1. Why are some fuel rails rectangular in shape? a. Increases fuel pressure b. Helps keep air out of the injectors c. Reduces noise d. Increases the speed of the fuel through the fuel rail
2. Which fuel-injection system uses the MAP sensor as the primary sensor to determine the base pulse width? a. Speed density b. Mass airflow c. Demand delivery d. Mechanical returnless
The correct answer and explanation is:
1. b. Helps keep air out of the injectors
2. a. Speed density
Detailed Explanation
Rectangular Fuel Rails:
Fuel rails are critical components in the fuel delivery system, ensuring that each injector receives a consistent supply of fuel. A key design challenge is the prevention of air pockets, which can lead to erratic fuel delivery, poor combustion, and even engine misfires. Rectangular fuel rails are engineered with a shape that minimizes “dead space” where air might accumulate. By using a non-circular cross-section, the design helps to maintain a steady, uninterrupted flow of fuel directly to each injector. This geometric configuration effectively “pushes out” any trapped air, ensuring that the injectors operate in a fully fuel-filled environment. Although options like increasing fuel pressure, reducing noise, or increasing fuel speed might seem plausible, the primary advantage of a rectangular design is its ability to keep air out of the system, which is essential for maintaining the precision of fuel injection and overall engine performance.
Speed Density Fuel-Injection System:
In fuel management, the precise calculation of fuel delivery is crucial. The speed density system achieves this by using the Manifold Absolute Pressure (MAP) sensor as its primary input for engine load. The MAP sensor measures the absolute pressure in the intake manifold, which, when combined with engine speed (RPM) and intake air temperature, allows the Engine Control Unit (ECU) to accurately compute the air mass entering the engine. This calculation is then used to determine the base fuel injector pulse width, ensuring the air-to-fuel ratio remains optimal for combustion. Unlike mass airflow systems that rely on direct measurement of air mass, speed density systems leverage mathematical models based on MAP data to drive fuel calculations. This method offers a simpler and more cost-effective solution while still providing reliable performance, making it a popular choice in many modern engines.
Below is an illustrative diagram representing a rectangular fuel rail feeding injectors and the connection of a MAP sensor on an engine’s intake manifold:
