Chapter 8: Induction Systems

A comprehensive set of Q&A flashcards covering fuel-air mixtures, carburetors, idle and lean/rich adjustments, carburetor icing, and modern fuel injection concepts.

What is the ideal fuel-to-air mixture ratio by weight for combustion in piston engines?

1 part fuel to 12 parts air (1:12).

What is the chemically correct mixture ratio and how does it compare to 1:12 for power output?

Chemically correct is about 1:15, but a 1:12 ratio has been found to give better power output.

Between which two ratios can combustion occur in the engine?

Between 1:9 (rich) and 1:18 (lean).

What are the two main induction systems found in piston engines?

Carburetor system and fuel injection system.

What is the starting point for both induction systems?

Collecting air from the outside via an air intake, through an air filter, with an alternate air source if the filter is blocked.

What is the float-type carburetor and why is it common?

A carburetor that uses a float in the float chamber to regulate fuel delivery; it is the most common type in older piston engines.

What is the role of the venturi in a float-type carburetor?

The venturi accelerates the air, causing a drop in pressure (Bernoulli’s principle), which draws fuel through the fuel jet from the float chamber.

Where is the fuel jet located and what does it do?

Located in the venturi; connected to the float chamber; it forces fuel to flow into the airstream to form the fuel-air mixture.

What can an atomizer and diffuser in the fuel jet do?

Help vaporize the fuel to improve mixing with the air.

How does the butterfly valve affect fuel flow in a float-type carburetor?

Its position, controlled by the throttle, determines how much air flows through the venturi and thus how much fuel is drawn through the jet.

What is the function of the accelerator pump?

Provides an extra squirt of fuel when the throttle is quickly opened to prevent a lag in power.

How does the float chamber regulate fuel level?

A float moves a needle valve to keep the fuel level constant; rising fuel raises the float and closes the needle, lowering fuel inflow, and vice versa.

What is the purpose of the idle system in a carburetor?

To deliver a small amount of fuel upstream of the butterfly valve so the engine can idle smoothly when air flow is low.

Why is carburetor calibration typically for sea level, and what happens at higher altitudes?

Because it’s calibrated for sea-level air density; at higher altitude, air is thinner, causing a tendency toward an over-rich mixture unless leaned.

How do pilots lean the mixture and what effect does it have on rpm?

Move the mixture control toward lean; rpm initially increases as the mixture becomes optimized, then may drop if too lean; the engine should be kept slightly on the rich side at high power for cooling.

What is the role of an exhaust gas temperature (EGT) gauge in leaning the mixture?

Used to find the optimal mixture position at peak EGT for efficiency.

What is idle cut-off in a carburetor system?

Closing the mixture so no fuel is delivered, causing the engine to stop.

What are the two main problems with float-type carburetors?

Poor response during abrupt maneuvers due to gravity-driven fuel flow, and carburetor ice around the venturi and butterfly valve.

What is carburetor ice and its typical temperature range?

Ice that forms around the venturi or butterfly due to cooling; commonly occurs around 5°C to 25°C (41°F to 77°F) but can occur with high humidity.

How can carburetor icing be prevented?

Use carburetor heat to introduce warm air and melt ice; avoid prolonged heating as it reduces engine power; perform run-ups to clear ice.

What is fuel injection and how does it differ from a carburetor in mixing air and fuel?

Fuel is injected under pressure either into the inlet manifold or directly into the cylinder, and is not mixed with air in a venturi; this eliminates carburetor icing.

What are the two main types of fuel pumps in a fuel injection system and their roles?

Engine-driven pump (runs when the engine runs) and electric boost/auxiliary pump (starts the engine and backs up the engine-driven pump).

What is FCU and FMU in a fuel injection system?

FCU (Fuel Control Unit) regulates how much fuel is delivered per cylinder based on throttle/mixture; FMU (Fuel Manifold Unit) distributes fuel to the cylinders.

What does continuous flow mean in a fuel injection system?

Fuel is continuously injected into the inlet manifold and drawn into each cylinder when the intake valve opens.

What is direct fuel injection and its advantage?

Fuel is sprayed directly into each cylinder via injector nozzles; allows precise fuel-air ratios per cylinder and reduces intake icing; typically used in larger engines.

Does fuel injection suffer from carburetor icing, and what icing can it experience instead?

It does not suffer from carburetor icing since fuel isn’t mixed in a venturi, but it can experience intake icing (impact icing) at the air intake.

What are hot starts and vapor lock in fuel-injected systems?

Hot start: difficulty starting after a recent shutdown due to vaporized fuel in lines; vapor lock is blockage from fuel vapor; a boost pump can help by pressurizing lines.

What is intake icing (impact icing) and when is it most likely?

Ice forming at the intake or inlet path, more likely above 0°C with visible moisture (clouds or rain); risk of reduced power or blade damage if ice breaks off.

What is the role of supercharging in addressing altitude-related performance changes?

Improves air density to maintain power at higher altitudes by forcing more air into the engine.