Internal combustion engine (ja304) chapter 3

INTERNAL COMBUSTION ENGINE (JA304)

CHAPTER 3
COMBUSTION AND FUEL
CHARACTERISTICS
BY

MOHD SAHRIL MOHD FOUZI
MECHANICAL ENGINEERING DEPARTMENT
UNGKU OMAR POLYTECHNIC

CHAPTER 3 COURSE LEARNING OUTCOME (CLO 3)


INTRODUCTION :

This topic covers the understanding of
combustion process in spark ignition as
well as compression ignition engine. It also
includes knocking phenomenon and fuel
characteristics.



COMBUSTION PROCESS TERMS

Normal Combustion
A combustion process which is initiated solely by a timed spark and
in which the flame front moves completely across the combustion
chamber in a uniform manner at a normal velocity.

Abnormal Combustion
A combustion process in which a flame front may be started by hot
combustion-chamber surfaces either prior to or after spark ignition,
or a process in which some part or all of the charge may be
consumed at extremely high rates.

Surface Ignition
Spark knock Hot spots-combustion-
chamber deposits


Spark Knock
A knock which is recurrent and repeatable in terms of audibility. It is
controllable by the spark advance; advancing the spark increases
the knock intensity and retarding the spark reduces the intensity.
Knock is the name given to the noise which is transmitted
through the engine structure when essentially spontaneous ignition
of a portion of the end gas—the fuel, air, residual gas, mixture ahead
of the propagating flame—occurs.
There is an extremely rapid release of most of the chemical
energy in the end-gas, causing very high local pressures and the
propagation of pressure waves of substantial amplitude across the
combustion chamber.



Surface Ignition
Surface ignition is ignition of the fuel-air charge by any hot surface
other than the spark discharge prior to the arrival of the normal
flame front. It may occur before the spark ignites the charge (pre-
ignition) or after normal ignition (post-ignition).
Surface Ignition is ignition of the fuel-air mixture by a hot
spot on the combustion chamber walls such as an overheated valve
or spark plug, or glowing combustion-chamber deposit: i.e., by any
means other than the normal spark discharge.

Following surface ignition, a flame develops at each surface-
ignition location and starts to propagate across the chamber in an
analogous manner to what occurs with normal spark-ignition.


Ignition
The ignition system is designed to ignite the air and fuel that have
been mixed in the fuel system. It is important to improve this system.
Each year, ignition is becoming more and more computerized.
Today’s ignition systems are almost totally computer controlled for
improved combustion.

Pre- Ignition
This Is one process where the spark is heated up before the ignition
begins. It causes rough running and in extreme cases, can do damage
to the engine.
Causes of pre-ignition include the following:
1. Carbon deposits form a heat barrier and can be a contributing factor to
pre-ignition.
2. Glowing carbon deposits on a hot exhaust
3. A sharp edge in the combustion chamber or on top of a piston
4. Sharp edges on valves that were reground improperly
5. An engine that is running hotter than normal due to a cooling system
problem
6. Auto-ignition of engine oil droplets.



Ignition Delay
Ignition delay is defined as the time (or crank angle interval) from
when the fuel injection starts to the onset of combustion.


Delay period is the commencement of injection and it is indicated
by the dot on the compression line 15° before dead centre. The
period 1 is the delay period during which ignition is being initiated,
but without any measurable departure of the pressure from the air
compression curve which is continued as a broken line in the
diagram as it would be recorded if there were no injection and
combustion.
6000
1
5000

4000
kN/m2
3000

2000

1000

75° 50° 25° 0° 25° 50° 75°

Degrees of crank angle

Figure of Delay period



Combustion
Combustion is one process in which air and fuel are burned after
being mixed at a correct ratio of 14.7 parts of air to 1 part of fuel.
The specific requirements for combustion including such
as air, fuel and ignition requirements, timing, air-fuel ratios,
compression ratio and engine efficiency.
The internal combustion engine has certain requirements
for efficient operation. There must be sufficient air for combustion,
correct amounts of fuel mixed with the air, and ignition to start
combustion.

AIR
FUEL

COMBUSTION



Timing
*One process of identifying when air, fuel and ignition occur in
relation to the crankshaft rotation.

This is a relationship between the position of the piston and
crankshaft. For the engine to operate efficiently, the air and fuel
mixture must enter the cylinder at the correct time.
This mean that the intake valve must be opened and closed
at the correct time. The exhaust valve must be opened and closed at
the correct time too. The ignition must also be timed.
The timing of the ignition can change with speed and load; if
the process is correctly timed, maximum power will be obtained in
converting chemical energy into mechanical energy.


Air-Fuel ratio
Air –fuel ratio is defined as the ratio of air to fuel mixed by the
carburetor or fuel injectors. The term air-fuel ratio is often called the
stoichiometric ratio.
The air and fuel must be thoroughly mixed. Each molecule of
fuel must have enough air surrounding it to be completely burned. If
the two are not mixed in the correct ratio, engine efficiency will drop,
and exhaust emission level will increase.


Advance Timing
Timing advance is required because it takes time to burn the air-fuel
mixture. Igniting the mixture before the piston reaches TDC will allow
the mixture to fully burn soon after the piston reaches TDC.
If the air-fuel mixture is ignited at the correct time, maximum
pressure in the cylinder will occur sometime after the piston reaches
TDC allowing the ignited mixture to push the piston down the cylinder
with the greatest force.
Ideally, the time at which the mixture should be fully burnt is
about 20 degrees ATDC. This will utilize the engine's power
producing potential.
If the ignition spark occurs at a position that is too advanced
relative to piston position, the rapidly expanding air-fuel mixture can
actually push against the piston still moving up, causing knocking
(pinging) and possible engine damage.


If the spark occurs too retarded relative to the piston
position, maximum cylinder pressure will occur after the piston is
already traveling too far down the cylinder. This results in lost power,
high emissions, and unburned fuel.


Retarded Timing
Retarded timing can be defined as; changing the timing so that fuel
ignition happens later than the manufacturer's specified time. For
example, if the timing specified by the manufacturer was set at 12
degrees BTDC initially and adjusted to 11 degrees BTDC, it would be
referred to as retarded. In a classic ignition system with breaker
points, the basic timing can be set statically using a test light or
dynamically using the timing marks and a timing light.


Graph engine pressure Vs crank angle
Refer to the graph of engine pressure Vs crank angle, this graph
shows how the pressure in the combustion chamber, between the
compression rings, and in the crankcase varies with crank angle in
an engine cycle.

Pressure ( P )

P1= cylinder pressure
P1
P2= pressure between

Compression rings.

P3 = crankcase pressure.

Reverse blow by
P2
Occur here
P3

BDC TDC BDC

Crank angle (θ)

Figure 4.2 : engine pressures Vs crank angle

There is a time delay in the pressure change from one
chamber to the next due to the restricted flow passage created by
the compression rings. Later in the power stroke, when the exhaust
valve opens, pressure between the compression rings will be
greater than in the combustion chamber, and some gases will be
forced back into the chamber. This is called reverse blow by.

Point of engine pressure Vs crank angle diagram.
The process of relationship between engine pressures as a function
of crank angle, is shown in Figure 4.2 the cylinder pressure ( P1),
pressure between piston compression rings ( P2 ), and pressure in
the crankcase ( P3 ).
There is a time delay for pressure change from one chamber
to the next due to the restricted flow passage past the pistons. When
the exhaust valve opens and blow down occurs, pressure in the
combustion chamber decreases quickly and opens P2 > P1. At this
point, reverse blow by occurs. The need for crankcase ventilation
can be seen by the pressure buildup in the crankcase.


Knocking
This is one process that happens within the combustion chamber. It
sounds like a small ticking or rattling noise within the engine. In
long term, the piston and ring can be damaged as well as the spark
plug and valve. Another name for knocking is detonation.

Valves
Spark plug

Second flame front from Knocking area
ignition of low-octane Flame front
gasoline

Figure 4.4.1: knocking results when two flame
fronts hit each other. One is from the spark plug
combustion, and one is from early fuel ignition
caused by poor-quality fuel.


Figure 4.4.1 shows what happens inside a combustion
chamber when knocking occurs. When the spark fires to ignite the
air-fuel mixture, it produces a flame front.
Shortly after ignition, a second explosion (flame front) and
ignition of fuel takes place on the other side of the combustion
chamber. This flame front is produced from low-octane fuel
combusting, or burning, too early. When these two energy fronts hit
each other, they cause a pinging or knocking within the engine.
Knocking is usually caused by poor-quality fuel. The
temperatures within the combustion chamber are high enough to
cause the air mixture to ignite without the spark plug. Fuel is made
to have anti-knocking characteristics. Actually, anti-knocking fuel
needs higher temperature to start burning.


In detail, the knocking for the cylinder pressure as a function of time
in typical SI engine combustion chamber is shown in the following
graphs ( Figure 4.4.5, 4.4.6, 4.4.7.)

Cylinder
Cylinder
Pressure
Pressure

Time Time
Figure 4.4.5 : Normal Combustion Figure 4.4.6 : Combustion with
with no Knocking light Knocking

Cylinder

Pressure

Time
Figure 4.4.7: Combustion with Heavy Knocking



Effects of knocking during engine process
The effects of knocking during engine process are ;-
1. a drop in engine performance.
2. pollution of gases from the combustion is incomplete.
3. high consumption of fuel.

Reduce knocking problem during engine process
In this case we have three options to reduce knocking during
engine process :-
1. Increase the ignition combustion engine.
2. Reduce the heat the final combustion.
3. Use high quality fuel.


Compression Ignition Combustion
The combustion process in a compression ignition engine starts
when the air-fuel mixture self-ignites due to high temperature in the
combustion chamber caused by high compression.

Ricardo Diagram
Ricardo diagram is another important term in combustion. This
diagram is important because it is used to study about the time
ignition of one combustion. If the fuel in the chamber burned very
fast, slow, or late; we can see this problem in the Ricardo diagram
(Figure 4.8.)



Pressure ( P ) Line 2

Line 1

Line 3

Crank angle ( TCD )

Figure 4.8 : Ricardo Diagram

Line 1 explains the good condition of combustion.
Line 2 explains the overhead and the curve in
Line 3 explains late ignition.


Catane Number
There is a delay between the time that fuel is injected into the
cylinder and the time that the hot gases ignite. This time period or
delay is expressed as a catane number. Catane number ranges from
30 to 60 on diesel fuel. Catane number is an indication of the ignition
quality of the diesel fuel.
The higher the catane number, the better the ignition
quality of the fuel. High catane numbers should be used to start an
engine in cold weather. A catane number of 85 to 96 is often used
for starting diesel engines in cold weather.
If a low catane number is used in diesel engine, some of the
fuel may not ignite. The fuel will then accumulate within the cylinder.
When combustion finally does occur, this excess fuel will explode
suddenly. This may result in a knocking sound as in gasoline.


Exercise
Question 1
Explain the term ―Knocking‖.

Question 2
List the effects of knocking during engine process

Question 3
List 3 options on how to reduce knocking problems during engine
process.

Question 4
What is combustion? Give three factors required to make
combustion?

Question 5
Draw and explain the Ricardo Diagram.


Internal combustion engine (ja304) chapter 3

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