Cars · Lesson 3 of 15Advanced6 min read

Air, Fuel and Spark

A gasoline engine needs three things at once: the right amount of air, the right amount of fuel for that air, and a spark at the right instant. A computer juggles all three, adjusting them constantly as you drive.

The recipe

Gasoline burns completely with about 14.7 pounds of air for every pound of fuel. That’s the chemically correct, or stoichiometric, mix. A mix with extra fuel is called rich; one with extra air is lean. Because the chemistry depends on how many molecules of air there are, the engine computer cares about the air’s mass, not just its volume.

In a gasoline engine you control power with air. The throttle is a valve in the intake: press the pedal and it opens, more air flows in, and the computer adds fuel to match. A diesel works the other way around, as you’ll see below.

The air side

Air enters through the air filter, which keeps sand, dust and lint out of the engine. Dirt that gets in causes engine wear and poor combustion. Next, an airflow sensor measures the air. A common type has a heated platinum wire in the air stream: the more air rushing past, the more power it takes to keep the wire hot, and that tells the computer how much air is coming in. The air then passes the throttle and the intake manifold, which splits it among the cylinders.

The fuel side

Late-model cars use an electric fuel pump, usually inside the tank. It pushes fuel through the lines and a filter to the fuel injectors. A pressure regulator holds the fuel at a set pressure. Some vehicles also have an inertia switch that shuts the pump off in a crash or rollover.

An injector is an electrically controlled valve. When the computer energizes its solenoid, the nozzle opens and sprays fuel; a spring snaps it shut. The computer sets the amount of fuel by changing how long each injector stays open. Some engines spray into the intake ports, others straight into the combustion chamber (direct injection).

The spark

A spark plug needs roughly 4,000 to 10,000 volts to jump its gap. The ignition coil turns the car’s 12 volts into 30,000 volts or more, plenty to fire the plug every time.

Timing matters as much as voltage. Normal combustion takes about three thousandths of a second, so the plug fires before the piston reaches the top. The goal is peak pressure just after top dead center, about 14 to 17 degrees of crank rotation past it in a typical gasoline engine, when the piston can use the push best.

The brain: closed-loop control

The electronic control module (ECM), the engine computer, manages the fuel mixture, ignition timing and emissions controls. It also watches over the engine, protects it, and records problems it finds.

Its most important helper is the oxygen sensor in the exhaust. A common type is a small ceramic cell that makes a voltage: high when the exhaust is rich (little leftover oxygen), low when it’s lean. The computer reads that signal and trims the injector open time, then checks the result and trims again. That feedback is called closed-loop control. A cold sensor doesn’t work well, so most have a built-in heater; until they’re ready, the computer runs on preset values (open loop).

Closed loop: the oxygen sensor reports whether the exhaust is rich or lean, and the computer corrects the injector open time, over and over as you drive. Sources: Clemson University Vehicular Electronics Laboratory; U.S. Navy Construction Mechanic Basic.

Parts and what they do
Part	What it does	Signs of trouble
Air filter	Keeps dust and grit out of the engine.	A filter caked with dirt; replace at the interval in your manual.
Airflow sensor	Measures the mass of air coming in.	Rough running, stalling, a check engine light.
Throttle	Lets in more or less air when you press the pedal.	Rough or high idle, slow response.
Fuel pump	Pushes fuel from the tank to the engine.	Hard starting, stalling, loss of power under load.
Fuel filter	Catches dirt before it reaches the injectors.	Loss of power when it clogs.
Injectors	Spray a measured squirt of fuel each cycle.	Misfire, rough idle, fuel smell.
Spark plugs	Make the spark inside each cylinder.	Misfire, rough idle, hard starting.
Ignition coils	Turn 12 volts into the high voltage the plugs need.	A misfire in one cylinder, a flashing check engine light.
Oxygen sensor	Tells the computer whether the exhaust is rich or lean.	Poor fuel economy, a check engine light.
Engine computer (ECM)	Runs fuel, spark and emissions controls; stores trouble codes.	Many symptoms at once; a technician diagnoses it.

How diesels differ

A diesel draws in air only and squeezes it hard, which makes it very hot. Then it sprays fuel into that hot air at a measured rate, and the fuel lights on contact. There are no spark plugs. Some diesels have glow plugs that heat the air in a small chamber to help cold starts. A diesel controls its speed with the amount of fuel injected, not by throttling the air.

A turbocharger is a fan spun by the exhaust that forces more air into the engine under pressure. It can turn up to 100,000 rpm, so engine oil lubricates its shaft bearings. Modern diesels also clean their exhaust: a particulate filter traps soot, an oxidation catalyst cleans other pollutants, and diesel exhaust fluid (a urea and water solution kept in its own tank) is sprayed in so a selective catalytic reduction unit can turn nitrogen oxides into nitrogen and water.

You canWhat you can safely do

Check and replace the engine air filter at the interval your owner’s manual lists. On most cars it sits in a box you can open by hand or with a screwdriver.
Use the fuel your manual or fuel door specifies, including the octane rating.
Make sure the gas cap clicks tight. A loose cap can turn on the check engine light (Lesson 5).

StopWhen to stop

Gasoline vapor catches fire easily. If you smell raw fuel strongly, or see fuel dripping, don’t start the car; have it checked. A flashing check engine light usually means a serious misfire: get it looked at right away and drive as little as possible.

Check yourself

Tap an answer to check it, or open “Show the answer.”

About how many pounds of air does a gasoline engine need to burn one pound of fuel completely?
Show the answer
14.7. About 14.7 pounds of air per pound of fuel. That’s the stoichiometric ratio.
What does the oxygen sensor tell the engine computer?
Show the answer
Whether the exhaust is rich or lean. It reports rich (high voltage) or lean (low voltage), and the computer corrects the fuel.
How does the engine computer change the amount of fuel injected?
Show the answer
By changing how long the injector stays open. It varies how long each injector nozzle stays open.
What lights the fuel in a diesel engine?
Show the answer
The heat of the squeezed air. Diesels use compression ignition: air is squeezed until it’s hot enough to light the fuel sprayed into it.

Go deeper

Clemson University Vehicular Electronics Laboratory: Oxygen Sensors How oxygen sensors work, narrowband and wideband.
Clemson University Vehicular Electronics Laboratory: Air Flow Sensors Air flow sensors and why air mass matters.
U.S. Department of Energy, Alternative Fuels Data Center: How Do Diesel Vehicles Work? Diesel vehicle parts, including the exhaust cleanup system.
U.S. Navy, Construction Mechanic Basic (NAVEDTRA 14264A), Chapter 5: Diesel Fuel Systems The Navy’s chapter on diesel fuel systems and turbochargers.

Sources

I finished this lessonSaved on this device only.

A free course from Teddy Rook Book. It explains how cars work in general and follows U.S. sources; your owner’s manual and your vehicle’s service information are the final word for your car, along with state and local rules.