Fuel system engine

(1) Gasoline engine

1) Overview

The fuel system using gasoline engine is a device which mixes gasoline with the air in a suitable proportion and provides this mixed gas inside the cylinder. Gasoline is sent from the fuel tank to the carburetor after it has garbage and others removed by the fuel filter.

By the use of the carburetor, Gasoline gushes in a cloudy form, is mixed with the air, and is input into the cylinder.

2) Fuel filter

The fuel filter (propellant filter) for the gasoline engine generally uses mesh metal filter net or an element made up of filter paper inside a plastic container.

3) Carburetor

The Carburetor (vaporizer) is a device which mixes the air and gasoline after turning them into particles based on the same principle as a mister. It creates an air-fuel mixture suited to the engine’s rotation speed, load, and accelerator operation, and provides it inside the cylinder. It is classifiable into the main system which handles mid to highspeed range and the slow system which handles low-speed range.

In case of the mister as shown in the figure, negative pressure (lower than the atmospheric pressure) generates in “B” due to the flow of the air blown out of “A”, and because of the pressure difference between the atmospheric pressure which affects “C”, water is blown out as it is sucked out of “B”. The faster the air flows, the greater the negative pressure becomes. In proportion to that, the amount of water sucked out
becomes greater, too.

In the carburetor, as shown in the figure, the air coming in from “A” is squeezed through the Venturi making it flow fast, and that amount is provided as a mist through a nozzle equivalent to“B” to deliver the fuel

[Float system]

The carburetor is equipped with a system that sucks out the gasoline inside the float chamber using the pressure difference between the one in the Venturi area and the one in the float chamber.

Consequently, if the height of the gasoline level inside the float chamber changes, the amount of gasoline’s eruptive volume changes as well, making the density of the air-fuel mixture unstable.

Therefore, in order to keep the gasoline level at a certain height, the inlet flow amount of gasoline from the fuel tank is controlled by the float and the float valve.

[Main system]

In the main system, the air-fuel mixture is created in the venturi area and by the main
nozzle. As the piston goes down to intake the air, the air passes through the venturi area of the carburetor. At this time, the air that passes through the venturi area generates negative pressure. This negative pressure mixes the gasoline inside the float chamber with the air from the main air bleed, and lets it spray out of the main nozzle. At that time, the volume of gasoline flowing is limited by the main jet

■ Air bleed

The air bleed serves the function of preventing change of the mixture ratio due to change of the engine speed by mixing gasoline and air to facilitate atomization. As the air bleed is under high pressure due to negative pressure generated in the venturi part, air flows into the main nozzle from the air bleed and mixes with gasoline after turning into air bubbles.

When the engine rotates at low speed, gasoline spouts out from the main nozzle by being pushed up by the air bubbles to support gasoline supply. When the engine rotates at high speed, because the pressure difference between the air bleed and the venturi part becomes large, a large amount of air enters from the air bleed to prevent the fuel-air mixture from thickening.

[Idle system]

The idle system generates fuel-air mixture by the low-speed port and the idle port located
near the throttle valve. When the engine rotates at low speed, because opening of the throttle valve is small and negative pressure generated in the venturi part is small, gasoline cannot spout out from the main nozzle. Therefore, during low-speed rotation when opening of the throttle valve is small, gasoline inside the float chamber is sucked out from the low-speed port and the idle port by negative pressure generated in the gap between the throttle valve and the inner wall of the carburetor.

■ Idle port and low-speed port

The idle port is a port for supplying gasoline during idling in which the throttle valve is almostfully closed, and the amount of gasoline may be adjusted by the pilot screw. The low-speed port is a port for supplying gasoline during lowspeed rotation in which the throttle valve is slightly open.

The amount of gasoline flowing out from the idle port, the low-speed port and the main nozzle, and the engine speed in a noload state are shown in the chart. The total amount of gasoline outflow gradually increases corresponding to opening of the throttle valve to supply a necessary amount of gasoline ranging from idling to the maximum rotation

[Choke system]

The choke system is equipped to improve engine startability. In general, the ideal mixture ratio between air and gasoline is 15:1 in the mass ratio. In the case of starting the engine when it is cold, the ratio of gasoline in the fuel-air mixture lowers due to liquefaction of gasoline, making ignition difficult.

Starting is facilitated by operating the choke valve to decrease the air intake and increase gasoline in order to increase the ratio between air and fuel to around 5:1.

The mixture ratio changes through increasing the speed of the airflow and increasing the
amount of fuel flowing from the main nozzle by closing the choke valve.