Air Intake System

engine valve stood on caliper
6mm diameter valve made from stainless steel

The air intake system channels clean air and fuel to an internal combustion engine. The primary function is to channel clean and cool air to the engine. In indirect injection engines the fuel is added at the last point before the air fuel mixture enters the cylinder head and passes through the air intake valve. In the case of direct injection engines no fuel is added until after the air is within the cylinder.

The four primary functions are:

  1. Channel air to the engine – Preferably cold air. Therefore, orifice best placed in the main airstream e.g. front of engine bay, behind headlamp unit.
  2. Filter particulates – The filter element needs to be large enough to filter the required volume of air and achieve necessary service life.
  3. Dynamic performance tuning – The length of the pipes (ram pipes) connecting the valve face to the plenum are tuned to improve the engine breathing at the desired engine speed.
  4. Reduce noise – Pulsating noise is produced as the engine draws air in (intake noise) for the combustion process.

If you following the air through the system from the point at which it is picked up then the following describes it′s progress:

  • Cool air is picked up at the air intake orifice.
  • The air is filtered by the filtration system to remove dirt and dust down to roughly 5µm.
  • The air filter is contained in a large box, this box has a secondary function of acting as an expansion chamber and so attenuates the acoustic pulses propagating from the opening of the intake valve out towards the intake orifice.
  • Flow rate is controlled by the throttle body.
  • Flow rate and air temperature are measured at the throttle body.
  • Air is distributed to each of the air intakes for each cylinder by the plenum chamber with runners to each air intake valve.
  • Often the runner length is tuned to an acoustic length to enhance the pressure wave entering the engine at a certain engine speed.

The air intake system channels clean cold air into the engine and the exhaust system channels the burnt gases away from the engine.

Air Intake Manifold

The air intake manifold of an internal combustion engine performs a number of functions.

  • The network of passages direct air or air-fuel mixture from the throttle body to the intake ports in the cylinder head.
  • The flow typically proceeds from the throttle body into a chamber called the plenum, which in turn feeds individual tubes, called runners, leading to each intake port.
  • Engine breathing is enhanced if the intake manifold is configured to optimize the pressure pulses in the intake system.
  • Supports the fuel injection system in an indirect injection engine.

Air Intake Noise – Pulsating noise is produced as the engine draws air in (intake noise) for the combustion process. In large engines this noise is attenuated by an expansion volume that doubles as the box that holds the air filtration element.

Manifold Absolute Pressure – the total pressure of air going into the engine, boost pressure plus atmospheric pressure.

Mass Air Flow Sensor – A device mounted in the engines air induction system that provides an electrical signal representing the instantaneous rate of mass air flow.

About Nigel 308 Articles
Have been making models since I was around 7 years old and using a lathe from the age of 11, a self taught engineer with a passion for making model engines.

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