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 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 throttle- body/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 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 Here, the coefficients a 2, a 1, and a 0 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 nth order

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 a 0. 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