DIRECT PETROL INJECTION

DIRECT PETROL INJECTION

This diagram shows the layout of the Bosch direct injection system. Direct injection systems differ from conventional port injection in several ways.

The fuel supply system uses two fuel pumps – a conventional electrical fuel pressure pump (in the past dubbed a high pressure pump but now referred to in this system as a low pressure pump) and a mechanically-driven high pressure pump. The low pressure pump works at pressures of 0.3 – 0.5 MPa while the high pressure pumps boost this very substantially to 5 – 12 MPa.

The high pressure fuel is stored in the fuel rail that feeds the injectors. The fuel rail is made sufficiently large that pressure fluctuations within it are minimised as each injector opens. The pressure of the fuel in the injector supply rail is controlled by an electronically-controlled bypass valve that can divert fuel from the high pressure pump outlet back to its inlet. The fuel bypass valve is varied in flow by being pulse-width modulated by the Electronic Control Unit (ECU). A fuel pressure sensor is used to monitor fuel rail pressure.

This diagram shows a cross-sectional view of an injector. Compared with a conventional port fuel injection system, the fuel injectors must be capable of working with huge fuel pressures and also injecting large amounts of fuel in very short periods. The reason for the much reduced time in which the injection can be completed is due to the fact that all the injection must sometimes occur within just a portion of the induction stroke. Conventional port fuel injectors have two complete rotations of the crankshaft in which to inject the fuel charge – at an engine speed of 6000 rpm, this corresponds to 20 milliseconds. However, in some modes, direct fuel injectors have only 5milliseconds in which to inject the full-load fuel. The fuel requirements at idle can drop the opening time to just 0.4 milliseconds. Direct injection fuel droplets are on average only one-fifth the droplet size of traditional injectors and one-third the diameter of a human hair.

The very lean air/fuel ratios at which direct injection systems can operate results in the production of large quantities of oxides of nitrogen (NOx). As a result, direct injected cars require both a primary catalytic converter fitted close to the engine, and also a main catalytic converter - incorporating a NOx accumulator - that is fitted further downstream.