chap22 pps Automotive technology at University of Cambridge

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Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

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OBJECTIVES:
After studying Chapter 22, the reader should
be able to:

• Prepare for ASE Engine Performance (A8)

•
•

certification test content area “C” (Air
Induction and Exhaust Systems Diagnosis and
Repair).
Discuss the purpose and function of intake
manifolds.
Explain the differences between throttle fuelinjection manifolds and port fuel-injection
manifolds.
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OBJECTIVES:

After studying Chapter 22, the reader should
be able to:

•
•

Describe the operation of the exhaust gas
recirculation system in the intake manifold.
List the materials used in exhaust manifolds
and exhaust systems.

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KEY TERMS:
Annealing
exhaust gas recirculation (EGR)
hangers • helmholtz resonator
micron
plenum

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AIR INTAKE FILTRATION
Gasoline must mix with air to 
form a combustible mixture. 
Air movement into an engine 
occurs due to vacuum being 
created in the engine. 
Like gas, air contains dirt and 
other materials which cannot
be allowed to reach the engine. 
Just as fuel filters are used to 
clean impurities from gas, an
air cleaner and filter are used
to remove contaminants from 
the air. 
Continued
Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition
By James D. Halderman

Figure 22–1 Downward movement of the piston
lowers the air pressure inside the combustion
chamber. The pressure differential between the
atmosphere and the inside of the engine forces
air into the engine.

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The three main jobs of the air cleaner and filter are to:
Clean the air before it is mixed with fuel
Silence intake noise
Act as a flame arrester in case of a backfire
The automotive engine uses about 9,000 gallons (34,069 liters) of 
air for every gallon of gasoline burned at a ratio of 14.7 to 1. 
Without proper filtering, dust and dirt in the air seriously damage 
engine parts and shorten engine life.
While abrasive particles can cause wear any place inside the engine 
where two surfaces move against each other, they first attack piston 
rings and cylinder walls. Contained in blowby gases, they pass by 
the piston rings and into the crankcase.
6

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From the crankcase, the particles circulate throughout the engine in 
the oil. Large amounts of abrasive particles in the oil can damage 
other moving engine parts.

Filter Replacement
Manufacturers recommend
cleaning or replacing air
filter elements at periodic
intervals, usually in terms
of distance driven or 
months of service. 
It is best to replace a filter 
element before it becomes 
too dirty to be effective.

Figure 22–2 Dust and dirt in the air are trapped in the
air filter so they do not enter the engine.

A dirty air filter passes contaminants that cause engine wear.
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Air Filter Elements  The paper air filter element is the most common type of filter, made 
of chemically treated paper stock that contains tiny passages in the fibers.

These passages form an indirect path for the airflow to follow. 
The airflow passes through several fiber surfaces, each of which 
traps microscopic particles of dust, dirt, and carbon.
NOTE: Do not attempt to clean a paper element filter by rapping it on a
sharp object to dislodge the dirt, or blowing compressed air through the
filter. This tends to clog the paper pores and further reduce the airflow
capability of the filter.

Most filters are capable of trapping dirt and other particles larger 
than 10 to 25 microns in size. One micron is equal to 0.000039 in.
NOTE: A person can only see objects that are 40 microns or larger in 
size. A human hair is about 50 microns in diameter.
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Remotely Mounted Air Filters and Ducts  Air cleaner and duct 
design depend on a number of factors such as the size, shape, and 
location of other engine compartment components, as well as the 
vehicle body structure.
Port fuel­injection systems 
generally use a horizontally 
mounted throttle body. 
Some systems also have
a mass airflow (MAF)
sensor between throttle
body and air cleaner. 
Remote air cleaners are 
connected by composite 
ducting. The ducting may
be rigid or flexible.

Figure 22–3 Most air filter housings are located on the
side of the engine compartment and use flexible rubber
hose to direct airflow to the throttle body of the engine.

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Always Check the Air Filter
Always inspect the air filter and the air intake system carefully during
routine service. Debris or objects deposited by animals can cause a
restriction to the airflow and can reduce engine performance.
Figure 22–4 (a) Note the discovery as the air
filter housing was opened during service on a
Pontiac Bonneville. The nuts were obviously
deposited by squirrels (or some other animal).

Figure 22–4 (b) Not only was the housing
filled with nuts, but also this air filter was
extremely dirty, indicating that this vehicle
had not been serviced for a long time.
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ENGINE AIR TEMPERATURE REQUIREMENTS
Some form of thermostatic control has been used on vehicles 
equipped with a throttle­body fuel injection to control intake air 
temperature for improved driveability.
In a throttle­body fuel injection system, fuel and air are combined 
above the throttle plate and must travel through the intake manifold 
before reaching the cylinders. Air temperature control is needed 
under these conditions to keep the gas & air mixture combined.
Heat from the exhaust manifold is retained and sent to the air 
cleaner inlet to provide heated air to the throttle body.
While the air control valve generally is located in the air cleaner 
snorkel, it may be in the air intake housing or ducting of remote air 

cleaners. Most fuel­injection systems don’t use temperature control.
11
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What Does This Tube Do? - Part 1
What is the purpose of the odd-shaped tube attached to the inlet duct
between the air filter and the throttle body, as seen here?

Figure 22–5
A resonance tube, called
a Helmholtz resonator, is
used on the intake duct
between the air filter and

the throttle body to reduce
air intake noise during
engine acceleration.

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What Does This Tube Do? - Part 2
The tube shape is designed to dampen out certain resonant frequencies
that can occur at certain engine speeds.
The length and shape of this tube are designed to absorb shock waves
that are created in the air intake system and to provide a reservoir for

the air that will then be released into the airstream during cycles of
lower pressure.
This resonance tube is often called a Helmholtz resonator, named for
the discoverer of the relationship between shape and value of frequency
Herman L. F. von Helmholtz (1821–1894) of the University of Hönizsberg
in East Prussia. The overall effect of these resonance tubes is to reduce
the noise of the air entering the engine.

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THROTTLE BODY INJECTION INTAKE

MANIFOLDS
The intake manifold is also called the inlet manifold. Smooth 
operation can only occur when each combustion chamber produces 
the same pressure as every other chamber in the engine. 
For this to be achieved, each cylinder must receive a charge 
exactly like the charge going into the other cylinders in quality 
and quantity.
The charges must have the same physical properties and the same 
air–fuel mixture. A throttle­body fuel injector forces finely 
divided droplets of liquid fuel into the incoming air to form a 
combustible air–fuel mixture. 
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For this to be achieved, each cylinder must receive a charge 
exactly like the charge going into the other cylinders in quality and 
quantity.

The charges must have the same physical properties and the same 
air–fuel mixture. A throttle­body fuel injector forces finely divided 
droplets of liquid fuel into the incoming air to form a combustible 
air–fuel mixture. 
These droplets start to evaporate 
as soon as they leave the throttle­
body injector nozzles.
The droplets stay in the charge as 
long as the charge flows at high 
velocities. These velocities may 
reach 300 feet per second. 
Figure 22–6 A throttle-body injection (TBI) unit
used on a GM V-6 engine.
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Separation of the droplets from the charge as it passes through the 
manifold occurs when the velocity drops below 50 feet per second. 
Intake charge velocities at idle speeds are often below this value. 
When separation occurs—at low engine speeds—extra fuel must be 
supplied to the charge in order to have a combustible mixture reach 
the combustion chamber. 
Manifold sizes represent a compromise. They must have a cross­
section large enough to allow charge flow for maximum power. 
The cross­section must be small enough that the flow velocities of 
the charge will be high enough to keep the fuel droplets in 
suspension. 
This is required so that equal mixtures reach each cylinder. 
Manifold cross­sectional size is one reason why engines designed 
especially for racing will not run at low engine speeds. Racing 
manifolds must be large enough to reach maximum horsepower.
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This size, however, allows the charge to move slowly, and the fuel 
will separate from the charge at low engine speeds. Fuel separation 
leads to poor accelerator response. Standard passenger vehicle 
engines are primarily designed for economy during light­load, 
partial­throttle operation. 

Their manifolds, therefore, have a much smaller cross­sectional 
area than do those of racing engines. This small size will help 
keep flow velocities of the charge high throughout the normal 
operating speed range of the engine.

Figure 22–7 Heavy fuel droplets

separate as they flow around an
abrupt bend in an intake manifold.

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Check the Intake If an Exhaust Noise
Because many V-type engines equipped with a throttle-body injection
and/or EGR valve use a crossover exhaust passage, a leak around this
passage will create an exhaust leak and noise. Always check for evidence
of an exhaust leak around the intake manifold whenever diagnosing an

exhaust sound.

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PORT FUEL-INJECTION INTAKE MANIFOLD
The size and shape of port fuel­injected engine intake manifolds 
can be optimized because the only thing in the manifold is air. 
The fuel injection is located in the intake manifold about 3 to 4 
inches (70 to 100 mm) from the intake valve. 
The runner length and shape are designed for tuning only. There is 
no need to keep an air–fuel mixture homogenized through its trip 

from TBI unit to intake valve. Long runners build low­rpm torque 
while shorter runners provide maximum high­rpm power. 
The runner length and shape are designed for tuning only. There is 
no need to keep an air–fuel mixture homogenized through its trip 
from the TBI unit to the intake valve. Typically, long runners build 
low­rpm torque while shorter runners provide maximum high­rpm 
power. 
19

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Figure 22–8 The graph shows the effect of sonic tuning of the intake manifold runners.

The longer runners increase the torque peak and move it to a lower rpm. The 600-mm-long
intake runner is about 24 inches long.

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Figure 22–9 Airflow through the large diameter upper intake manifold is distributed to smaller
diameter individual runners in the lower manifold in this two-piece manifold design.

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VARIABLE INTAKES
Many intake manifolds are designed to provide both short runners, 
best for higher engine speed power, and longer runners, best for 
lower engine speed torque.

The valve(s) that control
the flow of air through the 
passages of the intake
manifold are computer 
controlled. 
Figure 22–10 The air flowing into

the engine can be directed through
long or short runners for best
performance and fuel economy.

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PLASTIC INTAKE MANIFOLDS
Most thermoplastic intake manifolds are molded from fiberglass 
reinforced nylon. They can be cast or injection molded. Plastic 
manifolds are lighter than aluminum and can better insulate engine 
heat from fuel injectors.

Plastic manifolds
have smoother
interior surfaces
than other types,
resulting in
greater airflow.
Figure 22–11 Many plastic intake
manifolds are constructed using
many parts glued together to form
complex passages for airflow
into the engine.
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EXHAUST GAS RECIRCULATION PASSAGES
To reduce the emission of oxides of nitrogen (NOx), engines have 
been equipped with exhaust gas recirculation (EGR) valves. 
From 1973 until recently, they were used on almost all vehicles. 
Some engines use intake and exhaust valve overlap as a means of 
trapping some exhaust in the cylinder as an alternative to using an 
EGR valve. The EGR valve opens at speeds above idle on a warm 
engine. When open, the valve allows a small portion of the exhaust 
gas (5% to 10%) to enter the intake manifold. Here, the exhaust gas 
mixes with and takes the place of some of the intake charge. 
Recirculated exhaust gas is inert and does not enter the combustion 
process. The result is a lower peak combustion temperature and 
lowered production of oxides of nitrogen.
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The EGR system has means of interconnecting exhaust and intake 
manifolds. The passage is controlled by the EGR valve. On V­
type engines, the intake manifold crossover is used as a source of 
exhaust gas for the EGR system. A cast passage connects the 
exhaust crossover to the EGR valve. 
On inline­type engines,
an external tube is used
to carry exhaust gas to
the EGR valve. 
Figure 22–12
The exhaust gas recirculation
system is more efficient at
controlling Nox emissions if the
exhaust gases are cooled. A long
metal tube between the exhaust
manifold and the intake manifold
allows the exhaust gases to cool
before entering the engine.
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