Lecture-20
Prepared under
QIP-CD Cell Project
Internal Combustion Engines
Ujjwal K Saha, Ph.D.
Department of Mechanical Engineering
Indian Institute of Technology Guwahati
1
Fluid Motion in Combustion Chamber
Due to high velocities involved, all flows
into, out of and within cylinders are turbulent.
The exception to this are those flows in the
corners and small crevices of the combustion
chamber where the close proximity of the
walls dampens out the turbulence.
‰
As a result of turbulence, heat transfer,
evaporation, mixing and combustion rates all
increase. As the engine speed increases, flow
rate increases with a corresponding increase
in swirl, squish and turbulence. This increases
the rate of fuel evaporation, mixing of fuel
vapor and air, and combustion.
‰
2
Fluid Motion in Combustion Chamber
Turbulence in a cylinder is high during
intake, and decreases as the flow rate slows
near BDC. It increases again during
compression, as swirl, squish and tumble
increases near TDC.
‰
The high turbulence near TDC when ignition
occurs is very desirable for combustion. It
breaks up and spreads the flame front many
times faster. The air-fuel is consumed within a
short time, and self-ignition and knock are
avoided. The shape of the combustion
chamber plays an important role in
generating
maximum
turbulence
and
increasing the desired rapid combustion.
‰
3
Volumetric Efficiency
The inside surface of most intake
manifolds are usually made smooth to
maximize the volumetric efficiency.
‰
However, in some engines where high
power is not desirable, the inside surfaces of
manifolds are roughened to promote higher
turbulence levels to enhance evaporation
and air-fuel mixing.
‰
4
Two-stroke Engines
Turbulence
is
detrimental
in
the
scavenging process of two-stroke cycle
engines. This is because, the incoming air
mixes more with the exhaust gases, and a
greater exhaust residual will remain within
the cylinder.
‰
Another negative result occurs during
combustion
when
high
turbulence
enhances the convective heat transfer to
the walls in the combustion chamber. This
higher heat loss lowers the thermal
efficiency of the engine.
‰
5
Swirl
The rotational motion of the fluid mass within
the cylinder is called swirl. Swirl greatly
enhances the mixing of air and fuel to give a
homogeneous mixture within a short time. It is
also a main mechanism for very rapid spreading
of flame front during the combustion process.
‰
Swirl can be generated
by constructing the intake
system to give a tangential
component to the intake
flow as it enters the
cylinder. This is done by
shaping and contouring
intake manifolds, valve
ports and piston faces.
‰
6
Cylinder Swirl and its Generation
Swirl motion
Helical port
Tangential injection
Contoured valve
7
Swirl is used to:
‰ promote rapid combustion in SI engines
‰ rapidly mix fuel and air in gasoline direct
injection engines
‰ rapidly mix fuel and air in CI engines
8
Swirl Theory
Swirl can be simply modelled as solid body
rotation, i.e., cylinder of gas rotates at an angular
velocity, ω.
Swirl Ratio: It is a dimensionless parameter to
quantify the rotational motion within the cylinder,
and is defined in two different ways in technical
literature:
angular speed ω
=
( SR )1 =
engine speed
N
( SR )2
swirl tan gential speed ut
=
=
average piston speed U p
− − (1)
− − (2)
9
Swirl Ratio
angular speed ω
=
( SR )1 =
engine speed
N
( SR )2
swirl tan gential speed ut
=
=
average piston speed U p
− − (1)
− − (2)
Remark: The average values of either the angular
speed or tangential speed should be used in the
above equations. Angular motion is very nonuniform within the cylinder, being a maximum
away from the walls and being much less near
the walls due to viscous drag.
10
™ The
figure shows
how
swirl
ratio
changes through a
cycle of the engine.
During intake, it is
high, decreasing after
BDC
in
the
compression
stroke
due to viscous drag
with the cylinder walls.
™ Combustion expands the gases and increases
swirl to another maximum part way into the
power stroke. Expansion of the gases and viscous
drag quickly reduce this again before blowdown
occurs. Maximum swirl ratio as defined by
equation (1) can be on the order of 5 to 10 for
modern engine.
11
Paddle-wheel Model
The volume within the cylinder is idealized to
contain an imaginary paddle wheel that has no
mass. As the wheel runs, gas between the blade
turn with it with the result that all the gas rotate at
one angular velocity.
The mass moment of inertia (I) of cylinder gas is:
2
1 2 1 ⎛B⎞
mB 2
I = mr = m ⎜ ⎟ =
2
2 ⎝2⎠
8
where , m = mass of the gas,
and B = bore = diameter of rotating mass
The angular momentum (Γ) of the rotating gas is:
Γ = Iω
12
Swirl
Many engines have a wedge shape cylinder head
cavity or a bowl in the piston where the gas ends
up at TDC.
During the compression process as the piston
approaches TDC more of the air enters the cavity
and the air cylinder moment of inertia decreases and
the angular velocity (and thus the swirl) increases.
13
Squish
Squish is the radial flow occurring at the end of the
compression stroke in which the compressed
gases flow into the cavity in the piston or cylinder
head.
14
Tumble
As the piston reaches TDC the squish motion
generates a secondary flow called tumble, where
rotation occurs about a circumferential axis near
the outer edge of the cavity or piston bowl.
15
Squish and Tumble
Squish
Tumble
16
Crevice Flow
Within the engine combustion chamber, there are
tiny crevices which get filled with air, fuel and
exhaust gas during the cycle. These crevices
include:
‰ clearance between the piston and the
cylinder walls (80 % of total)
‰ imperfect fit in the threads of spark plug
or fuel injector (5 %)
‰ gaps in the gasket between head and
block (10-15 %)
‰ un-rounded corners at the edges of
combustion chamber and edges of valve
faces.
17
Crevice Flow
‰ Although this crevice volume is of the order
of 1-3 % of the total clearance volume, the flow
into and out of it greatly affects the engine
performance.
‰ In an SI engine, air-fuel mixture is forced into
these crevices, and some of the fuel ends up in
engine exhaust thereby lowering thermal
efficiency.
‰ As fuel is added towards the end of the
compression stroke in a CI engine, less fuel gets
into the crevice volume.
18
Piston Rings
‰ Most pistons have two or more compression
rings and atleast one oil ring. Compression rings
seal the clearance gap between the piston and
the cylinder walls. The oil ring scrape off most of
the lubricating oil splashed on the cylinder wall,
and return the oil to the crankcase.
19
Piston Rings
‰ Various designs of piston rings to minimize
the leakage flow through the gap where the
two ends meet.
20
Summary
‰ Efficient operation of an engine depends
upon high turbulence in the air-fuel mixture,
and the generated flows of swirl, squish and
tumble. Swirl is the rotational motion generated
in the cylinder during intake and compression,
squish is the radial inward motion that occurs as
the piston moves toward TDC, and tumble is
created by squish motion and the shape of the
clearance volume. All these motions enhance
proper operation of the engine.
21
Summary
‰ Crevice flow is another flow motion that
occurs during engine operation. Although
crevice volume is only a small percent of the
total combustion chamber volume, the flow
into and out of it affects combustion and
engine emissions. Some of the gas flow in the
crevice between the piston and cylinder
walls gets past the piston into the crankcase,
where it raises the crankcase pressure and
contaminates the lubricating oil.
22
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23
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