Induction Systems in Reciprocating Engines

Study Notes

The primary goal of the induction system within a reciprocating engine is to provide a sufficient amount of air needed for proper combustion.
Three main categories of reciprocating engine induction systems are:
- Normally aspirated
- Supercharged
- Turbocharged

The induction system ensures the delivery of air from the outside to the engine.
The pathway: outside air is directed through an air intake and then channeled to a carburetor or a similar fuel metering device.
The fuel/air mixture is then delivered into an intake manifold where it's routed to the cylinders for combustion.

A typical setup for a normally aspirated engine's induction system includes these four primary components:
- Air intake
- Induction air filter
- Fuel delivery system
- Induction manifold
Additional subsystems like:
- Alternate air source
- Ice removal/prevention systems
- Temperature indicating system, are designed to support the operation of these four primary components.

Also known as air scoops, are designed to direct air from the outside into a carburetor or fuel metering device.
Typically positioned to maximize utilization of ram air pressure.
Frequently placed within the propeller slipstream due to increased velocity.
Due to an increase in air pressure, a well-designed intake scoop can have a substantial influence on an engine's power output.

Commonly installed within air intake ducts.
Its primary purpose is to prevent entry of dust, sand, abrasive materials, or any other contaminants into the engine.
This system ensures engine performance, reliability, and a long service life.

A significant source of potential problems for aircraft engines.
Dust consists of small particles of hard, abrasive material.
Dust accumulation can contribute to:
- Accelerated wear on cylinder walls and piston rings.
- Silicon fouling of spark plugs.
- Contamination of engine oil

Leads to wear on bearings and gears.
In severe cases, dust accumulation can even clog oil passages, causing oil starvation.
Dust may also collect in fuel metering devices, compromising their effectiveness in providing the correct mixture ratio across all power settings.

A choice between two types of fuel delivery systems for normally aspirated engines:
- Carburetor
- Fuel injection system
The fuel delivery system's role is to accurately meter the amount of fuel and air that's delivered to the cylinders.
A detailed explanation of the fuel/air metering process is provided in fuel system and metering chapters.

Typically consists of ductwork.
Routes airflow from the fuel metering device to individual engine cylinders.
Acts as the connecting point for all individual pipes that deliver air or the fuel/air mixture to the cylinders.
One end of each cylinder's intake pipe is usually bolted to the cylinder intake port on each cylinder while the other end attaches to the manifold using a short section of synthetic rubber hose or rubber packing with a packing nut.
Both methods allow some movement between the intake pipes and the manifold as cylinders expand and contract.
In some cases, the intake manifold passes through the oil sump before branching out to each cylinder.
This process increases the temperature of the fuel/air mixture, which, in turn, improves fuel vaporization.

A crucial characteristic of an intake manifold is maintaining a gastight seal.
Any loss of seal leads to air leakage into the intake manifold, resulting in a lean mixture.
This can cause the engine to run rough.
Small leaks are most noticeable at idle because the pressure difference between the manifold and the atmosphere is the greatest at low rpm.