How Fuel System Of IC Engine Operates During Slow-Running And Cold-Starting Period
How Engine slow-running operates
At very low engine speeds, there is an insufficient vacuum to draw any fuel from the venturi outlet, but there is a high vacuum on the engine side of the throttle valve because the engine is trying to draw in air.
This high vacuum is used to suck a mixture of fuel and air through a separate idling circuit which has its outlet on the engine side of the throttle valve. In this way, the engine keeps running when the throttle is closed.
The flow from the idling circuit outlet is adjusted by a tapered screw to obtain the smoothest idling speed. The presence of an air entry point in the circuit prevents it from siphoning petrol when the engines have stopped.
There may be other small progression drillings adjacent to the throttle valve.
These bridge the gap between the slow-running circuit and the main outlet by supplying fuel which mixes with the first rush of air past the valve as it opens.
Because the idling circuit needs a high vacuum to operate it, once the throttle has opened past the progression drillings, it automatically stops working.
How the Engine cold starting works
To provide a very rich mixture for cold starting, a choke flap is used to block the entry of air into the barrel.
When the engine is cranked over, it creates a high vacuum below the flap, drawing a rich mixture from the outlets. As soon as the engine fires, the flap must be partially opened.
Most manually-operated choke flaps are spring-loaded into the shut position and automatically open a little when the engine starts. As the engine warms up, the choke should be gradually disengaged by the driver.
Cold engines will not idle at normal speed, so a mechanism is provided to open the throttle valve slightly and give a fast idle when the choke is in use.
Automatic chokes usually have a metallic coil attached to the choke flap spindle. When the engine is cold the spring contracts, holding the flap fully shut.
When the engine starts, heat from the exhaust, cooling system, or an electrical element will be directed at the spring.
The heat expands it, causing it to unwind and gradually open the flap. It will also have a linkage to provide a fast idling speed when the choke is in use.
A flap is used to partially block the barrel for cold starts. It increases the vacuum around the fuel outlet and draws more fuel to provide a rich mixture.
A temperature-sensitive spring, expanded by the warming-up of the exhaust or cooling system, is used to operate the choke flap on an automatic choke.
How the Accelerator Pumps work in the fuel system
The accelerator pump provides an additional supply of fuel which prevents the engine from temporarily cutting out or losing power when the throttle is suddenly opened for acceleration.
A piston or diaphragm, acting on a well filled with fuel, is connected to the throttle linkage and sprays a jet of neat fuel into the carburetor barrel when the throttle is opened.
The injected fuel mixes with the ‘gulp’ of the incoming air and provides a smooth increase in power by eliminating any fuel lag.
The accelerator pump squirts an enrichening shot of fuel down the barrel to provide a quick response when the throttle is opened quickly.
