Fuel injection is the system that delivers fuel into an engine as a controlled spray instead of a slow stream or drip. It matters because gasoline burns best when it is mixed with air in the right proportion and spread into tiny droplets. Modern engines use electronic control to decide exactly how much fuel to inject and when to inject it.
This precise fuel delivery improves power, fuel economy, starting, and emissions.
Key Facts
- A common target mixture for gasoline is the stoichiometric air to fuel ratio: 14.7:1 by mass.
- Fuel injectors atomize liquid fuel into tiny droplets so it can evaporate and mix with air quickly.
- The engine control unit adjusts injector pulse width, which is how long the injector stays open.
- Fuel mass delivered is approximately m = flow rate × pulse time.
- More air entering the cylinder usually requires more fuel to keep the mixture near the target ratio.
- Sensors such as oxygen, throttle position, air flow, and temperature sensors help the engine control unit correct fuel delivery.
Vocabulary
- Fuel injector
- A fuel injector is an electronically controlled valve that sprays fuel into the intake port or combustion chamber.
- Atomization
- Atomization is the breakup of liquid fuel into many tiny droplets to help it evaporate and burn efficiently.
- Air fuel ratio
- Air fuel ratio is the mass of air compared with the mass of fuel in the mixture entering the engine.
- Pulse width
- Pulse width is the amount of time an injector is commanded open during one injection event.
- Engine control unit
- The engine control unit is the computer that uses sensor data to control fuel injection, ignition timing, and other engine functions.
Common Mistakes to Avoid
- Thinking the injector pours fuel like a faucet is wrong because an injector sprays a fine mist that mixes with air and evaporates much faster.
- Ignoring the air side of the mixture is wrong because fuel delivery must match the amount of air entering the cylinder for efficient combustion.
- Assuming longer injector pulse width always improves performance is wrong because too much fuel makes the mixture rich, wastes fuel, and can increase emissions.
- Confusing spark timing with injection timing is wrong because spark timing controls when the mixture is ignited, while injection timing controls when fuel is sprayed.
Practice Questions
- 1 A cylinder receives 0.44 g of air during an intake event. Using a 14.7:1 air to fuel ratio, what mass of gasoline should be injected?
- 2 An injector flows 0.020 g of fuel per millisecond. How much fuel is delivered during a 4.5 ms pulse?
- 3 A cold engine needs extra fuel during startup. Explain why colder air and cold metal surfaces can require a richer mixture than a warm engine.