Exhaust Manifold

The intake manifold is the pipe network structure designed to distribute fresh air (and in some systems, the fuel-air mixture) evenly to the engine cylinders in an internal combustion engine.

Its function is to connect the throttle valve (or carburetor) with the intake ports on the cylinder head, ensuring that each cylinder receives a balanced amount of air or air-fuel mixture.

Schematic Diagram of the Intake Manifold (Source: Mechcontent)

Function and Operating Principle

The intake manifold is a critical component that directly affects engine performance. Its main functions are:

• Distributing air and fuel mixture evenly: Especially in carbureted engines (wet manifold), it is essential that the fuel remains uniformly mixed and reaches all cylinders without separating.
• Optimizing air flow: In modern fuel-injected engines (dry manifold), only air is carried, allowing for more complex and aerodynamic designs.
• Increasing volumetric efficiency: By managing the speed and density of air flow, the manifold enables the engine to produce more power.

After passing through the air filter and throttle valve, air enters a chamber known as the "plenum." From there, it is distributed through individual tubes called "runners," each leading to a cylinder’s intake port. The volume of the plenum and the length of the runners are decisive factors in determining the engine’s power characteristics.

Wet and Dry Manifolds

• Wet manifold: Used in carbureted engines. It carries the air-fuel mixture. Its internal surfaces must be smooth with gradual bends to prevent fuel separation.
• Dry manifold: Used in fuel-injected engines. It carries only air, enabling more complex and performance-oriented designs.

Structural Features

The intake manifold consists of several main components:

• Plenum: The central chamber that supplies air to all runners.
• Runners: Individual tubes connecting the plenum to the intake ports of each cylinder.
• Throttle Valves (in some designs): In variable-length manifold systems, internal mechanisms adjust airflow for different engine speeds.

Intake Manifold Structure (Source: Mechcontent)

Design Criteria

The primary criteria to consider when designing an intake manifold are:

• Airflow resistance: Wider pipes offer less resistance but can reduce airflow velocity at low engine speeds. Therefore, runner diameter is optimized according to engine dynamics.
• Runner length:
• • Long runners → Deliver higher torque at low engine speeds.
  • Short runners → Enable higher horsepower at high engine speeds.
• Plenum volume: A larger plenum → Enhances power at high engine speeds; a smaller plenum → Improves throttle response.
• Fluid mechanics principles: Losses at inlet and outlet points (major and minor losses) are minimized to maintain efficient airflow.
• Pressure waves and resonance effects: In high-performance engines, pressure waves generated during the intake process are harnessed to increase cylinder filling (wave tuning).