Bosch electronic fuel injection
The main difference between electronic injection and mechanical injection is that an electronic system is controlled by a complex microprocessor control unit (sometimes called an electronic control unit or ECU), which is basically a miniature computer.
This computer is fed with information from sensors mounted on the engine. These measure factors such as the air pressure and temperature in the air intake, the engine temperature, accelerator position and engine speed. All this information allows an electronic system to meter the fuel far more accurately than the simple mechanical system, which relies on sensing the airflow alone.
The computer compares the input signals from the sensors with information already programmed into it at the factory, and works out exactly how much fuel should be delivered to the engine. It then signals the on-off valve ig the injector to open and squirt fuel into the inlet port. All this happens in a fraction of a second, the control unit responding instantly to changes in accelerator position, temperature and air pressure.
As well as improved control over fuel flow, the electronic system also operates at lower pressure than a mechanical system – usually at around 25-30psi. This makes it run more quietly than a mechanical system does.
A typical system is the Bosch LJetronic, which is fitted to a wide range of European cars. In this system, fuel is drawn from the tank by an electric pump. It is then fed straight up pipes to the injectors. The system pumps more fuel than is needed for injection – a loop circuit returns the excess to the fuel tank via a pressure regulator which keeps the pressure in the pipes constant.
The injector valves are held closed by springs, and opened by solenoids(electromagnets) when signalled to do so by the control unit. The amount of fuel injected depends on how long the solenoid holds the injector open.