Carburetor Case Study

1) Starting with the piston at top dead center (TDC 0 degrees) ignition has occurred and the gasses in the combustion chamber are expanding and pushing down the piston. This pressurizes the crankcase causing the reed valve to close. At about 90 degrees after TDC the exhaust port opens ending the power stroke. A pressure wave of hot expanding gasses flows down the exhaust pipe. The blow-down phase has started and will end when the transfer ports open. The pressure in the cylinder must blow-down to below the pressure in the crankcase in order for the unburned mixture gasses to flow out the transfer ports during the scavenging phase.

2) Now the transfer ports are uncovered at about 120 degrees after TDC. The scavenging phase has begun. Meaning

The pipe is in the form of a venturi it narrows in section and then widens again, causing the airflow to increase in speed in the narrowest part. Below the venturi is a butterfly valve called the throttle valve a rotating disc that can be turned end-on to the airflow, so as to hardly restrict the flow at all, or can be rotated so that it (almost) completely blocks the flow of air. This valve controls the flow of air through the carburetor throat and thus the quantity of air/fuel mixture the system will deliver, thereby regulating engine power and speed. The throttle is connected, usually through a cable or a mechanical linkage of rods and joints or rarely by pneumatic link, to the accelerator pedal on a car or the equivalent control on other vehicles or

Fuel flow is adjusted by means of precisely calibrated orifices, referred to as jets, in the fuel path. Fig5 : Cross sectional schematic of carburetor

3.5 Types of carburetors

1. Natural or Side Draft carburetor: This carburetor is used where there is little space on top of the engine. The air horizontally into the manifold

2. Updraft carburetor: This type is placed low on the engine and uses a gravity fed-fuel supply. In other words, the tank is above the carburetor and the fuel falls to it. Even this carburetor uses gravity to receive the fuel from the tank, the air-fuel mixture must be forced upward into the engine

3. Downdraft carburetor: This carburetor operates with lower air velocities and larger passages. This is because gravity assists the air-fuel mixture flow to the cylinder. The downdraft carburetor can provide large volumes of fuel when needed for high speed and high power output.

, Volume 12, Issue 4 ver III (Jul-Aug 2015) , PP 58 –