Chapter 17 - The Carburettor Flashcards
What is the basic function of the Carburettor?
To mix fuel and air
What is the approximate chemically correct micture ratio for fuel and air?
1:12 (one part fuel to twelve parts air)
Describe the motion of the float and needle in the Float Type Carburettor when the fuel level increases inside the float chamber.
Float: rises
Needle: drops, blocking the needle valve, preventing more fuel entering the float chamber
What is the function of the butterfly valve in a Carburettor?
To control how much of the fuel/air mixture can pass into the cylinders
What is the butterfly valve directly linked to?
The throttle lever
What is a rich mixture?
After burning all of the air in the mixture, we have fuel left over
What is a lean mixture?
After burning all of the fuel in the mixture, we have air left over
What is a venturi? (physical description)
A choke (constricted area) in a passage
What is the venturi effect?
The decrease in static pressure as teh velocity of air increases through a contricted area
What is a weak-cut?
If the throttle is advanced too quickly, the fuel/air mixture can become too lean, cutting out the engine
This happens as fuel is denser than air and takes more time to pass through the carburettor
What can be fitted to a carburettor to avoid a weak cut?
An accelerator pump
How does an accelerator pump typically work?
A linkage on the throttle detects a rapid forward input, pushing a metal block which pumps fuel, under pressure, into the venturi throat
Why would we need an idling system?
To prevent the engine cutting out during low RPM conditions (such as ground operations)
How does an idling system work?
When the butterfly valve is closed, a small passage opens, bypassing the valve, releasing fuel into the area after the valve, using differnces in pressure
What are the two main types of fuel/air mixture control?
Back-suction and needle type
