Fuel and Air Systems

The intake stroke needs air and fuel in the right proportion. This lesson is how each gets there and what happens when the proportion is wrong.

The fuel path

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The order is the question. Tank, pump, filter, rail, injector - and the figure puts it on a banner because it is asked directly.

The filter is after the pump, not before. A pump pushes through a filter more reliably than it pulls through one, and a restricted filter on the suction side would starve the pump. A clogged filter starves the engine, showing up as poor power under load and hesitation, which is a diagnostic question.

The pump sits in the tank on most modern vehicles, where the fuel cools it and keeps it submerged. Running the tank very low can shorten a pump's life for that reason.

Fuel injection against carburetors

A carburetor relies on airflow to pull fuel in, using the pressure drop where the passage narrows - the same faster-means-lower-pressure effect from the fluids lesson.

Injection meters fuel electronically against sensor readings, so the mixture is right across temperatures, altitudes and loads. That precision is the reason for the change, and it is what a question about why injection replaced carburetors wants.

The air side

The throttle controls air, not fuel. Pressing the accelerator opens the throttle and lets more air in; the system then adds fuel to match. That is the right way round and it is asked.

A clogged air filter restricts airflow, which makes the mixture rich and costs power and economy.

Forced induction packs more air into the cylinder, so more fuel can be burnt and more power made. A turbocharger is driven by exhaust gas; a supercharger is driven by the engine through a belt or gears. That distinction is the question.

Air-fuel mixture

The mixture is the ratio of air to fuel, and for gasoline the ideal is about 14.7 parts air to 1 part fuel by weight.

Rich is wasteful; lean is dangerous. A lean mixture burns hotter, and sustained heat damages valves and pistons - which is why "lean runs hot" is worth remembering as a pair.

A choke enriches the mixture for cold starting on a carbureted engine, by restricting air. Fuel-injected engines do the same thing electronically and have no choke, which is a question about older engines.

Know it by sight

What you can skip

Across the 59 questions on this topic in our bank:

• Carburetor circuits. The choke never appears and the venturi once; the carburetor is asked mostly as the older alternative to injection.
• Injection system variants. Direct injection comes up once.
• The chemically correct mixture as a number. The 14.7 to 1 ratio is asked once. Rich and lean, and what each does, are what recur.

Where people lose points

Putting the filter before the pump.

Saying the throttle controls fuel. It controls air.

Swapping rich and lean. Rich is too much fuel.

Saying a lean mixture runs cool. It runs hot.

Confusing a turbocharger with a supercharger. Exhaust-driven against engine-driven.

Thinking high fuel consumption means a rich mixture. It is a ratio.

Work one in under a minute

A vehicle loses power under heavy load, hesitates on acceleration, but idles normally. What should be checked in the fuel system, and why does the symptom appear only under load?

A restricted fuel filter.

At idle the engine needs very little fuel, and even a partly blocked filter can pass that much. Under load the demand rises sharply, and the restriction cannot supply it, so the engine starves.

A fault that appears only at high demand points at a restriction rather than a failure, which is the same style of reasoning as the cooling system's idle-against-speed question.

Where this leads

The mixture this system delivers is what the ignition system lights, and what does not burn completely is what the emissions system has to deal with.