COURSE---Performing Engine Top Overhauling on Bench

GROUP---SEM (42)

INTRODUCTION TO ENGINE PARTS IDENTIFICATION

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CYLINDER HEAD
The cylinder head encloses one end of the engine cylinders and the
upper end of the combustion chambers. Where as the piston head and rings
form the lower end.
Cylinder heads are cast in one piece from gray iron alloy (iron mixed with other
metals) or aluminum alloy. But most cylinder heads are of cast iron.
The cylinder head includes water jackets, passages from the valve ports to the
openings in the manifolds (intake and exhaust) spark plug holes and in diesel
engines injector nozzle holes

Fig .cylinder head

COMBUSTION CHAMBERS:- Combustion chamber is the place where

burning of air fuel mixture is take pace. The cylinder head forms the top of the
combustion chamber. Are two types
SWIRL COMBUSTION CHAMBERS
Basic combustion – chamber shapes are:-
- Wedge
- Hemispheric (open)
- Cup (Bowl)
- Crescent (Pent-roof)
Some cylinder heads have additional features that help promote good combustion.
For example, there is a cylinder head which has turbulence generating port (TGP).
The purpose of this port is to produce high turbulence or swirl, of the air fuel mixture
during combustion.

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During compression; part of the air-fuel mixture is forced into the port.
On ignition, the mixture in the port starts burning first and Streams out at high
velocity. This helps spread the flame rapidly so that better combustion results.

. PRECOMBUSTION CHAMBERS:-
In this arrangement, there is a port with a small valve and the spark
plug in it.
The small valve works with the primary intake valve. When the two valves open, air-
fuel mixture enters. Lean mixture enters the combustion chamber But rich mixture
enters the pre combustion chamber.
Ignition takes place in the rich mixture in the pre combustion chamber. Then, the
burning of air-fuel mixture streams out into the main combustion chamber. This
produces good turbulence and rapid burning of the mixture.

EXHAUST MANIFOLD
The exhaust manifold is a set of tubes that carry the exhaust gases from the engine
cylinders.
In gasoline engine, the two manifolds (exhaust and intake) are attached to the same
side of the head. (Why?)
On some engines, the two manifolds are on opposite sides of the head. In in-line
engines, only one exhaust manifold is required. But in V-type engines, two are used,
one for each bank of cylinders attached to the out side of the heads.
Some exhaust manifolds for in-line engines have a heat control valve. The purpose of
this valve is to provide quick heating of air-fuel mixture when the engine is cold. This
improves fuel vaporization, provides better cold engine performance, and reduces
harmful emissions.

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 INTAKE MANIFOLD
- The intake manifold is a series of tubes that carry air, and in most systems
fuel vapor to the engine cylinders.
- The carburetor is a mixing device that mixes fuel vapor with the air going to
the cylinders to form a combustible mixture. It is mounted on top of the intake
manifold on a mounting pad. Other spark-ignition engines do not have a
carburetor. Instead, a fuel-injection system is used. This system has injection
valves that inject, or spray, fuel into the intake manifold. On the other hand,
the intake manifold for diesel engines is an assembly of tubes that carry air
from the air cleaner to the engine cylinders. The fuel is injected in to the
cylinders.

Fig . intake manifold

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CYLINDER COMPRESSION AND LEAKAGE TEST

Power loss:- Before a successful tune-up can be performed, it must be determined if the engine is in a
satisfactory mechanical condition. An engine that has burned or leaking valves, worn piston rings, sticking
valve lifters, leaking cylinder head gaskets or other mechanical malfunctions, will not perform efficiently
even after being tuned-up. These conditions must be corrected before an engine can be tuned to perform
satisfactorily or before emissions can be effectively limited.

Since the power developed by the engine on its power stroke is largely dependent on the efficiency of the
compression stroke, and because of the testing convenience afforded, the compression stroke is used for
testing engine condition.

During the compression stroke the air-fuel mixture is compressed in the tightly sealed combustion chamber.
Should any openings be created by burned valves, leaking gaskets, or worn piston rings, the reduced
amount of the air-fuel mixture would proportionately reduce the power output of the engine. Leaking at any
point in the combustion chamber will affect efficient engine operation. Leaking intake valves will allow a
portion of the air-fuel mixture to be pushed back into the intake manifold during the compression stroke.
During the power stroke, the expanding gases will leak past the burned valves and less fuel will be
available on the head of the piston. Also burned gases will be forced into the intake manifold to mix with
the air-fuel mixture. A diluted air-fuel mixture will then be available for the next intake stroke and
consequently the engine will develop less power. If the exhaust valve is burned, the expanding gases will
leak through it and less power will be available from the cylinder.

Any leakage past the piston rings will also affect the power of the engine. During the compression stroke,
part of the air-fuel mixture will be forced into the crankcase and cause oil contamination. The power stroke
will also force burned gases into the crankcase. These gases will overheat some of the oil, turning it into
carbon and the oil will become contaminated.

A leaking head gasket will permit water to be drawn into the cylinder during the intake stroke. During the
compression and power strokes, gases will be forced from the combustion chamber into the cooling system
and cause the engine to overheat. Also, a less dense air-fuel mixture will be available for the power stroke.

It is obvious that conditions of compression stroke leakage are proportionately reflected in engine power
loss and must be corrected before an engine can be properly tuned.

1) Material :
 Gasoline engines
 Repair manual
 Spark plug spanner
 Compression tester
2) Notice: a) Check vehicle manufacturers recommendations first
weather the engine has to be tested when ''hot or cold ''
Usually the test is executed with engine ''hot''!

3) Testing procedure :

a) Warm-up the engine to normal operating temperature.

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b) Remove the sparkplugs of all cylinders using the proper tool.
c) Disconnect terminal 15 of the ignition coil.
d) Crank the engine with starting motor speed for a few seconds only, this will blow out
residues of combustion through the plug hole.
e) Starting from the first cylinder, insert the compression tester into the spark plug hole.
f) Open the throttle vale fully to ensure that the maximum amount of air will enter the
cylinder.
g) Crank the engine through 5 to 10 compression strokes.
NB:- All cylinders should be tested with the same number of compression strokes.
h) Write down the maximum pressure indicated by the tester or gauge and compare
this figure with the normal one given by the vehicle manufacturer.
i) Continue with the next cylinder.

Results and Evaluation of compression test

Write down into the table below the nominal compression pressure values as
recommended by the vehicle manufacturer. Add the actual pressure values as measured
and determine the differences in compression pressure between each cylinders.

Take the  Nominal compression pressure: P = ----------- bar, and

Pressure difference between each cylinder: P = ----------- bar, from the repair manual.

Fill the table:-

Cylinder No. 1 2 3 4
Normal pressure, in bar
Actual pressure, in bar
Pressure difference, in bar

If low compression or high differences in pressure between cylinders are measured,

inject a small quantity of engine oil through the spark plug hole into the cylinder.

Adding oil oil helps to seal the piston rings temporarily so that the defective cylinder
shows one of the two possible results. These are:-
a) Adding oil increases the compression pressure.
Possible causes:-  Worn cylinder wall or piston.
 Broken or stuck piston rings.

b) Adding oil does not increases the compression pressure

Possible causes:-  Burned cylinder head gasket.
 Worn or burned valves and valve seats.
 Damaged or cracked cylinder head.
 Incorrect valve clearance adjustment.

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 THE CYLINDER LEAKAGE TEST

Purpose of cylinder leakage test

 To determine the cause of low compression.

 It helps to pinpoint places where air escapes or leakage occurs.
 It is an additional test executed when the compression test has given a bad result for
one or more cylinders.

During this test, the cylinder is put under air pressure while the piston is placed at TDC
position at the end of its compression stroke (both valves are closed). Escaping or leaking
air from the cylinder is indicated by a pressure gauge or meter. The test is executed with
engine ''hot'' that is, under normal operating temperature.

The leakage tester usually is connected to the workshop air supply system which in
general makes available compressed air of 5 to 10 bars. Air pressure from the shop
supply is applied in to the cylinder. A pressure drop caused by leakage is indicated by a
manometer. The manometer shows the percentage of air leaking from the cylinder. The
specification given by the testing equipment manufacturer should not be exceeded by the
test data.

Two methods of measuring leakage's are used :

a) Measuring of time in which the pressure has

dropped a certain limit
Example: Cylinder is filed with compressed air of about 7 bars pressure
Test result: Pressure drops from 7 to2 bars in,
 t  10 sec  cylinder in good condition,
 t = 8 sec  still acceptable condition,
 t < 8 sec  indicates excessive leakage

b) Measuring the percentage of the rate of pressure drop (p in %).

p  10 %  cylinder is in good condition.
p  25 %  still acceptable condition.
p < 25 %  possibly serious defect is indicated.

Testing procedure :

a) Warm-up the engine to normal operating temperature.

b) Remove the spark plugs, radiator cap, oil filter cap, oil dip stick, air cleaner.
c) Open fully the carburetor throttle valve.
d) Connect the leakage tester to the shop air supply and make sure the pressure regulator
on the tester is adjusted to “ zero “ .

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e) Select the appropriate adapter and fit into cylinder No.1.
f) Using a whistle, determine the TDC position.
g) After determination of TDC remove the whistle from the cylinder adapter.
h) Adjusting the pressure regulator to 100%, connect the leakage tester to the cylinder.
i) Lock the crankshaft or engine in the TDC position to avoid crankshaft rotation when
pressure is applied to the cylinder.
j) Check the places where excessive air leakage can be heard, felt or seen.
k) Proceed with the other cylinders.

Results and Evaluation of compression test

Cylinder leakage (%) Condition of cylinder

 10% Very good
 11 to 25% Still acceptable
> 25% Bad

A loss of compressed air ( drop of pressure in cylinders ) of more than 25% indicates an
excessive leakage which might be caused by serious defects inside the cylinder or engine.

Fill the table and write your own conclusion:-

Cylinder No. 1 2 3 4
P test in (%)
P drop in (%)

Place of excessive air

escape

Conclusion:-
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 NIFAS SILK TVET COLLEGE

Servicing exhaust system components Exam for SEM (42) second Year Students
Total time allowed 1hr
Tikimt 12/2004 E.C.

NAME:-_________________________________ I.D.No____________

I. Choose the best answer from the given alternatives

1. The self ignition temperature of gasoline fuel is:-

A/ 250-3000c B/350-4000c C/450-5000c D/550-6000c
2. When gasoline is burned completely in oxygen the result is
A/HC&O2 B/NOx&SO2 C/CO2&H2O D/CO&H2O
3. Volitility of gasoline engine is defined as:-
A/The burning of air fuel mixture C/The mixing of air fuel mixture
B/The ease with which gasoline vaporizes D/The incomplete burning of air fuel
mixture
4. The purpose of heat control valve is:-
A/To heat exhaust gas during high speed C/To heat air fuel mixture during
cold start
B/To cool the air fuel mixture during high speed D/All
5. Which of the following is used to dampen exhaust gas noise?
A/Air cleaner B/Tail pipe C/Exhaust manifold D/None
6. Which of the following is the best type silencer (free fro pollution)?
A/Absorption type silencer B/Interference silencer C/Reflection pattern
silencer D/ All
7. Which of the following is not a type of catalytic converter?
A/Three-way B/One-way C/Reduction D/Oxidation
8. NOx_________Z_ __________ N2 + O2, the catalyst that can be put in the place of
letter z is:-
A/Oxidation B/Reduction C/Two-way D/Three-way
9 .Which of the following fuel system problems create cylinder wall hammering (loud
sound)?
A/ Detonation B/Vapor lock C/Complete D/None
10. The type of gasoline fuel mixed at “SULULTA FUEL STATION” is:-
A/Leaded B/Gasohol C/LPG D/Pure gasoline

II. Give short Answer

1. Write all chemical reactions undergone inside the catalytic converter.

2. Write at least four characteristics possessed by a pure gasoline?
3. Show with simple sketches the combustion process inside the gasoline engine.
4. Write at least two methods of treating exhaust gas.
5. What is three-way catalytic converter?