Direct injection is a fuel delivery system that sprays gasoline or diesel fuel straight into the engine cylinder. This matters because the cylinder is where air, fuel, compression, and ignition come together to make power. Compared with older port injection systems, direct injection can control fuel timing and spray shape more precisely.
That precision can improve efficiency, power, and emissions when the system is well designed.
Key Facts
- Direct injection sprays fuel directly into the combustion chamber, not into the intake port.
- A four-stroke engine cycle is intake, compression, power, and exhaust.
- For gasoline engines, the spark plug ignites the air fuel mixture near the end of compression.
- Air fuel ratio = mass of air / mass of fuel.
- Stoichiometric gasoline combustion is about 14.7:1 by mass, meaning 14.7 parts air to 1 part fuel.
- Fuel pressure in gasoline direct injection is often about 50 to 350 bar, much higher than port injection.
Vocabulary
- Direct injection
- Direct injection is a fuel system that sprays fuel directly into the engine cylinder during the engine cycle.
- Combustion chamber
- The combustion chamber is the space above the piston where air and fuel burn to release energy.
- Fuel injector
- A fuel injector is an electronically controlled valve that atomizes fuel into a fine spray.
- Compression stroke
- The compression stroke is the part of the cycle when the piston moves upward and squeezes the air or air fuel mixture.
- Atomization
- Atomization is the process of breaking liquid fuel into tiny droplets so it can mix and burn more effectively.
Common Mistakes to Avoid
- Thinking direct injection sprays fuel into the intake manifold is wrong because the injector tip points into the cylinder and sprays into the combustion chamber.
- Ignoring spray timing is wrong because fuel injected too early or too late may mix poorly, hit the piston or cylinder wall, or burn inefficiently.
- Assuming more fuel always means more power is wrong because the engine also needs the correct amount of air and proper ignition timing.
- Confusing direct injection with a carburetor is wrong because a carburetor mixes fuel with air before the intake system, while direct injection uses electronically controlled high-pressure injectors.
Practice Questions
- 1 A gasoline engine cylinder receives 0.040 g of fuel during one cycle. Using a stoichiometric air fuel ratio of 14.7:1, what mass of air is needed for complete combustion?
- 2 A direct injector delivers fuel at 200 bar. If 1 bar is about 100,000 Pa, what is the injection pressure in pascals?
- 3 Explain why spraying fuel directly into the cylinder can improve control of combustion compared with spraying fuel into the intake port.