Design of Intake Systems for I.C. Engine
P M V Subbarao
Professor
Mechanical Engineering Department
Design Features of A Network of pipes for
effective breathing….
Engine Respiratory System
Intake System
• Intake system ensures optimum charge entry into each cylinder
of the engine. The design and positioning should be such that
it allows the maximum charge for a given application.
• At the entry of air filter, there can be a bell mouth which is a
duct of variable cross section area and is used to allow high
velocity air to enter from the inlet without flow separation
called as velocity stack.
Performance of an Air Stack
• The loss due to inefficient entry to the system can be explained in
more details as follows.
• Just after the vena contrecta formation when the flow area
increases, velocity decreases and the fluid fills the corners left
void at the vena contracta., and hence sudden lateral movement of
fluid exists.
• When the lateral flowing fluid strikes the wall, it causes turbulence
which increases the resistance to flow.
• Also since at the vena contracta, the velocity increases and just at
the exit reduces, unconfined mixing causes the loss in the
mechanical energy (there is relative difference in the velocity of
flow at and beyond the vena contracta).
The Effect of Vena-Contracta
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There are number of effects of vena contracta formation:
More work required for the air to enter
Flow separation
Lesser mass flow rate
The losses depend on the entrant diameter of the intake tract. The
following table shows typical values for pipe entry.
Hence, r/D  0.15 has to be
chosen.
Plenum
• Plenum is a volume between throttlebody/restrictor and runners.
• It provides a reserve to the individual intake
runners and distributes air equally in all the
runners/cylinders.
• This helps all cylinders generate equal power
which is a crucial factor for the safety of
engine.
• Plenum size and throttle body size are
dependent on each other with smaller
carburetors requiring larger plenum.
Advanced Design of Plenum
Tunnel ram: All the intake runners are straight and meet at a
common plenum (the tunnel).
This type of manifold gives excellent fuel distribution and top-end
power.
The large plenum area reduces signal strength and throttle response.
Minimum velocity in Intake system
• The velocity of charge in the intake system should not
decrease to such limits so that fuel will start falling out of
suspension.
• But the design should be careful so that fuel mixture is
sufficient for supporting peal torque RPM.
• For 3 to 6 cylinder engines the plenum volume of 0.8 to 1.5
works well while engines with 8, 10 and 12 cylinders run well
with even lesser.
• But with the restrictor, the plenum has to be slightly large.
• If the engine is left to the air purely from the T/B then at
higher RPM it may suffocate.
Effect of Plenum Volume on Engine Capability
Plenum Pressure Vs Plenum Size
Top down approach for the Design of manifold
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No warm air intake from the engine vicinity
Ram air benefits
Gravity pressure benefits
No 90° bends which reduces the losses considerably
Uniform charge distribution which is most crucial parameter
capable enough to fool the ECU.
This disrupt the operation of the entire engine affecting
ignition advance,
knocking,
power and so on.
Integrated Description of Wave Action
Dynamic Behaviour of A CV
• The behavior of a CV that exhibits linear behavior is mathematically
represented in the general form of expression given as
d2y
dy
...........a2 2  a1  a0 y  x(t )
dt
dt
Here, the coefficients a2, a1, and a0 are constants dependent on the
particular part of a intake/exhaust system.
The left hand side of the equation is known as the characteristic
equation.
It is specific to the internal properties of the CV and is not altered by
the way the engine is used.
• The specific combination of forcing function input and CV
characteristic equation collectively decides the combined
output response.
• Solution of the combined behavior is generally obtained using
Laplace transform methods to obtain the output responses in
the time or the complex frequency domain.
Behaviour of A CV
Zero order
First order
Second order
a0 y  x(t )
dy
a1
 a0 y  x(t )
dt
d2y
dy
a2 2  a1  a0 y  x(t )
dt
dt
nth order
dny
d n 1 y
d2y
dy
an n  an 1 n 1  ...........a2 2  a1  a0 y  x(t )
dt
dt
dt
dt
Behaviour of A CV
• Note that specific names have been given to each order.
• The zero-order situation is not usually dealt because it has no
time-dependent term and is thus seen to be trivial.
• It is an amplifier (or attenuator) of the forcing function with
gain of a0.
• It has infinite bandwidth without change in the amplification
constant.
• The highest order usually necessary to consider in first-cut CV
analysis is the second-order class.
• Higher-order systems do occur in.
• Computer-aided tools for systems analysis are used to study
the responses of higher order systems.
Generalized Model for A Second Order Cv
d2y
dy
2
2
 2n
 n y  kn x(t )
2
dt
dt