‫ قسم الهندسة الميكانيكية‬/ ‫ كلية الهندسة‬/ ‫جامعة الموصل‬
Department of Mechanical
Engineering
Subject: Internal Combustion Engines
Combustion in Internal Combustion Engines
Class: Third year
Lecture
First lecture: ‫ عبدالرحمن حبو محمد الحبو‬.‫د‬
Instructor: ‫ أحمد فؤاد النعمة‬.‫د‬
Combustion in Internal Combustion Engines
1- Combustion in Spark Ignition Engines:
Combustion in S.I. Engines can be set as follows:
A- Normal Combustion ( Advance, Retard , Ideal)
B- Abnormal combustion ( Knock, Surface Ignition)
A) Normal Combustion:
In a conventional spark ignition engine the fuel and air are mixed together
in the intake system through the intake valve into the cylinder, where
mixing with residual gas take place, and then compressed. Under normal
operating conditions, combustion is initiated towards the end of
compression stroke at the spark plug by electric discharge.
Combustion process in spark ignition engines can be divided in four
phases as follows:
1- Spark ignition
2- Early flame development
3- Flame propagation
4- Flame termination
‫عنوان البريد االليكتروني‬
Page | 56
‫رقم الهاتف‬
‫اسم التدريسي‬
‫ قسم الهندسة الميكانيكية‬/ ‫ كلية الهندسة‬/ ‫جامعة الموصل‬
This sort of combustion is called normal combustion since it is initiated by
spark. Towards the end of the compression stroke the cylinder contains a
more or less homogeneous mixture of fuel vapor and air. A spark is fired
and as it passes from electrode to the other it leaves a thin thread of flame.
Combustion spreads to the envelope of the mixture containing the thread
at a rate depending primarily on the temperature of the flame front and
secondarily on the temperature and density of the surrounding envelope.
As the flame front expands it travels across the chamber until finally the
whole mixture is combusted. Depending on the degree of turbulence in the
cylinder, the flame front wrinkles and perhaps breaks into eddies which
speed up the process. In a typical indicator diagram taken in the cylinder
as shown below in Fig. (1).
IVO: Intake Valve Opens
IVC: Intake Valve Closes
EVO: Exhaust Valve Opens
EVC: Exhaust Valve Closes
xb: Burned gas mole fraction
Figure (1): pressure versus crankshaft position in spark ignition engines.
‫عنوان البريد االليكتروني‬
Page | 57
‫رقم الهاتف‬
‫اسم التدريسي‬
‫ قسم الهندسة الميكانيكية‬/ ‫ كلية الهندسة‬/ ‫جامعة الموصل‬
IVO: Intake Valve Opens
IVC: Intake Valve Closes
EVO: Exhaust Valve Opens
EVC: Exhaust Valve Closes
ID: Ignition Delay
Fig. (2): pressure - volume and Pressure – crank angle diagrams of S.I. Engines.
B- Abnormal combustion
So far we have described normal combustion in which spark ignited the
fuel-air mixture and flame moves steadily across the combustion chamber
until the charge is fully consumed. However, several factors such as fuel
consumption, certain engine design and operating parameters and
combustion chamber deposit may prevent this normal combustion from
occurring, therefore another type of combustion may presence which is
called Abnormal combustion. Two type of abnormal combustion have
been identified in spark ignition engines, Knock and Surface ignition.
‫عنوان البريد االليكتروني‬
Page | 58
‫رقم الهاتف‬
‫اسم التدريسي‬
‫ قسم الهندسة الميكانيكية‬/ ‫ كلية الهندسة‬/ ‫جامعة الموصل‬
Knock is the most important abnormal combustion phenomenon. Its name
come from the noise that result from auto ignition of portion of the fuel, air
and residual gas ahead of the advancing flame. As the flame propagate
across the combustion chamber, the unburned mixture ahead of the flame
called the end gas. This end gas is compressed, causing its pressure and
temperature and density to increase. Some of the end gas fuel-air mixture
may undergo chemical reactions prior to normal combustion. The products
of these reactants may then auto ignite, i.e. spontaneously and rapidly,
release a large part or all their chemical energy.
When this happens, the end gas burns very rapidly, releasing its energy at
a rate of 5 to 25 times that characterized of normal combustion. This cause
high frequency oscillation inside the cylinder that produce the sharp
metallic noise called knock. The presence or absence of knock reflects the
outcome of a race between the advancing flame front and the
precombustion reaction in the unburned end gas. Knock will not occurs if
the flame front consumes the end gas before these reactions have time to
consume the fuel – air mixture to ignite. Knock will occur if the combustion
reactions produce auto ignition before the flame front arrives.
Figure 3(a) shows the top view of cylinder or combustion chamber for
normal condition and Fig. 3(b) shows the pressure distribution during
power stroke for ideal condition without knocking and Figs. 3(c) & 3(d)
shows actual condition with knocking.
It can clearly see that the normal condition generates a uniform pressure
wave which is required to drive vehicle. But when the flame front
compressed the fresh charge and the charge auto ignited, will create rapid
change in pressure wave. This rapid change in pressure wave creates
unwanted sound until the whole charge of opposite flame ignited. If the
compressed mixture does not reach its auto ignition temperature during
compression, the whole charge ignited by main flame front and the normal
combustion takes place.
‫عنوان البريد االليكتروني‬
Page | 59
‫رقم الهاتف‬
‫اسم التدريسي‬
‫ قسم الهندسة الميكانيكية‬/ ‫ كلية الهندسة‬/ ‫جامعة الموصل‬
Fig. 3(b)
Fig. 3(a)
Fig. 3(d)
Fig. 3(c)
Fig. 3: Show the top view of cylinder and the pressure distribution during power
stroke for ideal condition without knocking and actual condition with knocking.
‫عنوان البريد االليكتروني‬
Page | 60
‫رقم الهاتف‬
‫اسم التدريسي‬
‫ قسم الهندسة الميكانيكية‬/ ‫ كلية الهندسة‬/ ‫جامعة الموصل‬
When the spark advance is too big, it will cause knock, which will dispose
of power and waste fuel, as shown in Fig. (4).
Fig. 4: Spark advance is too large will cause a knock.
The other important abnormal combustion phenomenon is surface
ignition. Surface ignition is the ignition of the fuel – air charge by
overheated valve or spark plug, by glowing combustion chamber deposits
or by another hot spot in the engine combustion chamber. It may be occur
before the spark plug ignites the charge (preignition), or after normal
ignition (postignition). It may produce a single flame or many flame.
Surface ignition may result in knock, knock which occurs following normal
spark ignition is called spark knock to distinguish it from knock which has
been produced by surface ignition.
Table 1 summarize the influence of a number of operating factors on the
tendency to knock.
Factors
Effect
Compression ratio
Ignition timing
Increase
Advance
Full open
Partially open
Throttle setting
Mixture strength
Engine speed
Inlet air pressure
Cooling water temperature
‫عنوان البريد االليكتروني‬
Page | 61
Weak
Slightly rich
Rich
Increase
Decrease
Increase
Decrease
‫رقم الهاتف‬
Pro-knock Anti-knock
yes
yes
yes
Yes
Yes
yes
Yes
Yes
Yes
yes
Yes
‫اسم التدريسي‬
‫ قسم الهندسة الميكانيكية‬/ ‫ كلية الهندسة‬/ ‫جامعة الموصل‬
2-Combustion in Compression Ignition Engines
The essential features of the compression ignition or diesel engine
combustion process can be summarized as follows:- liquid fuel is injected
towards the end of compression stroke, so that the fuel is distributed in a
jet or in several jets throughout all or part of the combustion chamber.
Distribution of the fuel, mixing with air, evaporation and diffusion to
produce a gaseous mixture and chemical reaction to burn the fuel. This
process must be completed in extremely short period of time if the engine
is to be efficient.
The combustion in a C.I. Engine is considered to be taking place in four
stages as shown in Fig. (5). It is divided into the ignition delay period, the
period of rapid combustion, the period of controlled combustion and the
period of after-burning. The details are explained below as:
A is the point of passage of spark (say 20o bTDC), B is the point at which
the beginning of pressure rise can be detected (say 8o bTDC). C the
attainment of peak pressure. Thus AB represents the first stage, BC the
second stage and CD the third stage.
Figure (5): Four stages of combustion in C.I. Engines.
From the starting of injection the combustion process as deduced from a
study cylinder pressure diagrams, may be divided into several stages as
follows:‫عنوان البريد االليكتروني‬
Page | 62
‫رقم الهاتف‬
‫اسم التدريسي‬
‫ قسم الهندسة الميكانيكية‬/ ‫ كلية الهندسة‬/ ‫جامعة الموصل‬
1- Ignition delay:- The period between the start of fuel injection into the
combustion chamber and the start of combustion. Or may be defined as
the time (crank angle) interval between the start of injection and the start
of combustion.
2- Rapid combustion (Uncontrolled combustion):- In this stage
combustion of the fuel which has mixed with air to within the flammability
limits during the ignition delay period occurs rapidly in few crank angle
degrees.
3- Controlled combustion stage:- Once the fuel and air which premixed
during the ignition delay have been consumed throughout the combustion
process, the burning rate is controlled by the rate at which mixture
becomes available for burning.
4- Late combustion stage:- Heat release continues at lower rate into the
expansion stroke. There are several reasons for this:- a small fraction of
fuel may not burned, a fraction of the fuel energy is present in soot and
rich combustion products and can still release.
Four stages of combustion in C.I. Engines.
‫عنوان البريد االليكتروني‬
Page | 63
‫رقم الهاتف‬
‫اسم التدريسي‬