For most of the history of the internal combustion engine, the carburetor was used to get the necessary amount of fuel and air to the engine. Gasoline-powered lawn mowers, leaf blowers, chain saws, etc., still have carburetors, but unless your automobile was made before 1990, it doesn't. All modern automobiles use a more precise method to provide fuel and air to the engine: fuel injection.
History
As environmental concerns increased and emissions controls tightened, the requirements for closed-loop control of the fuel/air mixture produced more demands than the mechanical carburetor could meet. Eventually carburetors were replaced by several types of fuel injection systems. With fuel injection, sensors in the exhaust can monitor the amount of oxygen and send the information to the engine control unit (ECU). The ECU can then adjust the fuel injectors to optimize the use of fuel and minimize emissions.
Fuel Rails
Modern fuel injection systems are multiport, which means that each engine cylinder has its own fuel injector. Fuel is routed to each of the ports through a single header or pipe called a fuel rail. The fuel rail has output ports to each of the cylinders and to the cold start valve, a device that makes engine starting easier. When the accelerator is pressed, the throttle valve opens, letting in more air. The ECU adjusts the fuel input to match the increase in airflow and uses the amount of air input as well as the oxygen in the exhaust to fine-tune the fuel mixture.
Types
Single-point or throttle-body fuel injection delivers fuel to all the cylinders at once. It was used primarily to make the switch from carburetors easier. Multiport fuel injection systems (MFI) make use of the fuel rail to deliver fuel to a different injector for each cylinder, usually right at the intake valve. Sequential multiport fuel injection systems deliver fuel just before the cylinder's intake valve opens, further improving engine efficiency.
Pressure Transducers
Pressure transducers convert pressure into an electrical signal that can be transmitted to another instrument or control system. A common type of pressure transducer is one that uses a strain gauge, a piece of metal or other material that is deformed by strain, causing its electrical resistance to change. This change is converted to an electrical signal that indicates the pressure.
Role
The pressure in the fuel rail of a multiport fuel injection system is used by the ECU to adjust the output of the fuel pump to maintain stable fuel pressure. Stable fuel pressure is necessary to reduce vapors in the fuel system. The fuel rail pressure transducer measures the fuel pressure right at the injection site, ensuring that the fuel pressure value passed to the ECU is both accurate and precise.