with the pump are eliminated. However, the aspirator
functions only at idle and so admits significantly less air within a significantly
narrower range of engine speeds compared to a pump. This system is still
used on modern motorcycle engines, e.g. the Yamaha AIS (Air Injection
System).
 THERMAL REACTORS

A thermal reactor system for internal combustion engines having a reaction chamber provided
in the cylinder head. The reaction chamber is provided immediately behind the exhaust valve
and has a predetermined capacity for inducing the oxidation of harmful constituents of the
exhaust gases.

The gas-phase oxidation of carbon monoxide slows dramatically as combustion products cool,
but the reaction does not stop entirely.

In fact, carbon monoxide and hydrocarbons continue to react in the exhaust manifold. To
oxidize the hydrocarbons homogeneously requires a holding time of order 50 ms at
temperatures in excess of 900 K. Homogeneous oxidation of carbon monoxide requires higher
temperatures, in excess of 1000 K. The oxidation rate can be enhanced with a thermal reactor-
an enlarged exhaust manifold that bolts directly onto the cylinder head. The thermal reactor
increases the residence time of the combustion products at temperatures sufficiently high that
oxidation reactions can proceed at an appreciable rate. To allow for fuel-rich operation,
secondary air may be added and mixed rapidly with combustion products.

A multiple-pass arrangement is commonly used in thermal reactors to shield the hot core of
the reactor from the relatively cold surroundings. This is critical since the

8reactions require nearly adiabatic operation to achieve significant conversion, as illus-

trated in Figure 4.24. Typically, only about a factor of 2 reduction in emission levels for
CO and hydrocarbons can be achieved even with adiabatic operation. Higher tem-
peratures and long residence times are typically required to achieve better
conversions. The heat released in the oxidation reactions can result in a substantial
temperature rise and, thereby, promote increased conversion. Removal of 1.5 % CO
results in a temper-ature rise of about 490 K (Heywood, 1976). Hence thermal reactors
with fuel-rich cyl-inder exhaust gas and secondary air addition give greater fractional
reductions in CO and hydrocarbon levels than reactors with fuel-lean cylinder exhaust.
Incomplete com-bustion in the cylinder, however, does result in reduced fuel economy.
The attainable conversion is limited by incomplete mixing of gases exhausted at
various times in the cycle and any secondary air that is added.

Temperatures of the exhaust gases of automobile spark ignition engines can vary from
600 to 700 K at idle to 1200 K during high-power operation. Most of the time the
exhaust temperature is between 700 and 900 K, too low for effective homogeneous
ox-idation. Spark retard increases the exhaust temperature, but this is accompanied by
a significant loss in efficiency.
Noncatalytic processes for vehicular emission control can yield significant im-

provements in carbon monoxide and hydrocarbon emissions. The problem of NO< emission
control is not easily alleviated with such systems. Control of NO< emissions through
noncatalytic reduction by ammonia is feasible only in a very narrow window of temperature,
toward the upper limit of the normal exhaust temperature range, making joint control of products
of incomplete combustion and NO< a severe technological challenge.

Furthermore, the need to ensure a proper flow of ammonia presents a formidable

logis-tical problem in the implementation of such technologies for control of vehicular
emis-sions.

Pressure and void coefficients in thermal reactors

Thermal reactors with liquid coolant experience reactivity changes when voids in the coolant
undergo concentration changes (even liquid-cooled reactors can have small bubbles in the
coolant). The quantity of coolant in the core decreases as liquid boils, thereby reducing
absorptions in the coolant. This is a positive reactivity feedback. Also, the slowing down of
neutrons to thermal energy decreases because of a reduced moderator concentration. This is
a negative reactivity feedback. In under-moderated reactors the net effect is a negative void
coefficient.

The present invention relates to a thermal reactor for reducing the harmful constituents of the
exhaust emission from the engine.

It is another object of the present invention to provide a thermal reactor system which is
simple in construction.

It is still another object of the present invention to provide a thermal reactor system which
may reduce the harmful constituents to the required amount without providing an exhaust gas
purification system in the exhaust system.
The present invention is characterized in that a reaction chamber is provided in the cylinder
head behind the exhaust valve and oxidation of exhaust gases occurs in the reaction
chamber at a high temperature.
In the conventional exhaust thermal reactor system, the thermal reactor is positioned in the
exhaust passage after the outlet of cylinder head. In order to maintain the exhaust gases at a
high temperature in such a system, the exahust passage and thermal reactor are coated with
insulation material. According to inventor's experiments, it has been found that sufficient
oxidation cannot be expected in the termal reactor provided in the exhaust passage, because
of the exhaust gas temperature drop in the exhaust passage. In order to elevate the exhaust
gas temperature, the spark timing is retarded and in order to maintain the temperature at a high
level sufficient to induce the oxidation, a large scale insulation must be provided on a great part
of the exhaust system which will increase the cost of the system.
catalytic converter VIVEKANAND 2K17/EC193

SHOAIB RAFIQ 2K17/EC/153

Catalytic converters help clean car’s exhaust emissions using chemical reactions with
precious metals, but thefts are on the rise...

If we don’t know what a catalytic converter is, don’t worry. The technology isn’t new and
it’s present on virtually every car on the road today but there’s no real reason why
catalytic converters should be at the forefront of any motorist’s mind, most of the
time. They run along in the background using chemical reactions to clean harmful
gasses from your car’s exhaust emissions. Unless yours breaks or, as has become
increasingly common in recent years, somebody tries to steal it, there’s very little to
worry about.

How do catalytic converters work

Catalytic converters change harmful substances in a car’s exhaust gasses, such as
carbon monoxide, nitric oxide, nitrogen dioxide and hydrocarbons, into less harmful
substances like carbon dioxide and water vapour by means of chemical reactions.

The interior of the ‘cat’ is usually filled with a honeycomb structure onto which a coating
is applied that contains a catalyst - the substance that creates a reaction with the
exhaust gasses, changing their chemical structure.

Precious metals like palladium, rhodium and platinum are commonly used as the
catalyst and these have an intrinsic value that means they’re worth salvaging and
recycling when the car is scrapped. Unfortunately, these precious metals also make
catalytic converters a target for thieves.
Catalytic converters need to work at high temperatures of up to 400 degrees to
maximise their efficiency. To achieve this optimum operating temperature the first units
were positioned close to the car’s engine but this caused its own issues and the cat has
gradually been mover further down the exhaust system, away from the engine’s heat
source.

In today’s cars the catalytic converter is found underneath the vehicle towards the
exhaust outlet, a position that makes it accessible to thieves who can cut the whole unit
out from underneath the car.

Types of catalytic converters

There are various types of catalytic converter. A simple ‘two-way’ oxidation cat works to turn
carbon monoxide (CO) to carbon dioxide (CO2) and hydrocarbons, which are basically particles
of unburnt fuel, to carbon dioxide and water. More advanced ‘three-way’ catalytic converters
are fitted to modern cars and these do the above while also reducing emissions of nitric oxide
(NO) and nitrogen dioxide (NO2) which together are more commonly known as NOx, a major
cause of localised air pollution.

History of the catalytic converter

Catalytic converters have been around since the 19th century when metal cylinders containing
filters coated in platinum, Iridium and palladium were fitted to early French motor cars in an
attempt to clean up the smoke coming out of their exhausts. The technology was first patented
by Frenchman Eugene Houdry who relocated to Los Angeles in the 1930s and founded a
company called Oxy-Catalyst, which fitted catalytic converters to industrial chimneys to combat
smog.
 STRATIFIED CHARGING ENGINE

Constructors of gasoline engines are faced with higher and higher requirements as regards to
ecological issues and an increase in engine efficiency at a simultaneous decrease in fuel
consumption. Satisfaction of these requirements is possible owing to the recognition of the
phenomena occurring inside the engine cylinder, the choice of suitable optimal parameters
of the fuel injection process, and the determination of the geometrical shapes of the
combustion chamber and the piston head. All these parameters indeed have a considerable
impact on the improvement of gasoline engines performance, and they increase their
efficiency.
The increase in the engine efficiency is basically the result of the change in the fuel supply
method, that is by proper regulation of the petrol-air mixture composition depending on the
rotational speed and load. This is why the lean mixture combustion in the gasoline engine.
Further lowering of the temperature during the development of the fuel-air mixture, which is
an outcome of the heat being taken away from the evaporated spout by the surrounding air,
makes it possible to increase the compression ratio, which translates to the increase of the
ideal efficiency.
A stratified charge engine is a type of internal combustion engine, used in automobiles, in
which the fuel is injected into the cylinder just before ignition. This allows for higher
compression ratios without "knock," and leaner air/fuel ratio than in conventional internal
combustion engines.

Conventionally, a four-stroke (petrol or

gasoline) Otto cycle engine is fuelled by
drawing a mixture of air and fuel into the
combustion chamber during the intake
stroke. This produces a homogeneous
charge: a homogeneous mixture of air and
fuel, which is ignited by a spark plug at a
predetermined moment near the top of the
compression stroke.
In a homogeneous charge system, the air/fuel ratio is kept very close to stoichiometric,
meaning it contains the exact amount of air necessary for a complete combustion of the fuel.
This gives stable combustion, but it places an upper limit on the engine's efficiency: any
attempt to improve fuel economy by running a lean mixture with a homogeneous charge
results in unstable combustion; this impacts on power and emissions, notably of nitrogen
oxides or NOx.

Period t1 – from the fuel injection moment to contact of the stream with the piston head,
including air resistance.

Period t2 – from the moment of entry into curvature of the piston head to the half-length
of the curvature, including frictional resistance between the fuel stream and the piston
head.

Period t3 – from the half-length of the piston head curvature to the moment when the fuel
stream exits the head, including both frictional and air resistances for the evaporating fuel.

Period t4 – from exit the curvature of the piston head to the moment when the fuel stream
reaches the sparking plug points.
Advantages
Direct fuelling of petrol engines offers considerable advantages over port-fuelling, a type of
fuel injection in which the fuel injectors are placed in the intake ports, giving homogeneous
charges. Powerful electronic management systems mean that there is no significant cost
penalty. With the further impetus of tightening emissions legislation, the motor industry in
Europe and North America has now almost switched completely to direct fuelling for the
new petrol engines it is introducing.

High compression ratio: First, a higher mechanical compression ratio (or, with super-charged
or turbo-charged engines, maximum combustion pressure) may be used for better
thermodynamic efficiency. Since fuel is not present in the combustion chamber until virtually
the point at which combustion is required to begin, there is no risk of pre-ignition or engine
knock.
Lean burn: The engine may also run on a much leaner overall air/fuel ratio, using stratified
charge, in which a small charge of a rich fuel mixture is ignited first and used to improve
combustion of a larger charge of a lean fuel mixture.

Disadvantages
Disadvantages include:
 Increased injector cost and complexity
 Higher fuel pressure requirements
 Carbon build-up on the back of the intake valve[citation needed] due to the lack of
gasoline passing by the intake valve to act as a cleaning agent for the valve on
traditional multi-port injection designs
 Increased NOx formation, due to the presence of local extremely rich zones. These
zones are not present in a gasoline engine, because the air and fuel is better mixed.
 Combustion management
 Combustion can be problematic if a lean mixture is present at the spark plug.
However, fuelling a petrol engine directly allows more fuel to be directed towards
the spark-plug than elsewhere in the combustion-chamber. This results in a stratified
charge: one in which the air/fuel ratio is not homogeneous throughout the
combustion-chamber, but varies in a controlled (and potentially quite complex) way
across the volume of the cylinder.
 Charge stratification can also be achieved where there is no 'in cylinder' stratification: the
inlet mixture can be so lean that it is unable to be ignited by the limited energy provided by a
conventional spark plug. This exceptionally lean mixture can, however, be ignited by the use
of a conventional mixture strength of 12-15:1, in the case of a petrol fuelled engine, being
fed into a small combustion chamber adjacent to and connected to the main lean-mixture
chamber. The large flame front from this burning mixture is sufficient to combust the
charge. It can be seen from this method of charge stratification that the lean charge is 'burnt'
and the engine utilising this form of stratification is no longer subject to ' knock' or
uncontrolled combustion.

Comparison with diesel engine

It is worth comparing contemporary directly fuelled petrol engines with direct-injection
diesel engines. Petrol can burn faster than diesel fuel, allowing higher maximum engine
speeds and thus greater maximum power for sporting engines. Diesel fuel, on the other
hand, has a higher energy density, and in combination with higher combustion pressures
can deliver very strong torque and high thermodynamic efficiency for more "normal" road
vehicles.
This comparison of 'burn' rates is a rather simplistic view. Although Petrol and Diesel engines
appear similar in operation, the two types operate on entirely different principles. In earlier
manufactured editions the external characteristics were obvious. Most petrol engines were
carburetted, sucking the fuel/air mixture into the engine, while the diesel only sucked in air
and the fuel was directly injected at high pressure into the cylinder. In the conventional four-
stroke petrol engine the spark plug commences to ignite the mixture in the cylinder at up to
forty degrees before top dead centre while the piston is still travelling up the bore. Within this
movement of the piston up the bore, controlled combustion of the mixture takes place and the
maximum pressure occurs just after top dead centre, with the pressure diminishing as the
piston travels down the bore. i.e. the cylinder volume in relation to the cylinder pressure-time
generation remains essentially constant over the combustion cycle. Diesel motor operation on
the other hand inhales and compresses air only by the motion of the piston moving to top
dead centre. At this point maximum cylinder pressure has been reached. The fuel is now
injected into the cylinder and the fuel ' burn' or expansion is started at this point by the high
temperature of the, now compressed, air. As the fuel burns it expands exerting pressure on the
piston, which in turn develops torque at the crankshaft. It can be seen that the diesel motor is
operating at constant pressure. As the gas expands the piston is also moving down the
cylinder. By this process the piston and subsequently the crank experiences a greater torque,
which is also exerted over a longer time interval than its petrol equivalent.
CVCC- CVCC is a trademark by the Honda Motor Company for an
engine with reduced automotive emissions, which stood for "Compound
Vortex Controlled Combustion". The first mention of Honda developed
CVCC technology was done by Mr. Soichiro Honda February 12, 1971, at
the Federation of Economic Organizations Hall in Otemachi, Chiyoda-ku,
Tokyo. The first engine to be installed with the CVCC approach for
testing was a single-cylinder, 300 cc version of Honda's EA engine
installed in a modified Honda N600 hatchback in January 1970. A type of
stratified charge engine, it first appeared on the 1975 ED1 engine. As
emission laws advanced and required more stringent admissible levels,
Honda abandoned the CVCC method and introduced PGM-FI, or
Programmed Fuel Injection on all Honda vehicles.
Some vehicles in Japan had a combination of electronically controlled
carburetors, called PGM-Carb on specific, transitional Honda D, E and ZC
engines. Toyota briefly used a similar technology in the mid-to-late
seventies, called TTC-V. In 2007, the Honda CVCC technology was added
to the Mechanical Engineering Heritage of Japan.

Construction and operation: Honda CVCC engines have

normal inlet and exhaust valves, plus a small auxiliary inlet valve which
provides a relatively rich air–fuel mixture to a volume near the spark
plug. The remaining air–fuel charge, drawn into the cylinder through the
main inlet valve, is leaner than normal. The volume near the spark plug
is contained by a small perforated metal plate. Upon ignition flame
fronts emerge from the perforations and ignite the remainder of the air–
fuel charge. The remaining engine cycle is as per a standard four-stroke
engine.
This combination of a rich mixture near the spark plug, and a lean
mixture in the cylinder allowed stable running, yet complete combustion
of fuel, thus reducing CO (carbon monoxide) and hydrocarbon emissions.
This method allowed the engine to burn less fuel more efficiently
without the use of an exhaust gas recirculation valve or a catalytic
converter, although those methods were installed later to further
improve emission reduction.

Advantages over previous stratified charge engines:

Honda's big advancement with CVCC was that they were able to use
carburetors and they did not rely on intake swirl. Previous versions of
stratified charge engines needed costly fuel injection systems.
Additionally, previous engines tried to increase the velocity and swirl of
the intake charge in keeping the rich and lean mixtures separated.
Honda was able to keep the charges adequately separated by
combustion chamber shape.

Early design flaw: Some of the early CVCC engines had problems
with the auxiliary valves' retaining collars vibrating loose. Once
unscrewed, motor oil would leak from the valvetrain into the pre-
combustion chamber, causing a sudden loss of power and large amounts
of smoke to flow from the exhaust pipe. These symptoms usually
indicate a failure of critical oil seals in the motor that requires expensive
repairs, although the necessary repair was quite simple. Honda
corrected this problem with metal retaining-rings that slipped over the
valves' retaining collars and prevented them from backing out of their
threads.

CVCC-II: The 1983 Honda Prelude (the first year of the second
generation of Preludes) used a CVCC design and a catalytic converter to
reduce emissions, called CVCC-II, along with two separate sidedraft
carburettors (instead of a single, progressive twin-choke carburettor).
The following year a standard cylinder head design was used and the
center carburettor (providing the rich mixture) was dropped. The Honda
City AA, introduced in November 1981, also used a CVCC-II engine called
the ER.

List of CVCC equipped engines:

ED-The ED series introduced the CVCC technology. This group displaced
1,487 cc (1.487 L; 90.7 cu in) and used an SOHC 12-valve design. Output
with a 3 barrel carburetor was 52 hp (39 kW) at 5000 rpm and 68 lb·ft
(92 N·m) at 3000 rpm.

EF-The EF was an SOHC 12-valve (CVCC) engine, displacing 1.6 L

(1598 cc). Output was 68 hp (51 kW) at 5000 rpm and 85 lb·ft (115 N·m)
at 3000 rpm.
EJ-The EJ displaced 1,335 cc (1.3 L; 81.5 cu in) and was an SOHC 12-
valve CVCC engine with a 3 barrel carburetor. 4 intake valves, 4
exhaust valves, and 4 auxiliary valves. Output was 68 hp (51 kW) at
5000 rpm and 77 lb·ft (104 N·m) at 3000 rpm.
EK-The EK was an SOHC 12-valve (CVCC) engine, displacing 1.8 L
(1,751 cc). Output varied (see below) as the engine itself was refined.
EM-The EM displaced 1,487 cc (1.487 L; 90.7 cu in) and was an
SOHC 12-valve CVCC engine. Early versions produced 52 hp (39 kW)
at 5000 rpm and 68 lb·ft (92 Nm) at 3000 rpm, while later ones upped
the output to 63 hp (47 kW) at 5000 rpm and 77 lb·ft (104 N·m) at 3000
rpm. All used a 3 barrel carburetor.
EP-The EP displaced 1,601 cc (1.601 L; 97.7 cu in) and was an SOHC
8-valve engine with a 2 barrel carburetor. Output was 90 ps (66 kW) at
5500 rpm and 13.2 kg·m (129 N·m) at 3500 rpm.
EY-EY (1598 cc) 94 PS at 5800 rpm, 13.6 kg·m at 3500 rpm.
The development of Internal Combustion Engines is one of the most revolutionary
moves in the history of mechanical engineering. They have a wide range of applications
in numerous industries and fields including automotive, marine and agricultural usage,
etc. Although IC engines provide us with the locomotive needs in the automotive
industry, it somewhat has an adverse effect on the environment. In today’s scenario,
humans are facing the issue of the global pollution crisis and the authorities worldwide
have implemented stringent norms. Poor air quality has a hazardous impact on the lives
of the people. The harmful emissions from exhaust are one of the major causes of
degrading air quality. The major emissions from exhaust pipe contain Particulate Matter
(PM), Nitrogen Oxides (NOx), Carbon Monoxide (CO) and greenhouse gases which
exploit the environment due to which the air quality index is degrading. Furthermore,
these exhaust gases can trigger acute diseases in humans like headaches, nausea,
fatigue, etc. Thus, it is an essential task for design engineers to have exhaust emissions
in compliance with norms set by government globally.
 In internal combustion engines, exhaust gas recirculation (EGR) A properly operating EGR can theoretically increase the efficiency of gasoline
is a nitrogen oxide (NOx) emissions reduction technique used in engines via several mechanisms:
petrol/gasoline and diesel engines. EGR works by recirculating a
portion of an engine's exhaust gas back to the engine cylinders. • Reduced throttling losses:- The addition of inert exhaust gas into the intake
This dilutes the O2 in the incoming air stream and provides gases system means that for a given power output, the throttle plate must be
inert to combustion to act as absorbents of combustion heat to opened further, resulting in increased inlet manifold pressure and reduced
reduce peak in-cylinder temperatures. NOx is produced in high throttling losses.
temperature mixtures of atmospheric nitrogen and oxygen that • Reduced heat rejection:- Lowered peak combustion temperatures not only
occur in the combustion cylinder, and this usually occurs at reduces NOx formation, it also reduces the loss of thermal energy to
cylinder peak pressure. Another primary benefit of external EGR combustion chamber surfaces, leaving more available for conversion to
valves on a spark ignition engine is an increase in efficiency, as mechanical work during the expansion stroke.
charge dilution allows a larger throttle position and reduces
• Reduced chemical dissociation:-
associated pumping losses. In a gasoline engine, this inert exhaust
The lower peak temperatures
displaces some amount of combustible charge in the cylinder,
result in more of the released
effectively reducing the quantity of charge available for
energy remaining as sensible
combustion without affecting the air : fuel ratio. In a diesel engine,
energy near Top Dead Centre
the exhaust gas replaces some of the excess oxygen in the pre-
(TDC), rather than being bound up
combustion mixture Because NOx forms primarily when a mixture
(early in the expansion stroke) in
of nitrogen and oxygen is subjected to high temperature, the
the dissociation of combustion
lower combustion chamber temperatures caused by EGR reduces
products. This effect is minor
the amount of NOx the combustion generates. Gases re-
compared to the first two.
introduced from EGR systems will also contain near equilibrium
concentrations of NOx and CO; the small fraction initially within
the combustion chamber inhibits the total net production of these
and other pollutants when sampled on a time average. Chemical
properties of different fuels limit how much EGR may be used. For Fig. EGR System
example methanol is more tolerant to EGR than gasoline.