How does fuel get from the tank into the engine and mix with air to burn?
The petrol fuel system from tank to cylinder, the role of the air-fuel mixture and the carburettor or fuel injection, and the air filter and exhaust system.
A CCEA GCSE Motor Vehicle and Road User Studies answer on the petrol fuel system: tank, pump, filter and the air-fuel mixture made by the carburettor or fuel injection, plus the air filter and exhaust system.
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What this dot point is asking
CCEA wants you to trace the petrol fuel system from tank to cylinder, explain how the air-fuel mixture is made by a carburettor or fuel injection, and describe the air filter and the exhaust system. This links the four-stroke cycle (which needs the mixture) to the environmental topic (which deals with the emissions that leave the exhaust).
The answer
The job of the fuel system
The engine burns a mixture of petrol and air. The fuel system's job is to deliver petrol, clean it, and mix it with clean air in the right proportion for efficient combustion.
The path of petrol from tank to cylinder
Making the air-fuel mixture
- A carburettor (used on older engines) draws fuel into a stream of air using the pressure drop as air speeds up.
- Fuel injection (used on modern engines) sprays a precisely metered amount of fuel, controlled electronically. It gives better fuel economy, smoother running and lower emissions than a carburettor.
The air filter
The air filter cleans the air entering the engine, trapping dust and grit so they cannot enter the cylinders and cause wear. A blocked air filter restricts airflow, making the mixture too rich and harming performance and economy.
The exhaust system
After combustion, the exhaust system carries the burnt gases away from the engine, quietening the noise through the silencer (muffler). The gases are mainly carbon dioxide and water vapour, plus pollutants - carbon monoxide, oxides of nitrogen and unburnt hydrocarbons. A catalytic converter in the exhaust reduces these harmful gases.
Worked example: a fault in the fuel system
Examples in context
Example 1. Why injection replaced the carburettor. Electronic fuel injection adjusts the mixture continuously for the conditions, so modern cars are cleaner and more economical than older carburettor cars.
Example 2. The MOT emissions check. The exhaust gases are tested at the MOT; a faulty mixture or catalytic converter can cause an emissions failure.
Try this
Q1. What two things are mixed to form the combustible mixture? [1 mark]
- Cue. Petrol (fuel) and air.
Q2. What does the air filter do? [1 mark]
- Cue. Cleans the air entering the engine, removing dust and grit.
Q3. Name one advantage of fuel injection over a carburettor. [1 mark]
- Cue. Any one of: better fuel economy, smoother running, lower emissions.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA style4 marksDescribe the path of petrol through the fuel system from the tank to the cylinder, naming the main parts it passes through.Show worked answer →
Petrol is stored in the fuel tank. A fuel pump draws it from the tank along the fuel line/pipe, passing through a fuel filter that removes dirt. It then reaches the carburettor (older engines) or the fuel injection system (modern engines), where it is mixed with air in the correct ratio. The air-fuel mixture passes through the inlet manifold and inlet valve into the cylinder to be burnt.
Markers reward the correct order: tank, pump, filter, carburettor/injection (mixing with air), inlet manifold/valve, cylinder.
CCEA style4 marksExplain the purpose of (a) the air filter and (b) the carburettor or fuel injection system, and state what comes out of the exhaust system.Show worked answer →
(a) The air filter cleans the air drawn into the engine, removing dust and grit so they do not enter the cylinders and cause wear. A blocked air filter restricts airflow and harms performance and economy.
(b) The carburettor (or fuel injection) mixes petrol with air in the correct proportion to form a combustible air-fuel mixture; fuel injection sprays a precisely metered amount of fuel, giving better economy and lower emissions than a carburettor.
The exhaust system carries away the burnt gases (exhaust emissions) - mainly carbon dioxide and water vapour, plus pollutants such as carbon monoxide, nitrogen oxides and unburnt hydrocarbons - and the catalytic converter reduces the harmful ones.
Markers reward: air filter cleans the air, carburettor/injection mixes fuel and air, exhaust removes burnt gases (with the catalytic converter reducing pollutants).
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