Understanding the Carburettor in an SI Engine

An SI engine stands for Spark Ignition engine. These are commonly used in petrol vehicles. For a petrol engine to run, it needs a precise mixture of air and fuel (petrol) to be supplied to the combustion chamber where it is ignited by a spark plug.

Role of the Carburettor

The carburettor is a key component in older SI engines (newer ones often use fuel injection). Its primary function is to create the correct air-fuel mixture required for combustion. It mixes atomized fuel with air in the right proportion before sending it to the engine cylinders.

• The carburettor draws air in.
• It then meters the flow of fuel.
• Finally, it mixes the air and fuel together into a combustible mixture.

This air-fuel mixture then enters the combustion chamber during the intake stroke.

Analyzing the Statements

Let's look at the given options based on the function of a carburettor in an SI engine:

• Statement 1: "In an SI engine, the carburettor supplies grease to the cylinder." This is incorrect. Grease is a lubricant and is not supplied by the carburettor; engine oil handles lubrication.
• Statement 2: "In an SI engine, the carburettor supplies both air and grease mixture to the cylinder." This is incorrect for the same reason as statement 1 regarding grease.
• Statement 3: "In an SI engine, the carburettor supplies only fuel to the cylinder." This is incorrect. While it meters fuel, it must mix it with air to create a combustible mixture. Supplying only fuel would not allow the engine to run correctly.
• Statement 4: "In an SI engine, the carburettor supplies both air and fuel mixture to the cylinder." This statement accurately describes the main function of a carburettor in an SI engine – to mix air and fuel and supply this mixture to the cylinder.

Therefore, the correct statement is the one that says the carburettor supplies both air and fuel mixture to the cylinder.